awaiter using as a TResult monad

MonadicAwaiter.cs

using System.Runtime.CompilerServices;


var user1 = await GetUserById(id: 12);
var address1 = await GetUserAddress(user1);

Console.Out.WriteLine(address1.ToUpper());

var user2 = await GetUserById(id: 13);
var address2 = await GetUserAddress(user2);

Console.Out.WriteLine(address2.ToUpper());


Result<int, string> GetUserById(int id) => id switch
{
    12 => Result<int, string>.Success(1234),
    _ => Result<int, string>.Failure("not found"),
};

Result<string, string> GetUserAddress(int id) => id switch
{
    1234 => Result<string, string>.Success("Moscow"),
    _ => Result<string, string>.Failure("not found"),
};


record Result<TValue, TError>
{
    public TValue? Value { get; private set; }
    public TError? Error { get; private set; }

    public bool IsSuccess => Value != null;

    public static Result<TValue, TError> Success(TValue value)
        => new Result<TValue, TError>() { Value = value };

    public static Result<TValue, TError> Failure(TError error)
        => new Result<TValue, TError>() { Error = error };

    public ResultAwaiter<TValue, TError> GetAwaiter()
        => new ResultAwaiter<TValue, TError>(this);
}

class ResultAwaiter<TValue, TError> : INotifyCompletion
{
    private readonly Result<TValue, TError> _result;

    public ResultAwaiter(Result<TValue, TError> result) => _result = result;
    
    public bool IsCompleted => true;

    public void OnCompleted(Action continuation)
    {
        if (_result.IsSuccess)
            continuation();
        else
            throw new Exception(_result.Error!.ToString());
    }

    public TValue GetResult() => _result.IsSuccess switch
    {
        true => _result.Value!,
        _ => throw new Exception(_result.Error!.ToString()),
    };
}

The output:

MOSCOW
Unhandled exception. System.Exception: not found