Why won't a class derived from an abstract class with a where clause cast to its lowest common class

Question

Some code to replicate the issue:

using System;

public abstract class Response { }
public abstract class Request<T> where T : Response { }
public class LoginResponse : Response { }
public class LoginRequest : Request<LoginResponse> { }

public class Program
{
    static void Main(string[] args)
    {
        LoginRequest login = new LoginRequest();


        /* Error: Cannot implicitly convert type 'LoginRequest' to 'Request' */
        Request<Response> castTest = login;


        /* No Error */
        Request<LoginResponse> castTest2 = login;
    }
}

As far as i can tell the LoginRequest class is a Request<Response> because is inherits from Request<T> and LoginResponse inherits from Response so can anybody enlighten me as to why i get the compiler error?

note: i have also tried an explicit cast

Answer 1

You're getting the error because Request<Response> and Request<LoginResponse> are not covariant.

Just because LoginResponse inherits from Response doesn't mean that Request<LoginResponse> can be treated the same as Request<Response>. Give this article a read:

MSDN - Covariance and Contravariance in Generics