Tim Van Wassenhove home

I always seem to forget about the mechanics of implementing a Callback method that can be used in the Asynchronous Programming Model (APM). So here is a simple example in C# 3.0 using the Func<T, TResult> delegate

class Program
	static void Main(string[] args)
		Func<string, string> echoDelegate = Echo;
		IAsyncResult asyncResult = echoDelegate.BeginInvoke("some input", EchoCallback, "x");

		WaitHandle.WaitAll(new WaitHandle[] {asyncResult.AsyncWaitHandle});

		Console.Write("{0}Press any key to continue...", Environment.NewLine);

	static string Echo(string input)
		return string.Format("Echoing {0}", input);

	static void EchoCallback(object state)
		IAsyncResult r = (IAsyncResult)state;
		Debug.Assert(r.AsyncState == "x");

		AsyncResult asyncResult = (AsyncResult)state;
		Func<string, string> echoDelegate = (Func<string, string>)asyncResult.AsyncDelegate;
		string echoResult = echoDelegate.EndInvoke(asyncResult);
		Debug.Assert(echoResult == "Echoing some input");

		Console.WriteLine("EchoCallback completed.");