PromiseOnCSharp.cs

using System;
using System.Threading.Tasks;

/// <summary>
/// TaskCompletionSource를 이용한 비동기 반환 클래스 입니다.
/// JavaScript의 Promise를 모방한 클래스입니다.
/// </summary>
/// <remarks>
/// JavaScript에서의 Promise (MDN Docs): 
/// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
/// </remarks>
/// <example>
/// https://gist.github.com/hwahyang1/b49d256717a0aeb2a9413092e9aabf4e
/// </example>
public class Promise<T>
{
	private TaskCompletionSource<T> taskCompletionSource;

	public Promise()
	{
		taskCompletionSource = new TaskCompletionSource<T>();
	}

	public void Resolve(T result)
	{
		taskCompletionSource.SetResult(result);
	}

	public void Reject(string message)
	{
		taskCompletionSource.SetException(new Exception(message));
	}

	public void Reject(Exception e)
	{
		taskCompletionSource.SetException(e);
	}

	public Task<T> Fetch => taskCompletionSource.Task;
}

public class MyClass
{
	public Promise<int> PerformAsyncOperation()
	{
		// Return Type 지정
		Promise<int> promise = new Promise<int>();

		Task.Delay(5000).ContinueWith(_ =>
		{
			Random random = new Random();
			if (random.Next(0, 2) == 0)
			{
				int result = random.Next(1, 101);
				
				// Return Value
				promise.Resolve(result);
			}
			else
			{
				// Throw Error
				promise.Reject("Failed to perform the async operation.");
				// OR,
				//promise.Reject(new Exception("Failed to perform the async operation."));
			}
		});

		return promise;
	}
}

public class Program
{
	public static async Task Main(string[] args)
	{
		MyClass myClass = new MyClass();
		Promise<int> promise = myClass.PerformAsyncOperation();

		try
		{
			int result = await promise.Fetch;
			Console.WriteLine("Async operation result: " + result);
		}
		catch (Exception e)
		{
			Console.WriteLine("Async operation failed: " + e.Message);
		}

		// OR,

		try
		{
			int result = await myClass.PerformAsyncOperation().Fetch;
			Console.WriteLine("Async operation result: " + result);
		}
		catch (Exception e)
		{
			Console.WriteLine("Async operation failed: " + e.Message);
		}
	}
}