What does ?? mean in C#? [duplicate]

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Possible Duplicate:

What do two question marks together mean in C#?

What does the ?? 开发者_高级运维mean in this C# statement?

int availableUnits = unitsInStock ?? 0;

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if (unitsInStock != null)
    availableUnits = unitsInStock;
else
    availableUnits = 0;

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This is the null coalescing operator. It translates to: availableUnits equals unitsInStock unless unitsInStock equals null, in which case availableUnits equals 0.

It is used to change nullable types into value types.

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The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand.

?? Operator (C# Reference)

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according to MSDN, The ?? operator is called the null-coalescing operator and is used to define a default value for a nullable value types as well as reference types. It returns the left-hand operand if it is not null; otherwise it returns the right operand.

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

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it means the availableUnits variable will be == unitsInStock unless unitsInStock == 0, in which case availableUnits is null.