using System;

public class Program

{

    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; }

    }

}