?? [null-coalescing operator] in C#

null-coalescing operator  in C#

The ?? operator is called the null-coalescing operator and 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.

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";

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.