null (C# Reference)
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, except for nullable value types.
The following example demonstrates some behaviors of the null
keyword:
class Program
{
class MyClass
{
public void MyMethod() { }
}
static void Main()
{
// 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 accessible 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.
Console.WriteLine("t.Equals(s) is {0}", t.Equals(s));
// 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.
}
}
C# language specification
For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.
See also
Collaborate with us on GitHub
The source for this content can be found on GitHub, where you can also create and review issues and pull requests. For more information, see our contributor guide.