pm.cs

using System;

namespace ConsoleApp1
{

    abstract class Result {
        public static Result Ok<T>(T value) => new Ok<T> { Value = value };
        public static Result Error<T>(T value) => new Error<T> { Value = value };
    }

    sealed class Ok<T> : Result
    {
        public T Value { get; set; }
    }

    sealed class Error<T> : Result
    {
        public T Value { get; set; }
    }

    class Program
    {
        static void ParseResult(Result r)
        {
            switch (r)
            {
                case Ok<int> s:
                    Console.WriteLine($"SUCCESS! '{s.Value}'.");
                    break;
                case Error<string> e:
                    Console.WriteLine($"ERROR! '{e.Value}'.");
                    break;
                default:
                    Console.WriteLine("URGH.");
                    break;
            }
        }

        static void Main(string[] args)
        {
            var good = Result.Ok(123);
            ParseResult(good);
            var bad = Result.Error("Oh no!");
            ParseResult(bad);
            var ugly = Result.Error(99); // shouldn't compile
            ParseResult(ugly);
        }
    }
}