?? Operator in C#

The ?? operator is called the null-coalescing operator and is used to define a default value for nullable value types or reference types. It returns the left-hand operand if the operand 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.

int? x = null;
// y = x, unless x is null, in which case y = -1. 
int y = x ?? -1;

Useful links

Nullable Types

?? Operator

The C# ?? null coalescing operator (and using it with LINQ)