c#中的？？

1.使用场景

2.参考文章及示例代码下载

---

1. 使用场景

msdn：It returns the left-hand operand if it is not null; otherwise it returns the right operand.如果左侧值不为空的话，返回左侧的值，否则返回右面的值，下面是一个测试示例：

class NullCoalesce { 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"); } }

2.参考文章及示例代码下载

参考文章：

http://msdn.microsoft.com/en-us/library/ms173224.aspx