namespace AbstractFactory
{
   public class BaseUser
    {
        protected string login;
        protected string pass;

        public string GetPass() { return pass; }
        public string GetLogin() { return login; }

        public string Login
        {
            set { login = value; }
            get { return login; }
        }

        public string Pass
        {
            set { pass = value; }
            get { return pass; }
        }

        public BaseUser(string login, string pass) { Login = login; Pass = pass; }
    }

   sealed public class SimpleUser : BaseUser
    {
        public SimpleUser(string login, string pass):base(login,pass) {}
        public void Opera() { MessageBox.Show("SimpleUser!"+login+" -> "+pass); }
    }

   sealed public class AdminUser : BaseUser
    {
        public AdminUser(string login, string pass):base(login,pass) {}
        public void Opera() { MessageBox.Show("AdminUser!" + login + " -> " + pass); }
    }

    public class FactoryUser
    {
        public static BaseUser CreateUser(int type, string login, string pass)
        {
            switch (type)
            {
                case 1: return new SimpleUser(login, pass);
                case 2: return new AdminUser(login, pass);
            }
            return new BaseUser("", "");
        }
    }

}

AbstractFactory.AdminUser admin = AbstractFactory.FactoryUser.CreateUser(2, "User1", "12345");
AbstractFactory.SimpleUser user = AbstractFactory.FactoryUser.CreateUser(1, "User2", "1234567890");

...

user.Opera();

Cannot implicitly convert type 'AbstractFactory.BaseUser' to 'AbstractFactory.AdminUser'. An explicit conversion exists (are you missing a cast?)

AdminUser admin = new SimpleUser(login, pass); // Fail

BaseClass/*Interface*/ Obj = new ChildClass(args);

public void SomeMethod(BaseUser user)
{
    user.Opera(); // Fail метод отсутсвует
}

SomeMethod(new AdminUser());

public void SomeMethod(AdminUser user)
{
    user.Opera();
}

SomeMethod(new BaseUser()); // Fail метод отсутсвует

public abstract class BaseUser
{
    public string Login { get; protected set; }
    public string Pass { get; protected set; }

    public BaseUser(string login, string pass)
    {
        Login = login; Pass = pass;
    }

    public abstract void Opera();
}

sealed public class SimpleUser : BaseUser
{
    public SimpleUser(string login, string pass)
        : base(login, pass)
    {
    }

    public override void Opera()
    {
        Console.WriteLine("SimpleUser!" + Login + " -> " + Pass);
    }
}

sealed public class AdminUser : BaseUser
{
    public AdminUser(string login, string pass)
        : base(login, pass)
    {
    }

    public override void Opera()
    {
        Console.WriteLine("AdminUser!" + Login + " -> " + Pass);
    }
}

public class FactoryUser
{
    public static BaseUser CreateUser(int type, string login, string pass)
    {
        switch (type)
        {
            case 1: return new SimpleUser(login, pass);
            case 2: return new AdminUser(login, pass);
        }

        throw new NotImplementedException();
    }
}

public interface UserFactory
{
    BaseUser CreateUser(string login, string pass);
}

public class AdminUserFactory : UserFactory
{
    public BaseUser CreateUser(string login, string pass)
    {
        return new AdminUser(login, pass);
    }
}

public class SimpleUserFactory : UserFactory
{
    public BaseUser CreateUser(string login, string pass)
    {
        return new SimpleUser(login, pass);
    }
}