Covariance Contravariance Generics Interfaces - why doesn't this work?

Question

Thank you in advance. Here is my code:

public class ApplicationUser : IdentityUser
{
    public ApplicationUser()
    {

    }
}

public class AppUserManager : UserManager<ApplicationUser>
{
...
}

public interface IOwinManager
{
    UserManager<IdentityUser> UserManager { get; }
}

Why is this not working?

public class OwinManager : IOwinManager
{        
    public UserManager<IdentityUser> UserManager
    {
        get { return new AppUserManager(); }
    }
}

Since ApplicationUser inherits from IdentityUser and AppUserManager from UserManager, why is the combined generic not accepted? Thanks!

Answer 1

Both contravariance and covariance on generic type parameters for classes isn't supported.

Simplifying your issue:

// Compiler error!
UserManager<IdentityUser> userManager = new AppUserManager();

• AppUserManager inherits UserManager<ApplicationUser>.
• Thus, you're trying to set an UserManager<ApplicationUser>-derived reference on a UserManager<IdentityUser> reference. This is the problem! They're different types.

OP said...:

which, essentially means, I can't use concrete classes and their generics in an interface and expect them to be implemented by their children?

Interfaces support variance. Thus, you can design your interface as follows:

public interface IOwinManager<out TUser, out TManager>
    where TUser : IdentityUser 
    where TManager : UserManager<TUser>
{
    TManager UserManager { get; }
}

...and once you've implemented this interface, your implementation will declare a property of the concrete TManager type.