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What is a NullReferenceException, and how do I fix it?

Tech C# .Net
浮世絵空事
作者
浮世絵空事
所谓灵魂 几行代码

Copy from: http://stackoverflow.com/a/4660186/4190277

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. 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 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
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Some common scenarios where the exception can be thrown:

Generic
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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.

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.

Array
#

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

Array Elements
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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:
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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
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// 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
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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 Control Creation Order and Events
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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.

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"))