?? Operator (null-coalescing operator)

?? 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.

A nullable type can contain a value, or it can be undefined. 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.