namespace Testing

{

    public class Result<T, E>

    {

        private readonly T value;

        private readonly E error;

        private readonly bool isSuccess;

​

        public Result(T value, E error, bool isSuccess)

        {

            this.value = value;

            this.error = error;

            this.isSuccess = isSuccess;

        }

​

        public static Result<T, E> AsOk(T value)

        {

            return new Result<T, E>(value, default, true);

        }

​

        public static Result<T, E> AsError(E error)

        {

            return new Result<T, E>(default, error, false);

        }

​

        public T Value => this.value;

​

        public E Error => this.error;

​

        public bool IsSuccess => this.isSuccess;

​

        public bool IsFailure => !this.isSuccess;

    }

​

    public class Foo

    {

        public Foo(string tag)

        {

            this.Tag = tag;

        }

​

        public string Tag { get; }

    }

​

    public class Bar : Foo

    {

        public Bar(string tag, int id)

            : base(tag)

        {

            this.Id = id;

        }

​

        public int Id { get; }

    }

​

    public static class Program

    {

        public static void Run()

        {

            Foo foo = new Foo("tag1");

            Bar bar = new Bar("tag2", 2);

            Foo barAsFoo = new Bar("tag3", 3);

​

            Result<Foo, int> triedFoo = Result<Foo, int>.AsOk(foo);

            Result<Bar, int> triedBar = Result<Bar, int>.AsOk(bar);

​

            // Now this assignment will work using covariance

            Result<Foo, int> triedBarAsTriedFoo = triedBar;

        }

    }

}

​