- C# code
string id = Request["id"] ?? string.Empty;
这里两个问号是什么意思呀。？
------解决方案--------------------------------------------------------
如果Request["id"] 不存在
string id = string.Empty;
------解决方案--------------------------------------------------------
??
判断??左边表达式是否为null，如是null则取右边表达式的值，否则就取左边表达式的值
------解决方案--------------------------------------------------------
The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand.
- C# code
// nullable_type_operator.csusing System;class MainClass{ static int? GetNullableInt() { return null; } static string GetStringValue() { return null; } static void 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"); }}