ResultValErr.cs

using System;

namespace MonadTest
{
    public interface IResult<T, E>
    {
        T Outcome { get; }
        E Error { get; }
    }

    public abstract class Result<T, E> : IResult<T, E>
    {
        public abstract T Outcome { get; }
        public abstract E Error { get; }

        public static IResult<T, E> Ok(T outcome) => new Ok<T, E>(outcome);
        public static IResult<T, E> Err(E error) => new Err<T, E>(error);
    }

    public sealed class Ok<T, E> : Result<T, E>
    {
        public override T Outcome { get; }
        public override E Error => throw new NotSupportedException();

        public Ok(T outcome)
        {
            Outcome = outcome;
        }
    }

    public sealed class Err<T, E> : Result<T, E>
    {
        public override T Outcome => throw new NotSupportedException();
        public override E Error { get; }

        public Err(E error)
        {
            Error = error;
        }
    }
    
    class Program
    {
        public static IResult<int,string> GetResult(bool isOk)
        {
            return isOk ? 
            Result<int, string>.Ok(1):
            Result<int, string>.Err("Ooops we got error");
        }

        static void Main(string[] args)
        {
            var res = GetResult(true);

            switch (res)
            {
                case Ok<int, string> ok:
                    Console.WriteLine($"Lucky number is {ok.Outcome}");
                    break;
                case Err<int, string> err:
                    Console.WriteLine(err.Error);
                    break;
            }
        }
    }
}