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null (C# Reference)

Updated: July 2008

The null keyword is a literal that represents a null reference, one that does not refer to any object. null is the default value of reference-type variables. Ordinary value types cannot be null. However, C# 2.0 introduced nullable value types. See Nullable Types (C# Programming Guide).

The following example demonstrates some behaviors of the null keyword:

class Program
{
    class MyClass
    {
        public void MyMethod() { }
    }

    static void Main(string[] args)
    {
        // Set a breakpoint here to see that mc = null. 
        // However, the compiler considers it "unassigned." 
        // and generates a compiler error if you try to 
        // use the variable.
        MyClass mc;

        // Now the variable can be used, but...
        mc = null;

        // ... a method call on a null object raises  
        // a run-time NullReferenceException. 
        // Uncomment the following line to see for yourself. 
        // mc.MyMethod(); 

        // Now mc has a value.
        mc = new MyClass();

        // You can call its method.
        mc.MyMethod();

        // Set mc to null again. The object it referenced 
        // is no longer accsessible and can now be garbage-collected.
        mc = null;

        // A null string is not the same as an empty string. 
        string s = null;
        string t = String.Empty; // Logically the same as "" 

        // Equals applied to any null object returns false. 
        bool b = (t.Equals(s));
        Console.WriteLine(b);

        // Equality operator also returns false when one 
        // operand is null.
        Console.WriteLine("Empty string {0} null string", s == t ? "equals": "does not equal");

        // Returns true.
        Console.WriteLine("null == null is {0}", null == null);


        // A value type cannot be null 
        // int i = null; // Compiler error! 

        // Use a nullable value type instead: 
        int? i = null;

        // Keep the console window open in debug mode.
        System.Console.WriteLine("Press any key to exit.");
        System.Console.ReadKey();

    }
}

For more information, see the following sections in the C# Language Specification:

  • 2.4.4.6 The null literal

Date

History

Reason

July 2008

Added code example.

Content bug fix.

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