I have some code and when it executes, it throws a NullReferenceException, saying:

Object reference not set to an instance of an object.

What does this mean, and what can I do to fix this error?

8 upvote
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in case framework based error refer my below answer //allinonescript.com/questions/779091/… – Vijay Kumbhoje
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The exception helper in VS 2017 will be more helpful in diagnosing the cause of this exception -- blogs.msdn.microsoft.com/visualstudio/2016/11/28/… under New Exception Helper. – Zev Spitz
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Cant we just say "object has not been initialized"? So if you have a variable declaration: SomeClass myVariable; this will create myVariable with reference to SomeClass, but it is Not initialized and will be equal to null. You have to invoke the class initialization by doing SomeClass myVariable = new SomeClass(); Or, if you are returning a reference to another variable: SomeClass myVariable = anotherVariableDeclared(); – Arvin Amir
1 upvote
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@Arvin what about `MfClass – John Saunders
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Dear future visitors, the answers to this question equally apply to an ArgumentNullException. If your question has been closed as a duplicate of this one, and you are experiencing an ANE, please follow the directions in the answers to debug and fix your problem. – Will
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@will ANE should only happen if a null is passed as a parameter. Can you give an example if an ANE question closed as a duplicate of this one? – John Saunders
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It came up on Meta, but I'd have to go digging for the link. But as to that comment, an ANE is simply a NRE but someone added a preemptive check, and you at least know exactly what is null (the argument name is provided), so it's a little easier to diagnose than a straight up NRE. – Will

33 Answers 11

It means your code used an object reference variable that was set to null (i.e. it did not reference an actual object instance).

To prevent the error, objects that could be null should be tested for null before being used.

if (myvar != null)
{
    // Go ahead and use myvar
    myvar.property = ...
}
else
{
    // Whoops! myvar is null and cannot be used without first
    // assigning it to an instance reference
    // Attempting to use myvar here will result in NullReferenceException
}

It means that the variable in question is pointed at nothing. I could generate this like so:

SqlConnection connection = null;
connection.Open();

That will throw the error because while I've declared the variable "connection", it's not pointed to anything. When I try to call the member "Open", there's no reference for it to resolve, and it will throw the error.

To avoid this error:

  1. Always initialize your objects before you try to do anything with them.
  2. If you're not sure whether the object is null, check it with object == null.

JetBrains' Resharper tool will identify every place in your code that has the possibility of a null reference error, allowing you to put in a null check. This error is the number one source of bugs, IMHO.

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JetBrains' Resharper tool will identify every place in your code that has the possibility of a null reference error. This is incorrect. I have a solution without that detection, yet the code occasionally results to the exception. I suspect it's occasionally undetectable - by them at least - when multithreading is involved, but I can't comment further because I didn't identify the location of my bug yet. – j riv
up vote 1905 down vote accepted

What is the cause?

Bottom Line

You are trying to use something that is null (or Nothing in VB.NET). This means you either set it to null, or you never set it to anything at all.

Like anything else, null gets passed around. If it is null in method "A", it could be that method "B" passed a null to method "A".

The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException.

More Specifically

The runtime throwing a NullReferenceException always means the same thing: you are trying to use a reference, and the reference is not initialized (or it was once initialized, but is no longer initialized).

This means the reference is null, and you cannot access members (such as methods) through a null reference. The simplest case:

string foo = null;
foo.ToUpper();

This will throw a NullReferenceException at the second line because you can't call the instance method ToUpper() on a string reference pointing to null.

Debugging

How do you find the source of a NullReferenceException? Apart from looking at the exception itself, which will be thrown exactly at the location where it occurs, the general rules of debugging in Visual Studio apply: place strategic breakpoints and inspect your variables, either by hovering the mouse over their names, opening a (Quick)Watch window or using the various debugging panels like Locals and Autos.

If you want to find out where the reference is or isn't set, right-click its name and select "Find All References". You can then place a breakpoint at every found location and run your program with the debugger attached. Every time the debugger breaks on such a breakpoint, you need to determine whether you expect the reference to be non-null, inspect the variable and and verify that it points to an instance when you expect it to.

By following the program flow this way, you can find the location where the instance should not be null, and why it isn't properly set.

Examples

Some common scenarios where the exception can be thrown:

Generic

ref1.ref2.ref3.member

If ref1 or ref2 or ref3 is null, then you'll get a NullReferenceException. If you want to solve the problem, then find out which one is null by rewriting the expression to its simpler equivalent:

var r1 = ref1;
var r2 = r1.ref2;
var r3 = r2.ref3;
r3.member

Specifically, in HttpContext.Current.User.Identity.Name, the HttpContext.Current could be null, or the User property could be null, or the Identity property could be null.

Class instances

When creating a variable of a reference (class) type, it is by default set to null.

public class Book {
    public string Title { get; set; }
}
public class Example {
    public void Foo() {
        Book b1;
        string title = b1.Title; // You never initialized the b1 variable.
                                    // there is no book to get the title from.
    }
}

Class type variables must either be initialized or set to an already existing class instance. Initialization is done by using the new keyword.

Book b1 = new Book();

Indirect

public class Person {
    public int Age { get; set; }
}
public class Book {
    public Person Author { get; set; }
}
public class Example {
    public void Foo() {
        Book b1 = new Book();
        int authorAge = b1.Author.Age; // You never initialized the Author property.
                                       // there is no Person to get an Age from.
    }
}

If you want to avoid the child (Person) null reference, you could initialize it in the parent (Book) object's constructor.

Nested Object Initializers

The same applies to nested object initializers:

Book b1 = new Book { Author = { Age = 45 } };

This translates to

Book b1 = new Book();
b1.Author.Age = 45;

While the new keyword is used, it only creates a new instance of Book, but not a new instance of Person, so the Author the property is still null.

Nested Collection Initializers

public class Person {
    public ICollection<Book> Books { get; set; }
}
public class Book {
    public string Title { get; set; }
}

The nested collection initializers behave the same:

Person p1 = new Person {
    Books = {
        new Book { Title = "Title1" },
        new Book { Title = "Title2" },
    }
};

This translates to

Person p1 = new Person();
p1.Books.Add(new Book { Title = "Title1" });
p1.Books.Add(new Book { Title = "Title2" });

The new Person only creates an instance of Person, but the Books collection is still null. The collection initializer syntax does not create a collection for p1.Books, it only translates to the p1.Books.Add(...) statements.

Array

int[] numbers = null;
int n = numbers[0]; // numbers is null. There is no array to index.

Array Elements

Person[] people = new Person[5];
people[0].Age = 20 // people[0] is null. The array was allocated but not
                   // initialized. There is no Person to set the Age for.

Jagged Arrays

long[][] array = new long[1][];
array[0][0] = 3; // is null because only the first dimension is yet initialized.
                 // Use array[0] = new long[2]; first.

Collection/List/Dictionary

Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
                               // There is no Dictionary to perform the lookup.

Range Variable (Indirect/Deferred)

public class Person {
    public string Name { get; set; }
}
var people = new List<Person>();
people.Add(null);
var names = from p in people select p.Name;
string firstName = names.First(); // Exception is thrown here, but actually occurs
                                  // on the line above.  "p" is null because the
                                  // first element we added to the list is null.

Events

public class Demo
{
    public event EventHandler StateChanged;

    protected virtual void OnStateChanged(EventArgs e)
    {        
        StateChanged(this, e); // Exception is thrown here 
                               // if no event handlers have been attached
                               // to StateChanged event
    }
}

Bad Naming Conventions:

If you named fields differently from locals, you might have realized that you never initialized the field.

public class Form1 {
    private Customer customer;

    private void Form1_Load(object sender, EventArgs e) {
        Customer customer = new Customer();
        customer.Name = "John";
    }

    private void Button_Click(object sender, EventArgs e) {
        MessageBox.Show(customer.Name);
    }
}

This can be solved by following the convention to prefix fields with an underscore:

private Customer _customer;

ASP.NET Page Life cycle:

public partial class Issues_Edit : System.Web.UI.Page
{
    protected TestIssue myIssue;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            // Only called on first load, not when button clicked
            myIssue = new TestIssue(); 
        }
    }

    protected void SaveButton_Click(object sender, EventArgs e)
    {
        myIssue.Entry = "NullReferenceException here!";
    }
}

ASP.NET Session Values

// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();

ASP.NET MVC empty view models

If the exception occurs when referencing a property of @Model in an ASP.NET MVC view, you need to understand that the Model gets set in your action method, when you return a view. When you return an empty model (or model property) from your controller, the exception occurs when the views access it:

// Controller
public class Restaurant:Controller
{
    public ActionResult Search()
    {
         return View();  // Forgot the provide a Model here.
    }
}

// Razor view 
@foreach (var restaurantSearch in Model.RestaurantSearch)  // Throws.
{
}

<p>@Model.somePropertyName</p> <!-- Also throws -->

WPF Control Creation Order and Events

WPF controls are created during the call to InitializeComponent in the order they appear in the visual tree. A NullReferenceException will be raised in the case of early-created controls with event handlers, etc. , that fire during InitializeComponent which reference late-created controls.

For example :

<Grid>
    <!-- Combobox declared first -->
    <ComboBox Name="comboBox1" 
              Margin="10"
              SelectedIndex="0" 
              SelectionChanged="comboBox1_SelectionChanged">
        <ComboBoxItem Content="Item 1" />
        <ComboBoxItem Content="Item 2" />
        <ComboBoxItem Content="Item 3" />
    </ComboBox>

    <!-- Label declared later -->
    <Label Name="label1" 
           Content="Label"
           Margin="10" />
</Grid>

Here comboBox1 is created before label1. If comboBox1_SelectionChanged attempts to reference `label1, it will not yet have been created.

private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    label1.Content = comboBox1.SelectedIndex.ToString(); // NullReference here!!
}

Changing the order of the declarations in the XAML (i.e., listing label1 before comboBox1, ignoring issues of design philosophy, would at least resolve the NullReferenceException here.

Cast with as

var myThing = someObject as Thing;

This doesn't throw an InvalidCastException but returns a null when the cast fails (and when someObject is itself null). So be aware of that.

LINQ FirstOrDefault() and SingleOrDefault()

The plain versions First() and Single() throw exceptions when there is nothing. The "OrDefault" versions return null in that case. So be aware of that.

foreach

foreach throws when you try to iterate null collection. Usually caused by unexpected null result from methods that return collections.

 List<int> list = null;    
 foreach(var v in list) { } // exception

More realistic example - select nodes from XML document. Will throw if nodes are not found but initial debugging shows that all properties valid:

 foreach (var node in myData.MyXml.DocumentNode.SelectNodes("//Data"))

Ways to Avoid

Explicitly check for null and ignore null values.

If you expect the reference sometimes to be null, you can check for it being null before accessing instance members:

void PrintName(Person p) {
    if (p != null) {
        Console.WriteLine(p.Name);
    }
}

Explicitly check for null and provide a default value.

Methods call you expect to return an instance can return null, for example when the object being sought cannot be found. You can choose to return a default value when this is the case:

string GetCategory(Book b) {
    if (b == null)
        return "Unknown";
    return b.Category;
}

Explicitly check for null from method calls and throw a custom exception.

You can also throw a custom exception, only to catch it in the calling code:

string GetCategory(string bookTitle) {
    var book = library.FindBook(bookTitle);  // This may return null
    if (book == null)
        throw new BookNotFoundException(bookTitle);  // Your custom exception
    return book.Category;
}

Use Debug.Assert if a value should never be null, to catch the problem earlier than the exception occurs.

When you know during development that a method maybe can, but never should return null, you can use Debug.Assert() to break as soon as possible when it does occur:

string GetTitle(int knownBookID) {
    // You know this should never return null.
    var book = library.GetBook(knownBookID);  

    // Exception will occur on the next line instead of at the end of this method.
    Debug.Assert(book != null, "Library didn't return a book for known book ID.");

    // Some other code

    return book.Title; // Will never throw NullReferenceException in Debug mode.
}

Though this check will not end up in your release build, causing it to throw the NullReferenceException again when book == null at runtime in release mode.

Use GetValueOrDefault() for nullable value types to provide a default value when they are null.

DateTime? appointment = null;
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the default value provided (DateTime.Now), because appointment is null.

appointment = new DateTime(2022, 10, 20);
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the appointment date, not the default

Use the null coalescing operator: ?? [C#] or If() [VB].

The shorthand to providing a default value when a null is encountered:

IService CreateService(ILogger log, Int32? frobPowerLevel)
{
    var serviceImpl = new MyService(log ?? NullLog.Instance);

    // Note that the above "GetValueOrDefault()" can also be rewritten to use
    // the coalesce operator:
    serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5;
}

Use the null condition operator: ?. (available in C# 6 and VB.NET 14):

This is also sometimes called the safe navigation or Elvis (after its shape) operator. If the expression on the left side of the operator is null, then the right side will not be evaluated, and null is returned instead. That means cases like this:

var title = person.Title.ToUpper();

If the person does not have a title, this will throw an exception because it is trying to call ToUpper on a property with a null value.

In C# 5 and below, this can be guarded with:

var title = person.Title == null ? null : person.Title.ToUpper();

Now the title variable will be null instead of throwing an exception. C# 6 introduces a shorter syntax for this:

var title = person.Title?.ToUpper();

This will result in the title variable being null, and the call to ToUpper is not made if person.Title is null.

Of course, you still have to check title for null or use the null condition operator together with the null coalescing operator (??) to supply a default value:

// regular null check
int titleLength = 0;
if (title != null)
    titleLength = title.Length; // If title is null, this would throw NullReferenceException

// combining the `?` and the `??` operator
int titleLength = title?.Length ?? 0;
40 upvote
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Maybe this is a dumb comment but wouldnt the first and best way to avoid this problem be to initialize the object? For me if this error occurs it is usually because I forgot to initialize something like the array element. I think it is far less common to define the object as null and then reference it. Maybe give the way to solve each problem adjacent to the description. Still a good post. – JPK
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What if there is no object, but rather the return value from a method or property? – John Saunders
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The book/author example is a little weird.... How does that even compile? How does intellisense even work? What is this I'm not good with computar... – Will
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@Will: does my last edit help? If not, then please be more explicit about what you see as a problem. – John Saunders
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@JohnSaunders Oh, no, sorry, I meant the object initializer version of that. new Book { Author = { Age = 45 } }; How does the inner initialization even... I can't think of a situation where inner init would ever work, yet it compiles and intellisense works... Unless for structs? – Will
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@Will: interesting. I'm not sure exactly which syntax that is (I didn't write that example), but ReSharper says that it's equivalent to ` Book b1 = new Book(); b1.Author.Age = 45; `, which is clearly a problem. – John Saunders
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I checked--the syntax is valid for objects, but not for a struct property. Does work if you initialize Author. Freaky. – Will
4 upvote
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@Will: you mean valid for classes but not for structs. Struct instances are are objects too. – John Saunders
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My favourite cause of null reference exceptions is the incorrect use of the "As" cast operator. You should never use this without a null check. If you are certain about the type, use a normal cast, and if you are wrong you'll get an InvalidCastException. – Dominic Cronin
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@JonathanWood they actually do, and you get an Access Violation under the hood that gets turned into NullReferenceException, which is why these are really nasty. – Yishai Galatzer
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@Will, Better is "Everything is an Object"... structs are value-type objects, stored on the stack, not on the Heap, and cannot be null, (they are ALWAYS initialized-they have to be some value!) - whereas reference Objects are (in effect) pointer variables that "Point" to a memory structure on the Heap which may (initialized) or may not (not initialized) exist. – Charles Bretana
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@CharlesBretana Decision of storing a value-type on the "stack" has nothing to do with its type (value vs Reference). It is based on the expected life-time. A struct passed to a lambda is stored on the heap. The same applies to value-types used in iterator blocks. – user3185569
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I had this error. the reason was that, I wanted to create mapping between To object with same properties in name but different in type (from decimal? to List<decimal?> and vice versa). I used .ForMember(vm => vm.TheProperty, pr => pr.Ignore()) for fixing that. – Elnaz
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What if I have a list inside another class, like this: pastebin.com/aWhfu8UM. I get a Run-time exception (line 9): Object reference not set to an instance of an object. error – Si8
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@Si8 then you have to initialize it: dt.tList = new List<TheList>(); – John Saunders
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A couple things: firstly, you need to be careful with extension methods, because they look like instance methods but can still work if the instance is null. Secondly, VS2017's new exception helper finally tells you what was null when an NRE happens, so that's a big improvement in the debugging experience. – ArrowCase

Be aware that regardless of the scenario, the cause is always the same in .NET:

You are trying to use a reference variable whose value is Nothing/null. When the value is Nothing/null for the reference variable, that means it is not actually holding a reference to an instance of any object that exists on the heap.

You either never assigned something to the variable, never created an instance of the value assigned to the variable, or you set the variable equal to Nothing/null manually, or you called a function that set the variable to Nothing/null for you.

An example of this exception being thrown is: When you are trying to check something, that is null.

For example:

string testString = null; //Because it doesn't have a value (i.e. it's null; "Length" cannot do what it needs to do)

if (testString.Length == 0) // Throws a nullreferenceexception
{
    //Do something
} 

The .NET runtime will throw a NullReferenceException when you attempt to perform an action on something which hasn't been instantiated i.e. the code above.

In comparison to an ArgumentNullException which is typically thrown as a defensive measure if a method expects that what is being passed to it is not null.

More information is in C# NullReferenceException and Null Parameter.

Another scenario is when you cast a null object into a value type. For example, the code below:

    object o = null;
    DateTime d = (DateTime)o;

It will throw a NullReferenceException on the cast. It seems quite obvious in the above sample, but this can happen in more "late-binding" intricate scenarios where the null object has been returned from some code you don't own, and the cast is for example generated by some automatic system.

One example of this is this simple ASP.NET binding fragment with the Calendar control:

<asp:Calendar runat="server" SelectedDate="<%#Bind("Something")%>" />

Here, SelectedDate is in fact a property - of DateTime type - of the Calendar Web Control type, and the binding could perfectly return something null. The implicit ASP.NET Generator will create a piece of code that will be equivalent to the cast code above. And this will raise a NullReferenceException that is quite difficult to spot, because it lies in ASP.NET generated code which compiles fine...

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Great catch. One-liner way to avoid: DateTime x = (DateTime) o as DateTime? ?? defaultValue; – Serge Shultz

You are using the object that contains the null value reference. So it's giving a null exception. In the example the string value is null and when checking its length, the exception occurred.

Example:

string value = null;
if (value.Length == 0) // <-- Causes exception
{
    Console.WriteLine(value); // <-- Never reached
}

The exception error is:

Unhandled Exception:

System.NullReferenceException: Object reference not set to an instance of an object. at Program.Main()

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How profound! I never considered the 'null' constant a reference value. So this is how C# abstracts a "NullPointer" huh ? B/c as I recall in C++, a NPE can be caused by dereferencing an uninitialized pointer (ie, ref type in c#) whose default value happens to be an address that is not allocated to that process (many cases this would be 0, especially in later versions of C++ that did auto-initialization, which belongs to the OS - f with it and die beeotch (or just catch the sigkill the OS attacks your process with)). – samusarin

Another case where NullReferenceExceptions can happen is the (incorrect) use of the as operator:

class Book {
    public string Name { get; set; }
}
class Car { }

Car mycar = new Car();
Book mybook = mycar as Book;   // Incompatible conversion --> mybook = null

Console.WriteLine(mybook.Name);   // NullReferenceException

Here, Book and Car are incompatible types; a Car cannot be converted/cast to a Book. When this cast fails, as returns null. Using mybook after this causes a NullReferenceException.

In general, you should use a cast or as, as follows:

If you are expecting the type conversion to always succeed (ie. you know what the object should be ahead of time), then you should use a cast:

ComicBook cb = (ComicBook)specificBook;

If you are unsure of the type, but you want to try to use it as a specific type, then use as:

ComicBook cb = specificBook as ComicBook;
if (cb != null) {
   // ...
}
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This can happen a lot when unboxing a variable. I find it happens often in event handlers after I changed the type of the UI element but forget to update the code-behind. – Brendan

If you have not initialized a reference type, and you want to set or read one of its properties, it will throw a NullReferenceException.

Example:

Person p = null;
p.Name = "Harry"; // NullReferenceException occurs here.

You can simply avoid this by checking if the variable is not null:

Person p = null;
if (p!=null)
{
    p.Name = "Harry"; // Not going to run to this point
}

To fully understand why a NullReferenceException is thrown, it is important to know the difference between value types and reference types.

So, if you're dealing with value types, NullReferenceExceptions can not occur. Though you need to keep alert when dealing with reference types!

Only reference types, as the name is suggesting, can hold references or point literally to nothing (or 'null'). Whereas value types always contain a value.

Reference types (these ones must be checked):

  • dynamic
  • object
  • string

Value types (you can simply ignore these ones):

  • Numeric types
  • Integral types
  • Floating-point types
  • decimal
  • bool
  • User defined structs
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-1: since the question is "What is a NullReferenceException", value types are not relevant. – John Saunders
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@John Saunders: I disagree. As a software developer it is really important to be able to distinguish between value and reference types. else people will end up checking if integers are null. – Fabian Bigler
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True, just not in the context of this question. – John Saunders
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Thanks for the hint. I improved it a bit and added an example at the top. I still think mentioning Reference & Value Types is useful. – Fabian Bigler
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I think you haven't added anything that wasn't in the other answers, since the question pre-supposes a reference type. – John Saunders

Another general case where one might receive this exception involves mocking classes during unit testing. Regardless of the mocking framework being used, you must ensure that all appropriate levels of the class hierarchy are properly mocked. In particular, all properties of HttpContext which are referenced by the code under test must be mocked.

See "NullReferenceException thrown when testing custom AuthorizationAttribute" for a somewhat verbose example.

Adding a case when the class name for entity used in entity framework is same as class name for a web form code-behind file.

Suppose you have a web form Contact.aspx whose codebehind class is Contact and you have an entity name Contact.

Then following code will throw a NullReferenceException when you call context.SaveChanges()

Contact contact = new Contact { Name = "Abhinav"};
var context = new DataContext();
context.Contacts.Add(contact);
context.SaveChanges(); // NullReferenceException at this line

For the sake of completeness DataContext class

public class DataContext : DbContext 
{
    public DbSet<Contact> Contacts {get; set;}
}

and Contact entity class. Sometimes entity classes are partial classes so that you can extend them in other files too.

public partial class Contact 
{
    public string Name {get; set;}
}

The error occurs when both the entity and codebehind class are in same namespace. To fix this, rename the entity class or the codebehind class for Contact.aspx.

Reason I am still not sure about the reason. But whenever any of the entity class will extend System.Web.UI.Page this error occurs.

For discussion have a look at NullReferenceException in DbContext.saveChanges()

While what causes a NullReferenceExceptions and approaches to avoid/fix such an exception have been addressed in other answers, what many programmers haven't learned yet is how to independently debug such exceptions during development.

In Visual Studio this is usually easy thanks to the Visual Studio Debugger.


First, make sure that the correct error is going to be caught - see How do I allow breaking on 'System.NullReferenceException' in VS2010? Note1

Then either Start with Debugging (F5) or Attach [the VS Debugger] to Running Process. On occasion it may be useful to use Debugger.Break, which will prompt to launch the debugger.

Now, when the NullReferenceException is thrown (or unhandled) the debugger will stop (remember the rule set above?) on the line on which the exception occurred. Sometimes the error will be easy to spot.

For instance, in the following line the only code that can cause the exception is if myString evaluates to null. This can be verified by looking at the Watch Window or running expressions in the Immediate Window.

var x = myString.Trim();

In more advanced cases, such as the following, you'll need to use one of the techniques above (Watch or Immediate Windows) to inspect the expressions to determine if str1 was null or if str2 was null.

var x = str1.Trim() + str2.Trim();

Once where the exception is throw has been located, it's usually trivial to reason backwards to find out where the null value was [incorrectly] introduced --

Take the time required to understand the cause of the exception. Inspect for null expressions. Inspect the previous expressions which could have resulted in such null expressions. Add breakpoints and step through the program as appropriate. Use the debugger.


1 If Break on Throws is too aggressive and the debugger stops on an NPE in the .NET or 3rd-party library, Break on User-Unhandled can be used to limit the exceptions caught. Additionally, VS2012 introduces Just My Code which I recommend enabling as well.

If you are debugging with Just My Code enabled, the behavior is slightly different. With Just My Code enabled, the debugger ignores first-chance common language runtime (CLR) exceptions that are thrown outside of My Code and do not pass through My Code

A NullReferenceException is thrown when we are trying to access Properties of a null object or when a string value becomes empty and we are trying to access string methods.

For example:

  1. When a string method of an empty string accessed:

    string str = string.Empty;
    str.ToLower(); // throw null reference exception
    
  2. When a property of a null object accessed:

    Public Class Person {
        public string Name { get; set; }
    }
    Person objPerson;
    objPerson.Name  /// throw Null refernce Exception 
    
2 upvote
  flag
This is incorrect. String.Empty.ToLower() will not throw a null reference exception. It represents an actual string, albeit an empty one (i.e. ""). Since this has an object to call ToLower() on, it would not make sense to throw a null reference exception there. – Kjartan

I have a different perspective to answering this. This sort of answers "what else can I do to avoid it?"

When working across different layers, for example in an MVC application, a controller needs services to call business operations. In such scenarios Dependency Injection Container can be used to initialize the services to avoid the NullReferenceException. So that means you don't need to worry about checking for null and just call the services from the controller as though they will always to available (and initialized) as either a singleton or a prototype.

public class MyController
{
    private ServiceA serviceA;
    private ServiceB serviceB;

    public MyController(ServiceA serviceA, ServiceB serviceB)
    {
        this.serviceA = serviceA;
        this.serviceB = serviceB;
    }

    public void MyMethod()
    {
        // We don't need to check null because the dependency injection container 
        // injects it, provided you took care of bootstrapping it.
        var someObject = serviceA.DoThis();
    }
}
4 upvote
  flag
-1: this only handles a single scenario - that of uninitialized dependencies. This is a minority scenario for NullReferenceException. Most cases are simple misunderstanding of how objects work. Next most frequent are other situations where the developer assumed that the object would be initialized automatically. – John Saunders
2 upvote
  flag
All others have already been answered above. – Mukus
upvote
  flag
Dependency injection is not generally used in order to avoid NullReferenceException. I don't believe that you have found a general scenario here. In any case, if you edit your answer to be more in the style of //allinonescript.com/a/15232518/76337, then I will remove the downvote. – John Saunders

Simon Mourier gave this example:

object o = null;
DateTime d = (DateTime)o;  // NullReferenceException

where an unboxing conversion (cast) from object (or from one of the classes System.ValueType or System.Enum, or from an interface type) to a value type (other than Nullable<>) in itself gives the NullReferenceException.

In the other direction, a boxing conversion from a Nullable<> which has HasValue equal to false to a reference type, can give a null reference which can then later lead to a NullReferenceException. The classic example is:

DateTime? d = null;
var s = d.ToString();  // OK, no exception (no boxing), returns ""
var t = d.GetType();   // Bang! d is boxed, NullReferenceException

Sometimes the boxing happens in another way. For example with this non-generic extension method:

public static void MyExtension(this object x)
{
  x.ToString();
}

the following code will be problematic:

DateTime? d = null;
d.MyExtension();  // Leads to boxing, NullReferenceException occurs inside the body of the called method, not here.

These cases arise because of the special rules the runtime uses when boxing Nullable<> instances.

NullReference Exception — Visual Basic

The NullReference Exception for Visual Basic is no different from the one in C#. After all, they are both reporting the same exception defined in the .NET Framework which they both use. Causes unique to Visual Basic are rare (perhaps only one).

This answer will use Visual Basic terms, syntax, and context. The examples used come from a large number of past Stack  Overflow questions. This is to maximize relevance by using the kinds of situations often seen in posts. A bit more explanation is also provided for those who might need it. An example similar to yours is very likely listed here.

Note:

  1. This is concept-based: there is no code for you to paste into your project. It is intended to help you understand what causes a NullReferenceException (NRE), how to find it, how to fix it, and how to avoid it. An NRE can be caused many ways so this is unlikely to be your sole encounter.
  2. The examples (from Stack  Overflow posts) do not always show the best way to do something in the first place.
  3. Typically, the simplest remedy is used.

Basic Meaning

The message "Object not set to an instance of Object" means you are trying to use an object which has not been initialized. This boils down to one of these:

  • Your code declared an object variable, but it did not initialize it (create an instance or 'instantiate' it)
  • Something which your code assumed would initialize an object, did not
  • Possibly, other code prematurely invalidated an object still in use

Finding The Cause

Since the problem is an object reference which is Nothing, the answer is to examine them to find out which one. Then determine why it is not initialized. Hold the mouse over the various variables and Visual Studio (VS) will show their values - the culprit will be Nothing.

IDE debug display

You should also remove any Try/Catch blocks from the relevant code, especially ones where there is nothing in the Catch block. This will cause your code to crash when it tries to use an object which is Nothing. This is what you want because it will identify the exact location of the problem, and allow you to identify the object causing it.

A MsgBox in the Catch which displays Error while... will be of little help. This method also leads to very bad Stack  Overflow questions, because you can't describe the actual exception, the object involved or even the line of code where it happens.

You can also use the Locals Window (Debug -> Windows -> Locals) to examine your objects.

Once you know what and where the problem is, it is usually fairly easy to fix and faster than posting a new question.

See also:

Examples and Remedies

Class Objects / Creating an Instance

Dim reg As CashRegister
...
TextBox1.Text = reg.Amount         ' NRE

The problem is that Dim does not create a CashRegister object; it only declares a variable named reg of that Type. Declaring an object variable and creating an instance are two different things.

Remedy

The New operator can often be used to create the instance when you declare it:

Dim reg As New CashRegister        ' [New] creates instance, invokes the constructor

' Longer, more explicit form:
Dim reg As CashRegister = New CashRegister

When it is only appropriate to create the instance later:

Private reg As CashRegister         ' Declare
  ...
reg = New CashRegister()            ' Create instance

Note: Do not use Dim again in a procedure, including the constructor (Sub New):

Private reg As CashRegister
'...

Public Sub New()
   '...
   Dim reg As New CashRegister
End Sub

This will create a local variable, reg, which exists only in that context (sub). The reg variable with module level Scope which you will use everywhere else remains Nothing.

Missing the New operator is the #1 cause of NullReference Exceptions seen in the Stack  Overflow questions reviewed.

Visual Basic tries to make the process clear repeatedly using New: Using the New Operator creates a new object and calls Sub New -- the constructor -- where your object can perform any other initialization.

To be clear, Dim (or Private) only declares a variable and its Type. The Scope of the variable - whether it exists for the entire module/class or is local to a procedure - is determined by where it is declared. Private | Friend | Public defines the access level, not Scope.

For more information, see:


Arrays

Arrays must also be instantiated:

Private arr as String()

This array has only been declared, not created. There are several ways to initialize an array:

Private arr as String() = New String(10){}
' or
Private arr() As String = New String(10){}

' For a local array (in a procedure) and using 'Option Infer':
Dim arr = New String(10) {}

Note: Beginning with VS 2010, when initializing a local array using a literal and Option Infer, the As <Type> and New elements are optional:

Dim myDbl As Double() = {1.5, 2, 9.9, 18, 3.14}
Dim myDbl = New Double() {1.5, 2, 9.9, 18, 3.14}
Dim myDbl() = {1.5, 2, 9.9, 18, 3.14}

The data Type and array size are inferred from the data being assigned. Class/Module level declarations still require As <Type> with Option Strict:

Private myDoubles As Double() = {1.5, 2, 9.9, 18, 3.14}

Example: Array of class objects

Dim arrFoo(5) As Foo

For i As Integer = 0 To arrFoo.Count - 1
   arrFoo(i).Bar = i * 10       ' Exception
Next

The array has been created, but the Foo objects in it have not.

Remedy

For i As Integer = 0 To arrFoo.Count - 1
    arrFoo(i) = New Foo()         ' Create Foo instance
    arrFoo(i).Bar = i * 10
Next

Using a List(Of T) will make it quite difficult to have an element without a valid object:

Dim FooList As New List(Of Foo)     ' List created, but it is empty
Dim f As Foo                        ' Temporary variable for the loop

For i As Integer = 0 To 5
    f = New Foo()                    ' Foo instance created
    f.Bar =  i * 10
    FooList.Add(f)                   ' Foo object added to list
Next

For more information, see:


Lists and Collections

.NET collections (of which there are many varieties - Lists, Dictionary, etc.) must also be instantiated or created.

Private myList As List(Of String)
..
myList.Add("ziggy")           ' NullReference

You get the same exception for the same reason - myList was only declared, but no instance created. The remedy is the same:

myList = New List(Of String)

' Or create an instance when declared:
Private myList As New List(Of String)

A common oversight is a class which uses a collection Type:

Public Class Foo
    Private barList As List(Of Bar)

    Friend Function BarCount As Integer
        Return barList.Count
    End Function

    Friend Sub AddItem(newBar As Bar)
        If barList.Contains(newBar) = False Then
            barList.Add(newBar)
        End If
    End Function

Either procedure will result in an NRE, because barList is only declared, not instantiated. Creating an instance of Foo will not also create an instance of the internal barList. It may have been the intent to do this in the constructor:

Public Sub New         ' Constructor
    ' Stuff to do when a new Foo is created...
    barList = New List(Of Bar)
End Sub

As before, this is incorrect:

Public Sub New()
    ' Creates another barList local to this procedure
     Dim barList As New List(Of Bar)
End Sub

For more information, see List(Of T) Class.


Data Provider Objects

Working with databases presents many opportunities for a NullReference because there can be many objects (Command, Connection, Transaction, Dataset, DataTable, DataRows....) in use at once. Note: It does not matter which data provider you are using -- MySQL, SQL Server, OleDB, etc. -- the concepts are the same.

Example 1

Dim da As OleDbDataAdapter
Dim ds As DataSet
Dim MaxRows As Integer

con.Open()
Dim sql = "SELECT * FROM tblfoobar_List"
da = New OleDbDataAdapter(sql, con)
da.Fill(ds, "foobar")
con.Close()

MaxRows = ds.Tables("foobar").Rows.Count      ' Error

As before, the ds Dataset object was declared, but an instance was never created. The DataAdapter will fill an existing DataSet, not create one. In this case, since ds is a local variable, the IDE warns you that this might happen:

img

When declared as a module/class level variable, as appears to be the case with con, the compiler can't know if the object was created by an upstream procedure. Do not ignore warnings.

Remedy

Dim ds As New DataSet

Example 2

ds = New DataSet
da = New OleDBDataAdapter(sql, con)
da.Fill(ds, "Employees")

txtID.Text = ds.Tables("Employee").Rows(0).Item(1)
txtID.Name = ds.Tables("Employee").Rows(0).Item(2)

A typo is a problem here: Employees vs Employee. There was no DataTable named "Employee" created, so a NullReferenceException results trying to access it. Another potential problem is assuming there will be Items which may not be so when the SQL includes a WHERE clause.

Remedy

Since this uses one table, using Tables(0) will avoid spelling errors. Examining Rows.Count can also help:

If ds.Tables(0).Rows.Count > 0 Then
    txtID.Text = ds.Tables(0).Rows(0).Item(1)
    txtID.Name = ds.Tables(0).Rows(0).Item(2)
End If

Fill is a function returning the number of Rows affected which can also be tested:

If da.Fill(ds, "Employees") > 0 Then...

Example 3

Dim da As New OleDb.OleDbDataAdapter("SELECT TICKET.TICKET_NO,
        TICKET.CUSTOMER_ID, ... FROM TICKET_RESERVATION AS TICKET INNER JOIN
        FLIGHT_DETAILS AS FLIGHT ... WHERE [TICKET.TICKET_NO]= ...", con)
Dim ds As New DataSet
da.Fill(ds)

If ds.Tables("TICKET_RESERVATION").Rows.Count > 0 Then

The DataAdapter will provide TableNames as shown in the previous example, but it does not parse names from the SQL or database table. As a result, ds.Tables("TICKET_RESERVATION") references a non-existent table.

The Remedy is the same, reference the table by index:

If ds.Tables(0).Rows.Count > 0 Then

See also DataTable Class.


Object Paths / Nested

If myFoo.Bar.Items IsNot Nothing Then
   ...

The code is only testing Items while both myFoo and Bar may also be Nothing. The remedy is to test the entire chain or path of objects one at a time:

If (myFoo IsNot Nothing) AndAlso
    (myFoo.Bar IsNot Nothing) AndAlso
    (myFoo.Bar.Items IsNot Nothing) Then
    ....

AndAlso is important. Subsequent tests will not be performed once the first False condition is encountered. This allows the code to safely 'drill' into the object(s) one 'level' at a time, evaluating myFoo.Bar only after (and if) myFoo is determined to be valid. Object chains or paths can get quite long when coding complex objects:

myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename")

It is not possible to reference anything 'downstream' of a null object. This also applies to controls:

myWebBrowser.Document.GetElementById("formfld1").InnerText = "some value"

Here, myWebBrowser or Document could be Nothing or the formfld1 element may not exist.


UI Controls

Dim cmd5 As New SqlCommand("select Cartons, Pieces, Foobar " _
     & "FROM Invoice where invoice_no = '" & _
     Me.ComboBox5.SelectedItem.ToString.Trim & "' And category = '" & _
     Me.ListBox1.SelectedItem.ToString.Trim & "' And item_name = '" & _
     Me.ComboBox2.SelectedValue.ToString.Trim & "' And expiry_date = '" & _
     Me.expiry.Text & "'", con)

Among other things, this code does not anticipate that the user may not have selected something in one or more UI controls. ListBox1.SelectedItem may well be Nothing, so ListBox1.SelectedItem.ToString will result in an NRE.

Remedy

Validate data before using it (also use Option Strict and SQL parameters):

Dim expiry As DateTime         ' for text date validation
If (ComboBox5.SelectedItems.Count > 0) AndAlso
    (ListBox1.SelectedItems.Count > 0) AndAlso
    (ComboBox2.SelectedItems.Count > 0) AndAlso
    (DateTime.TryParse(expiry.Text, expiry) Then

    '... do stuff
Else
    MessageBox.Show(...error message...)
End If

Alternatively, you can use (ComboBox5.SelectedItem IsNot Nothing) AndAlso...


Visual Basic Forms

Public Class Form1

    Private NameBoxes = New TextBox(5) {Controls("TextBox1"), _
                   Controls("TextBox2"), Controls("TextBox3"), _
                   Controls("TextBox4"), Controls("TextBox5"), _
                   Controls("TextBox6")}

    ' same thing in a different format:
    Private boxList As New List(Of TextBox) From {TextBox1, TextBox2, TextBox3 ...}

    ' Immediate NRE:
    Private somevar As String = Me.Controls("TextBox1").Text

This is a fairly common way to get an NRE. In C#, depending on how it is coded, the IDE will report that Controls does not exist in the current context, or "cannot reference non-static member". So, to some extent, this is a VB-only situation. It is also complex because it can result in a failure cascade.

The arrays and collections cannot be initialized this way. This initialization code will run before the constructor creates the Form or the Controls. As a result:

  • Lists and Collection will simply be empty
  • The Array will contain five elements of Nothing
  • The somevar assignment will result in an immediate NRE because Nothing doesn't have a .Text property

Referencing array elements later will result in an NRE. If you do this in Form_Load, due to an odd bug, the IDE may not report the exception when it happens. The exception will pop up later when your code tries to use the array. This "silent exception" is detailed in this post. For our purposes, the key is that when something catastrophic happens while creating a form (Sub New or Form Load event), exceptions may go unreported, the code exits the procedure and just displays the form.

Since no other code in your Sub New or Form Load event will run after the NRE, a great many other things can be left uninitialized.

Sub Form_Load(..._
   '...
   Dim name As String = NameBoxes(2).Text        ' NRE
   ' ...
   ' More code (which will likely not be executed)
   ' ...
End Sub

Note this applies to any and all control and component references making these illegal where they are:

Public Class Form1

    Private myFiles() As String = Me.OpenFileDialog1.FileName & ...
    Private dbcon As String = OpenFileDialog1.FileName & ";Jet Oledb..."
    Private studentName As String = TextBox13.Text

Partial Remedy

It is curious that VB does not provide a warning, but the remedy is to declare the containers at the form level, but initialize them in form load event handler when the controls do exist. This can be done in Sub New as long as your code is after the InitializeComponent call:

' Module level declaration
Private NameBoxes as TextBox()
Private studentName As String

' Form Load, Form Shown or Sub New:
'
' Using the OP's approach (illegal using OPTION STRICT)
NameBoxes = New TextBox() {Me.Controls("TextBox1"), Me.Controls("TestBox2"), ...)
studentName = TextBox32.Text           ' For simple control references

The array code may not be out of the woods yet. Any controls which are in a container control (like a GroupBox or Panel) will not be found in Me.Controls; they will be in the Controls collection of that Panel or GroupBox. Nor will a control be returned when the control name is misspelled ("TeStBox2"). In such cases, Nothing will again be stored in those array elements and an NRE will result when you attempt to reference it.

These should be easy to find now that you know what you are looking for: VS shows you the error of your ways

"Button2" resides on a Panel

Remedy

Rather than indirect references by name using the form's Controls collection, use the control reference:

' Declaration
Private NameBoxes As TextBox()

' Initialization -  simple and easy to read, hard to botch:
NameBoxes = New TextBox() {TextBox1, TextBox2, ...)

' Initialize a List
NamesList = New List(Of TextBox)({TextBox1, TextBox2, TextBox3...})
' or
NamesList = New List(Of TextBox)
NamesList.AddRange({TextBox1, TextBox2, TextBox3...})

Function Returning Nothing

Private bars As New List(Of Bars)        ' Declared and created

Public Function BarList() As List(Of Bars)
    bars.Clear
    If someCondition Then
        For n As Integer = 0 to someValue
            bars.Add(GetBar(n))
        Next n
    Else
        Exit Function
    End If

    Return bars
End Function

This is a case where the IDE will warn you that 'not all paths return a value and a NullReferenceException may result'. You can suppress the warning, by replacing Exit Function with Return Nothing, but that does not solve the problem. Anything which tries to use the return when someCondition = False will result in an NRE:

bList = myFoo.BarList()
For Each b As Bar in bList      ' EXCEPTION
      ...

Remedy

Replace Exit Function in the function with Return bList. Returning an empty List is not the same as returning Nothing. If there is a chance that a returned object can be Nothing, test before using it:

 bList = myFoo.BarList()
 If bList IsNot Nothing Then...

Poorly Implemented Try/Catch

A badly implemented Try/Catch can hide where the problem is and result in new ones:

Dim dr As SqlDataReader
Try
    Dim lnk As LinkButton = TryCast(sender, LinkButton)
    Dim gr As GridViewRow = DirectCast(lnk.NamingContainer, GridViewRow)
    Dim eid As String = GridView1.DataKeys(gr.RowIndex).Value.ToString()
    ViewState("username") = eid
    sqlQry = "select FirstName, Surname, DepartmentName, ExtensionName, jobTitle,
             Pager, mailaddress, from employees1 where username='" & eid & "'"
    If connection.State <> ConnectionState.Open Then
        connection.Open()
    End If
    command = New SqlCommand(sqlQry, connection)

    'More code fooing and barring

    dr = command.ExecuteReader()
    If dr.Read() Then
        lblFirstName.Text = Convert.ToString(dr("FirstName"))
        ...
    End If
    mpe.Show()
Catch

Finally
    command.Dispose()
    dr.Close()             ' <-- NRE
    connection.Close()
End Try

This is a case of an object not being created as expected, but also demonstrates the counter usefulness of an empty Catch.

There is an extra comma in the SQL (after 'mailaddress') which results in an exception at .ExecuteReader. After the Catch does nothing, Finally tries to perform clean up, but since you cannot Close a null DataReader object, a brand new NullReferenceException results.

An empty Catch block is the devil's playground. This OP was baffled why he was getting an NRE in the Finally block. In other situations, an empty Catch may result in something else much further downstream going haywire and cause you to spend time looking at the wrong things in the wrong place for the problem. (The "silent exception" described above provides the same entertainment value.)

Remedy

Don't use empty Try/Catch blocks - let the code crash so you can a) identify the cause b) identify the location and c) apply a proper remedy. Try/Catch blocks are not intended to hide exceptions from the person uniquely qualified to fix them - the developer.


DBNull is not the same as Nothing

For Each row As DataGridViewRow In dgvPlanning.Rows
    If Not IsDBNull(row.Cells(0).Value) Then
        ...

The IsDBNull function is used to test if a value equals System.DBNull: From MSDN:

The System.DBNull value indicates that the Object represents missing or non-existent data. DBNull is not the same as Nothing, which indicates that a variable has not yet been initialized.

Remedy

If row.Cells(0) IsNot Nothing Then ...

As before, you can test for Nothing, then for a specific value:

If (row.Cells(0) IsNot Nothing) AndAlso (IsDBNull(row.Cells(0).Value) = False) Then

Example 2

Dim getFoo = (From f In dbContext.FooBars
               Where f.something = something
               Select f).FirstOrDefault

If Not IsDBNull(getFoo) Then
    If IsDBNull(getFoo.user_id) Then
        txtFirst.Text = getFoo.first_name
    Else
       ...

FirstOrDefault returns the first item or the default value, which is Nothing for reference types and never DBNull:

If getFoo IsNot Nothing Then...

Controls

Dim chk As CheckBox

chk = CType(Me.Controls(chkName), CheckBox)
If chk.Checked Then
    Return chk
End If

If a CheckBox with chkName can't be found (or exists in a GroupBox), then chk will be Nothing and be attempting to reference any property will result in an exception.

Remedy

If (chk IsNot Nothing) AndAlso (chk.Checked) Then ...

The DataGridView

The DGV has a few quirks seen periodically:

dgvBooks.DataSource = loan.Books
dgvBooks.Columns("ISBN").Visible = True       ' NullReferenceException
dgvBooks.Columns("Title").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Author").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Price").DefaultCellStyle.Format = "C"

If dgvBooks has AutoGenerateColumns = True, it will create the columns, but it does not name them, so the above code fails when it references them by name.

Remedy

Name the columns manually, or reference by index:

dgvBooks.Columns(0).Visible = True

Example 2 — Beware of the NewRow

xlWorkSheet = xlWorkBook.Sheets("sheet1")

For i = 0 To myDGV.RowCount - 1
    For j = 0 To myDGV.ColumnCount - 1
        For k As Integer = 1 To myDGV.Columns.Count
            xlWorkSheet.Cells(1, k) = myDGV.Columns(k - 1).HeaderText
            xlWorkSheet.Cells(i + 2, j + 1) = myDGV(j, i).Value.ToString()
        Next
    Next
Next

When your DataGridView has AllowUserToAddRows as True (the default), the Cells in the blank/new row at the bottom will all contain Nothing. Most attempts to use the contents (for example, ToString) will result in an NRE.

Remedy

Use a For/Each loop and test the IsNewRow property to determine if it is that last row. This works whether AllowUserToAddRows is true or not:

For Each r As DataGridViewRow in myDGV.Rows
    If r.IsNewRow = False Then
         ' ok to use this row

If you do use a For n loop, modify the row count or use Exit For when IsNewRow is true.


My.Settings (StringCollection)

Under certain circumstances, trying to use an item from My.Settings which is a StringCollection can result in a NullReference the first time you use it. The solution is the same, but not as obvious. Consider:

My.Settings.FooBars.Add("ziggy")         ' foobars is a string collection

Since VB is managing Settings for you, it is reasonable to expect it to initialize the collection. It will, but only if you have previously added an initial entry to the collection (in the Settings editor). Since the collection is (apparently) initialized when an item is added, it remains Nothing when there are no items in the Settings editor to add.

Remedy

Initialize the settings collection in the form's Load event handler, if/when needed:

If My.Settings.FooBars Is Nothing Then
    My.Settings.FooBars = New System.Collections.Specialized.StringCollection
End If

Typically, the Settings collection will only need to be initialized the first time the application runs. An alternate remedy is to add an initial value to your collection in Project -> Settings | FooBars, save the project, then remove the fake value.


Key Points

You probably forgot the New operator.

or

Something you assumed would perform flawlessly to return an initialized object to your code, did not.

Don't ignore compiler warnings (ever) and use Option Strict On (always).


MSDN NullReference Exception

On the matter of "what should I do about it", there can be many answers.

A more "formal" way of preventing such error conditions while developing is applying design by contract in your code. This means you need to set class invariants, and/or even function/method preconditions and postconditions on your system, while developing.

In short, class invariants ensure that there will be some constraints in your class that will not get violated in normal use (and therefore, the class will not get in an inconsistent state). Preconditions mean that data given as input to a function/method must follow some constraints set and never violate them, and postconditions mean that a function/method output must follow the set constraints again without ever violating them. Contract conditions should never be violated during execution of a bug-free program, therefore design by contract is checked in practice in debug mode, while being disabled in releases, to maximize the developed system performance.

This way, you can avoid NullReferenceException cases that are results of violation of the constraints set. For example, if you use an object property X in a class and later try to invoke one of its methods and X has a null value, then this will lead to NullReferenceException:

public X { get; set; }

public void InvokeX()
{
    X.DoSomething(); // if X value is null, you will get a NullReferenceException
}

But if you set "property X must never have a null value" as method precondition, then you can prevent the scenario described before:

//Using code contracts:
[ContractInvariantMethod]
protected void ObjectInvariant () 
{
    Contract.Invariant ( X != null );
    //...
}

For this cause, Code Contracts project exists for .NET applications.

Alternatively, design by contract can be applied using assertions.

UPDATE: It is worth mentioning that the term was coined by Bertrand Meyer in connection with his design of the Eiffel programming language.

1 upvote
  flag
I thought to add this as no one mentioned this, and as far as it exists as an approach, my intention was to enrich the topic. – Nick L.
1 upvote
  flag
Thank you for enriching the topic. I have given my opinion of your addition. Now others can do the same. – John Saunders
1 upvote
  flag
I thought this was a worthwhile addition to the topic given that this is a highly viewed thread. I've heard of code contracts before and this was a good reminder to consider using them. – VoteCoffee

TL;DR: Try using Html.Partial instead of Renderpage


I was getting Object reference not set to an instance of an object when I tried to render a View within a View by sending it a Model, like this:

@{
    MyEntity M = new MyEntity();
}
@RenderPage("_MyOtherView.cshtml", M); // error in _MyOtherView, the Model was Null

Debugging showed the model was Null inside MyOtherView. Until I changed it to:

@{
    MyEntity M = new MyEntity();
}
@Html.Partial("_MyOtherView.cshtml", M);

And it worked.

Furthermore, the reason I didn't have Html.Partial to begin with was because Visual Studio sometimes throws error-looking squiggly lines under Html.Partial if it's inside a differently constructed foreach loop, even though it's not really an error:

@inherits System.Web.Mvc.WebViewPage
@{
    ViewBag.Title = "Entity Index";
    List<MyEntity> MyEntities = new List<MyEntity>();
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
}
<div>
    @{
        foreach(var M in MyEntities)
        {
            // Squiggly lines below. Hovering says: cannot convert method group 'partial' to non-delegate type Object, did you intend to envoke the Method?
            @Html.Partial("MyOtherView.cshtml");
        }
    }
</div>

But I was able to run the application with no problems with this "error". I was able to get rid of the error by changing the structure of the foreach loop to look like this:

@foreach(var M in MyEntities){
    ...
}

Although I have a feeling it was because Visual Studio was misreading the ampersands and brackets.

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You wanted Html.Partial, not @Html.Partial – John Saunders
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Also, please show which line threw the exception, and why. – John Saunders
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The error occurred in MyOtherView.cshtml, which I did not include here, because the Model was not being properly sent in (it was Null), so I knew the error was with how I was sending the Model in. – Travis Heeter

Well, in simple terms:

You are trying to access an object that isn't created or currently not in memory.

So how to tackle this:

  1. Debug and let the debugger break... It will directly take you to the variable that is broken... Now your task is to simply fix this.. Using the new keyword in the appropriate place.

  2. If it is caused on some database commands because the object isn't present then all you need to do is do a null check and handle it:

    if (i == null) {
        // Handle this
    }
    
  3. The hardest one .. if the GC collected the object already... This generally occurs if you are trying to find an object using strings... That is, finding it by name of the object then it may happen that the GC might already cleaned it up... This is hard to find and will become quite a problem... A better way to tackle this is do null checks wherever necessary during the development process. This will save you a lot of time.

By finding by name I mean some framework allow you to FIndObjects using strings and the code might look like this: FindObject("ObjectName");

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If you have a reference to an object, then the GC never cleans it up – John Saunders
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if you use things like FindObject("Name of Object") there is no way GC will know before hand that you are going to refernece that object .. this is what is was trying to explaing .. these occur at runtime – Akash Chowdary
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There are some frameworks that Provide this functionality in C# such as Unity . the question has nothing related to BCl. Search the Internet before Criticizing there are a ton of functions like them and for ur kind information i even use it daily. Now please tell me how does the answer doesn't make anysense. – Akash Chowdary
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docs.unity3d.com/ScriptReference/… check the link and correct urself mr.expert :p – Akash Chowdary
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The examples I saw in your link assign the results of GameObject.Find to a member field. That's a reference and the GC will not collect it until the containing object is collected. – John Saunders
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In unity there is a concept of scenes .. So if you change a scene everything in the current scene will be collected by GC . But the scripts u write are same all the time in the project and wrongly referencing those objects will cause this problem . – Akash Chowdary
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Unity is built on top of .NET. There is no magic. When you switch a scene you are likely emptying the data structures which allow elements of the scene to be referenced by name – John Saunders

What can you do about it?

There is a lot of good answers here explaining what a null reference is and how to debug it. But there is very little on how to prevent the issue or at least make it easier to catch.

Check arguments

For example, methods can check the different arguments to see if they are null and throw an ArgumentNullException, an exception obviously created for this exact purpose.

The constructor for the ArgumentNullException even takes the name of the parameter and a message as arguments so you can tell the developer exactly what the problem is.

public void DoSomething(MyObject obj) {
    if(obj == null) 
    {
        throw new ArgumentNullException("obj", "Need a reference to obj.");
    }
}

Use Tools

There are also several libraries that can help. "Resharper" for example can provide you with warnings while you are writing code, especially if you use their attribute: NotNullAttribute

There's "Microsoft Code Contracts" where you use syntax like Contract.Requires(obj != null) which gives you runtime and compile checking: Introducing Code Contracts.

There's also "PostSharp" which will allow you to just use attributes like this:

public void DoSometing([NotNull] obj)

By doing that and making PostSharp part of your build process obj will be checked for null at runtime. See: PostSharp null check

Plain Code Solution

Or you can always code your own approach using plain old code. For example here is a struct that you can use to catch null references. It's modeled after the same concept as Nullable<T>:

[System.Diagnostics.DebuggerNonUserCode]
public struct NotNull<T> where T: class
{
    private T _value;

    public T Value
    {
        get
        {
            if (_value == null)
            {
                throw new Exception("null value not allowed");
            }

            return _value;
        }
        set
        {
            if (value == null)
            {
                throw new Exception("null value not allowed.");
            }

            _value = value;
        }
    }

    public static implicit operator T(NotNull<T> notNullValue)
    {
        return notNullValue.Value;
    }

    public static implicit operator NotNull<T>(T value)
    {
        return new NotNull<T> { Value = value };
    }
}

You would use very similar to the same way you would use Nullable<T>, except with the goal of accomplishing exactly the opposite - to not allow null. Here are some examples:

NotNull<Person> person = null; // throws exception
NotNull<Person> person = new Person(); // OK
NotNull<Person> person = GetPerson(); // throws exception if GetPerson() returns null

NotNull<T> is implicitly cast to and from T so you can use it just about anywhere you need it. For example, you can pass a Person object to a method that takes a NotNull<Person>:

Person person = new Person { Name = "John" };
WriteName(person);

public static void WriteName(NotNull<Person> person)
{
    Console.WriteLine(person.Value.Name);
}

As you can see above as with nullable you would access the underlying value through the Value property. Alternatively, you can use an explicit or implicit cast, you can see an example with the return value below:

Person person = GetPerson();

public static NotNull<Person> GetPerson()
{
    return new Person { Name = "John" };
}

Or you can even use it when the method just returns T (in this case Person) by doing a cast. For example, the following code would just like the code above:

Person person = (NotNull<Person>)GetPerson();

public static Person GetPerson()
{
    return new Person { Name = "John" };
}

Combine with Extension

Combine NotNull<T> with an extension method and you can cover even more situations. Here is an example of what the extension method can look like:

[System.Diagnostics.DebuggerNonUserCode]
public static class NotNullExtension
{
    public static T NotNull<T>(this T @this) where T: class
    {
        if (@this == null)
        {
            throw new Exception("null value not allowed");
        }

        return @this;
    }
}

And here is an example of how it could be used:

var person = GetPerson().NotNull();

GitHub

For your reference I made the code above available on GitHub, you can find it at:

https://github.com/luisperezphd/NotNull

Related Language Feature

C# 6.0 introduced the "null-conditional operator" that helps with this a little. With this feature, you can reference nested objects and if any one of them is null the whole expression returns null.

This reduces the number of null checks you have to do in some cases. The syntax is to put a question mark before each dot. Take the following code for example:

var address = country?.State?.County?.City;

Imagine that country is an object of type Country that has a property called State and so on. If country, State, County, or City is null then address will benull. Therefore you only have to check whetheraddressisnull`.

It's a great feature, but it gives you less information. It doesn't make it obvious which of the 4 is null.

Built-in like Nullable?

C# has a nice shorthand for Nullable<T>, you can make something nullable by putting a question mark after the type like so int?.

It would be nice if C# had something like the NotNull<T> struct above and had a similar shorthand, maybe the exclamation point (!) so that you could write something like: public void WriteName(Person! person).

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Never throw NullReferenceException – John Saunders
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@JohnSaunders dare I ask why? (Seriously though why?) – Luis Perez
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NullReferenceException is meant to be thrown by the CLR. It means that a reference to a null has occurred. It does not mean that a reference to a null would occur except that you cleverly checked first. – John Saunders
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I see your point about how that would be confusing. I've updated it to a regular exception for this example and a custom exception in GitHub. – Luis Perez
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Great answer for such a basic question. It's not so bad when it is your code that is failing. It's horrible when it's coming from deep inside some a commercial third party library you are relying on, and the customer support keeps insisting that it has to be your code that is causing the problem. And your not entirely sure it's not and the whole project is ground to halt.. I actually think this might make an appropriate epitaph for my tombstone: "Object reference not set to an instance of an object." – Darrel Lee
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@LuisPerez There is the ArgumentNullException which you can throw specifically for this situation. – DavidG

NullReferenceException or Object reference not set to an instance of an object occurs when an object of the class you are trying to use is not instantiated. For example:

Assume that you have a class named Student.

public class Student
{
    private string FirstName;
    private string LastName;
    public string GetFullName()
    {
        return FirstName + LastName;
    }
}

Now, consider another class where you are trying to retrieve the student's full name.

public class StudentInfo
{      
    public string GetStudentName()
    {
        Student s;
        string fullname = s.GetFullName();
        return fullname;
    }        
}

As seen in the above code, the statement Student s - only declares the variable of type Student, note that the Student class is not instantiated at this point. Hence, when the statement s.GetFullName() gets executed, it will throw the NullReferenceException.

The error line "Object reference not set to an instance of an object. " states that you have not assigned instance object to a object reference and still you are accessing properies/methods of that object.

for example: let say you have a class called myClass and it contains one property prop1.

public Class myClass
{
   public int prop1 {get;set;}
}

Now you are accessing this prop1 in some other class just like below:

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref.prop1 = 1;  //This line throws error
     }
}

above line throws error because reference of class myClass is declared but not instantiated or an instance of object is not assigned to referecne of that class.

To fix this you have to instantiate (assign object to reference of that class).

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref = new myClass();
        ref.prop1 = 1;  
     }
}
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This answer has already been answered. – John Saunders

Interestingly, none of the answers on this page mention the two edge cases, hope no one minds if I add them:

Edge case #1: concurrent access to a Dictionary

Generic dictionaries in .NET are not thread-safe and they sometimes might throw a NullReference or even (more frequent) a KeyNotFoundException when you try to access a key from two concurrent threads. The exception is quite misleading in this case.

Edge case #2: unsafe code

If a NullReferenceException is thrown by unsafe code, you might look at your pointer variables, and check them for IntPtr.Zero or something. Which is the same thing ("null pointer exception"), but in unsafe code, variables are often cast to value-types/arrays, etc., and you bang your head against the wall, wondering how a value-type can throw this exception.

(Another reason for non-using unsafe code unless you need it, by the way)

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Your dictionary example is not an edge case. If the object is not thread safe, then using it from multiple threads produces random results. Your unsafe code example differs from null in which way? – John Saunders

It means you are trying to manipulate something which has reference but not yet initialized
The first thing to do here is check every instance created.

Use breakpoints , watches , inspect your varibale values.
Follow stack trace and search for exact row and column which is creating problem

There is a scenario that can happen that is Class related. The question ended up getting closed prior to my stating the resolution: //allinonescript.com/questions/43348009/unable-to-instantiate-class

Beware of classes not instantiating: If any part of your constructor in a class throws a null reference exception the class does not instantiate. In my case it was trying to get a connection string from the web.config that that did not exist.

I instantiated a class:

ClassName myClass = new ClassName();
myClass.RunSomeMethod();

Inside the class itself was a call to get a connection string from the web.config. This part of the constructor threw an null value exception so myClass was null.

If you ever have a situation where a class in not instantiating, try making sure that no part of the class constructor Is throwing a null value exception. F-11 and step through the class and make sure there are no nulls.

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Constructors never return null. They either instantiate an object or throw an exception. – CodeCaster
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@CodeCaster I reworded the entire answer hopefully satistying your concerns. The point being that I haven't seen in other answers is that if any part of the class constructor throws an exception than it will cause the class to not instantiate. Although this makes a lot of sense to me it didn't come as an obvious choice for me. – logixologist
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Again, this can't happen. If this constructor throws, the next line won't execute. – CodeCaster
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@logixologist Totally agree with CC here, this answer doesn't make any sense as it stands. – DavidG

You are trying to access an object that isn't created or currently not in memory.

When I get this error, I add the following code to any event handler that triggers the error.

if (!IsLoaded) return;

The ! means "not", FYI. So if the object in question isn't loaded, the program terminates the execution and prevents the crash.

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This makes no sense at all. – DavidG
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It works for me when an event handler fires but the object it's looking for hasn't been created yet. Last time it was a TextChanged event for a textbox. – Nathan

This is basically is a Null reference exception. As Microsoft states-

A NullReferenceException exception is thrown when you try to access a member of a type whose value is null.

What does that mean?

That means if any member which doesn’t hold any value and we are making that member to perform certain task then the system will undoubtedly toss a message and say-

“Hey wait, that member has no values so it can’t perform the task which you are handing it over.”

The exception itself says that something is being referred but whose value is not being set. So this denotes that it only occurs while using reference types as Value types are non-nullable.

NullReferenceException won't occur if we are using Value type members.

class Program
{
    static void Main(string[] args)
    {
        string str = null;
        Console.WriteLine(str.Length);
        Console.ReadLine();
    }
}

The above code shows simple string which is assigned with a null value.

Now, when I try to print the length of the string str, I do get An unhandled exception of type ‘System.NullReferenceException’ occurred message because member str is pointing to null and there can’t be any length of null.

NullReferenceException’ also occurs when we forget to instantiate a reference type.

Suppose I have a class and member method in it. I have not instantiated my class but only named my class. Now if I try to use the method, the compiler will throw an error or issue a warning (depending on the compiler).

class Program
{
    static void Main(string[] args)
    {
        MyClass1 obj;
        obj.foo();  //Use of unassigned local variable 'obj'
    }
}

public class MyClass1
{
    internal void foo()
    {
        Console.WriteLine("hello from foo");

    }
}

Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing. Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing.

Why it occurs?

  • NullReferenceException arises due to our fault for not checking the object’s value. We often leave the object values unchecked in the code development.

  • It also arises when we forget to instantiate our objects. Using methods, properties, collections etc. which can return or set null values can also be the cause of this exception.

How can it be avoided?

There are various ways and methods to avoid this renowned exception:

  1. Explicit Checking: We should adhere to the tradition of checking the objects, properties, methods, arrays, and collections whether they are null. This can be simply implemented using conditional statements like if-else if-else etc.

  2. Exception handling: One of the important ways of managing this exception. Using simple try-catch-finally blocks we can control this exception and also maintain a log of it. This can be very useful when your application is on production stage.

  3. Null operators: Null Coalescing operator and null conditional operators can also be used in handy while setting values to objects, variables, properties and fields.

  4. Debugger: For developers, we have the big weapon of Debugging with us. If have we face NullReferenceException during the development face we can use the debugger to get to the source of the exception.

  5. In-built method: System methods such as GetValueOrDefault(),IsNullOrWhiteSpace() and IsNullorEmpty() checks for nulls and assign the default value if there is a null value.

There are many good answers already here. You can also check more detailed description with examples on my blog.

Hope this helps too!

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You basically copied half of that blog post and added nothing new that existing answers don't address. – CodeCaster
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@codecaster Is it said copying when you rewrite a summary from your own blog. I know there is nothing new in my answer and nothing new that previous answers dont have but I wish to contribute in more sophisticated way and let others understand the way I understood. Will be glad even if it helps a single person. In good faith. – Wasim

If we consider common scenarios where this exception can be thrown, accessing properties withing object at the top.

Ex:

string postalcode=Customer.Address.PostalCode; 
//if customer or address is null , this will through exeption

in here , if address is null , then you will get NullReferenceException.

So, as a practice we should always use null check, before accessing properties in such objects (specially in generic)

string postalcode=Customer?.Address?.PostalCode;
//if customer or address is null , this will return null, without through a exception
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This has already been answered several times. – John Saunders

If one is getting this message during saving or compiling the build, just close all the files and then open any file to compile and save.

For me the reason was that I had rename the file and old file was still open.

To use methods and member of an object you first have to create that object. If you didn't create it (variable that should hold the object is not initialized), but you try to use it's methods or variables you'll get that error.

Sometime you may just forgot to do initialization.

Edited: new can't return null, but fire's exception when failed. Long time ago it was the case in some languages, but not any more. Thanks @John Saunders for pointing that out.

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new never returns null – John Saunders
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@John did you mean to accept this answer? – CodeCaster
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@code no. Thanks for the catch. Phone app. – John Saunders

Literally the easiest way to fix a NullReferenceExeption has two ways. If you have a GameObject for example with a script attached and a variable named rb (rigidbody) this variable will start null when you start your game.
This is why you get a NullReferenceExeption because the computer does not have data stored in that variable.

I'll be using a RigidBody variable as an example.
We can add data really easily actually in a few ways:

  1. Add a RigidBody to your object with AddComponent > Physics > Rigidbody
    Then go into your script and type rb = GetComponent<Rigidbody>();
    This line of code works best under your Start() or Awake() functions.
  2. You can add a component programmatically and assign the variable at the same time with one line of code: rb = AddComponent<RigidBody>();

Further Notes: If you want unity to add a component to your object and you might have forgotten to add one, you can type [RequireComponent(typeof(RigidBody))] above your class declaration (the space below all of your usings).
Enjoy and have fun making games!

Reference types default to null to indicate that they are not referencing any object. Hence, if you try and access the object that is being referenced and there isn't one, you will get a NullReferenceException.

For Ex:

SqlConnection connection = null;
connection.Open();

When you run this code, you will get :

System.NullReferenceException: Object reference not set to an instance of an object.

You can avoid this error by coding like this:

if (connection != null){
    connection.Open();
}

Note: In order to avoid this error you should always initialize your objects before you try to do anything with them.

You can fix NullReferenceException in a clean way using Null-conditional Operators in c#6 and write less code to handle null checks.

It's used to test for null before performing a member access (?.) or index (?[) operation.

Example

  var name = p?.Spouse?.FirstName;

is equivalent to:

    if (p != null)
    {
        if (p.Spouse != null)
        {
            name = p.Spouse.FirstName;
        }
    }

The result is that the name will be null when p is null or when p.Spouse is null.

Otherwise, the variable name will be assigned the value of the p.Spouse.FirstName.

For More details : Null-conditional Operators

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