What is the “??” operator for? [duplicate]

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It’s the null coalescing operator. It was introduced in C# 2.

The result of the expression a ?? b is a if that’s not null, or b otherwise. b isn’t evaluated unless it’s needed.

Two nice things:

  • The overall type of the expression is that of the second operand, which is important when you’re using nullable value types:

    int? maybe = ...;
int definitely = maybe ?? 10;

    (Note that you can’t use a non-nullable value type as the first operand – it would be
    pointless.)

  • The associativity rules mean you can chain this really easily. For example:

    string address = shippingAddress ?? billingAddress ?? contactAddress;

That will use the first non-null value out of the shipping, billing or contact address.

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