最近在看Asp.net MVC時，遇到了這麼一段代碼
public AccountController(IFormsAuthentication formsAuth, IMembershipService service)
{
FormsAuth = formsAuth ?? new FormsAuthenticationService();
MembershipService = service ?? new AccountMembershipService();
}
FormsAuth = formsAuth ?? new FormsAuthenticationService();
該代碼的語義是
If (formsAuth != null)
FormsAuth = formsAuth;
else
FormsAuth = FormsAuthenticationService();
對“？？”MSDN上的解釋是：
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.
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.
For more information, see Nullable Types (C# Programming Guide).
The result of a ?? operator is not considered to be a constant even if both its arguments are constants.