I have some code and when it executes, it throws a
NullReferenceException
Object reference not set to an instance of an object.
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
.
The runtime throwing a NullReferenceException
always means the same thing: you are trying to use a reference. The reference is not initialized (or it was initialized, but is no longer initialized).
This means the reference is null
, and you cannot access members 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
.
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.
Some common scenarios where the exception can be thrown:
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.
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.
}
}
The same applies to nested object initializers:
Book b1 = new Book { 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
.
int[] numbers = null;
int n = numbers[0]; // numbers is null. There is no array to index.
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.
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.
Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
// There is no Dictionary to perform the lookup.
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.
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
}
}
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;
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!";
}
}
// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();
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 accesses 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 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 (ie: listing label1
before comboBox1
, ignoring issues of design philosophy, would at least resolve the NullReferenceException
here.
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.
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.
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);
}
}
null
, and provide a default value.Methods calls 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;
}
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;
}
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 actually 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.
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
??
[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;
}
?.
(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;