Did you know that C# now has a new operator ?? ? It has to do with the support for nullable data types. 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 type to a non-nullable type without using the ?? operator, you will generate a compile-time error. If you use a cast, and the nullable type is currently undefined, an InvalidOperationException exception will be thrown.
// define a nullable integer
int? x = null;
// Now assign it to a not nullable integer avoiding the errors
// y = x, unless x is null, in which case y = -1.
int y = x ?? -1;
For more information, see Nullable Types (C# Programming Guide).