C＃泛型协变

Question

I ran into an issue with generics which I believed can be solved with covariance, but don't fully understand how covarience works and how it can be declared properly. Let's say I have the following interfaces and classes:

public interface IOwnedObject<TUser>
where TUser : IBaseUser
{
    string UserId { get; set; }
    TUser User { get; set; }
}

public interface IBaseUser
{
    string Id { get; set; }
}

public class User : IBaseUser
{
    public string Id { get; set; }
}

public class SomeOwnedObject : IOwnedObject<User>
{
    public string UserId { get; set; }
    public User User { get; set; }
}

Then consider the following code:

var obj = new SomeOwnedObject();
            if(obj is IOwnedObject<IBaseUser> o)
                Console.WriteLine("Success"); // This never executes

The above if statement does not evaluate to true. However, in IOwnedObject it's only ever possible for TUser to be IBaseUser .

The following code evaluates to true:

var obj = new SomeOwnedObject();
            if(obj is IOwnedObject<User> o)
                Console.WriteLine("Success"); // This executes

since User implements IBaseUser , shouldn't IOwnedObject<IBaseUser> technically be a base class of IOwnedObject<User> . Is it possible to make that first statement evaluate to true without referencing the concrete implementation User ?

Answer 1

You can get the first statement to evaluate to true with a few modifications to your interface. We'll want to mark the TUser type as covariant with the out keyword, and in doing so we'll have to remove the property setter.

public interface IOwnedObject<out TUser> where TUser : IBaseUser
{
    string UserId { get; set; }
    TUser User { get; }
}

Now evaluating

var obj = new SomeOwnedObject();
if (obj is IOwnedObject<IBaseUser> o)
    Console.WriteLine("Success");

will result in "Success" being printed to the console.

Answer 2

Either do @Jonathon's suggestion or the below. Making it covariant you need to use an invariant type, which is IBaseUser.

public static void Main()
{
    var obj = new SomeOwnedObject();
        if(obj is IOwnedObject<IBaseUser> o)
            Console.WriteLine("Success"); // This never executes
}

public interface IOwnedObject<out TUser> where TUser : IBaseUser
{
    string UserId { get; set; }
    IBaseUser User { get; set; }
}

public interface IBaseUser
{
    string Id { get; set; }
}

public class User : IBaseUser
{
    public string Id { get; set; }
}

public class SomeOwnedObject : IOwnedObject<User>
{
    public string UserId { get; set; }
    public IBaseUser User { get; set; }
}

https://dotnetfiddle.net/PAE3CW