Asp.net MVC中的"??"

最近在看Asp.net MVC时，遇到了这么一段代码

 

 

public AccountController(IFormsAuthentication formsAuth, IMembershipService service)
{
    FormsAuth = formsAuth ?? new FormsAuthenticationService();
    MembershipService = service ?? new AccountMembershipService();
}

FormsAuth = formsAuth ?? new FormsAuthenticationService();
该代码的语义是

If (formsAuth != null)
  FormsAuth = formsAuth;
else
  FormsAuth = FormsAuthenticationService();

对“？？”MSDN上的解释是：

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.