null-coalescing operator

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.

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.

  
string partner = Request.QueryString["GoogleId"] ??

    Request.QueryString["PartnerId"] ??

    Request.QueryString["UserKey"] ??

    string.Empty;