What does ?? mean in C#?

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What do two question marks together mean in C#?

What does the ?? 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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