c#中的？？

1.使用场景

2.参考文章及示例代码下载

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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");
    }
}

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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