what does double question marks mean in c# [duplicate]

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开发者_开发技巧 What is the “??” operator for?

Debugging some code and found ?? inside the code. What does this mean?

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?? is the null-coalescing operator for nullable types.

object obj = canBeNull ?? alternative;

// equivalent to:
object obj = canBeNull != null ? canBeNull : alternative;

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http://msdn.microsoft.com/en-us/library/ms173224.aspx refer to this for description. it's an operator

The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type.

    // ?? 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");