null (C# 參考)
null
關鍵字是表示 Null 參考的常值,不參考任何物件的值。 null
是參考型別變數的預設值。 一般實值型別不可為 Null,但可為 Null 的實值型別除外。
下例範例示範 null
關鍵字的一些行為:
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# 語言規格
如需詳細資訊,請參閱<C# 語言規格>。 語言規格是 C# 語法及用法的限定來源。