The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type. If you try to assign a nullable value type to a non-nullable value type without using the ?? operator, you will generate a compile-time error. If you use a cast, and the nullable value type is currently undefined, an InvalidOperationException exception will be thrown. 
eg:
// y = x, unless x is null, in which case y = -1.
int y = x ?? -1;