c# 两个问号

http://topic.csdn.net/u/20090326/15/586b9281-314d-43b3-b1c0-a795d24fffc3.html The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand.

C# code

// nullable_type_operator.cs using System; class MainClass { staticint? GetNullableInt() { returnnull; } staticstring GetStringValue() { returnnull; } staticvoid Main() { // ?? operator example. int? x =null; // y = x, unless x is null, in which case y = -1. int y = x ??-1; // Assign i to return value of method, unless // return value is null, in which case assign // default value of int to i. int i = GetNullableInt() ??default(int); string s = GetStringValue(); // ?? also works with reference types. // Display contents of s, unless s is null, // in which case display "Unspecified". Console.WriteLine(s ??"Unspecified"); } }