Result public

Program.cs

// See https://aka.ms/new-console-template for more information
using ConsoleApp1;

var nameEntities = new[] { "", null, "Dawid", "Kacper", "Wojciech", "Michał M", "Michał B", "Rafał", "Anna", "Floris" }
    .Select(CreateUser).ToList();

var index = 0;
foreach (var ne in nameEntities)
{
    if (ne.TryGetValue(out var u)) Console.WriteLine($"{index++}. User created: {u}");
    if (ne.TryGetError(out var err)) Console.Error.WriteLine($"{index++}. Failed: {err}");
}


//static Result<MyEnum> Parse(string name) => name switch
//{
//    "A" => MyEnum.A,
//    "B" => MyEnum.B,
//    "C" => MyEnum.C,
//    _ => MyApiExpectedErrors.EnumCannotBeParsed
//};


static Result<User> CreateUser(string? name)
{
    if (string.IsNullOrEmpty(name)) return MyApiExpectedErrors.ValidationFailed;

    if (Random.Shared.NextDouble() > 0.8) return MyApiExpectedErrors.DatabaseNotOnline;

    try
    {
        if (Random.Shared.NextDouble() > 0.5) throw new Exception("Something went wrong in upstream");

        return new User(Random.Shared.Next(), name);
    }
    catch (Exception e)
    {
        // TODO use logger
        var fore = Console.ForegroundColor;
        Console.ForegroundColor = ConsoleColor.Red;
        Console.Error.WriteLine(e);
        Console.ForegroundColor = fore;

        return (Error)e.Message;
    }
}

enum MyEnum { A, B, C }

public static class MyApiExpectedErrors
{
    public static readonly Error DatabaseNotOnline = new("DatabaseNotOnline", "Cannot connect to DB");
    public static readonly Error ValidationFailed = new("ValidationFailed", "Some data is wrong");
    public static readonly Error EnumCannotBeParsed = new("EnumParseFailed", $"Cannot parse enum. Valid labels are: {string.Join(", ", Enum.GetNames<MyEnum>())}");
}

public record User(int Id, string Name);

Result.cs

#nullable enable
using System.Diagnostics.CodeAnalysis;

namespace ConsoleApp1;

public sealed record Error(string Code, string? Description = null)
{
    private const string UNEXPECTED = "UNEXPECTED";

    public static explicit operator Error(string description) => new(UNEXPECTED, description);
}

public class Result<TValue>
{
    private readonly TValue? Value;
    private readonly Error? Error;

    [MemberNotNullWhen(true, nameof(Value))]
    [MemberNotNullWhen(false, nameof(Error))]
    public bool IsSuccess { get; }


    private Result(TValue value)
    {
        Value = value;
        Error = default;

        IsSuccess = true;
    }

    private Result(Error error)
    {
        Value = default;
        Error = error;

        IsSuccess = false;
    }

    public static Result<TValue> Success(TValue value) => new(value);
    public static Result<TValue> Failure(Error error) => new(error);

    public static implicit operator Result<TValue>(TValue result) => new(result);
    public static implicit operator Result<TValue>(Error error) => new(error);

    public void Invoke(Action<TValue> success, Action<Error>? failure = null)
    {
        if (IsSuccess) success(Value);
        else failure?.Invoke(Error);
    }

    public bool TryGetValue([NotNullWhen(true)] out TValue? value)
    {
        if (IsSuccess)
        {
            value = Value;
            return true;
        }
        else
        {
            value = default;
            return false;
        }
    }

    public bool TryGetError([NotNullWhen(true)] out Error? error)
    {
        if (!IsSuccess)
        {
            error = Error;
            return true;
        }
        else
        {
            error = default;
            return false;
        }
    }

    public override string ToString()
    {
        if (IsSuccess) return Value.ToString() ?? "";
        else return Error.ToString() ?? "";
    }
}