The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand. For example:
int? x = null;
// y = x, unless x is null, in which case y = -1.
int y = x ?? -1;
The ?? operator also works with reference types:
//message = param, unless param is null
//in which case message = "No message"
string message = param ?? "No message";