C# 3.0 Cookbook, 3rd Edition

18.4. Being Notified of the Completion of an Asynchronous Delegate

Problem

You need a way of receiving notification from an asynchronously invoked delegate that it has finished. This scheme must allow your code to continue processing without having to constantly call IsCompleted in a loop or to rely on the WaitOne method. Since the asynchronous delegate will return a value, you must be able to pass this return value back to the invoking thread.

Solution

Use the BeginInvoke method to start the asynchronous delegate, but use the first parameter to pass a callback delegate to the asynchronous delegate, as shown in Example 18-3.

Example 18-3. Getting notification on completion of an anonymous delegate

using System;
using System.Threading;

public class AsyncAction2
{
    public void CallbackAsyncDelegate( )
    {
        AsyncCallback callBack = DelegateCallback;

        AsyncInvoke method1 = TestAsyncInvoke.Method1;
        Console.WriteLine("Calling BeginInvoke on Thread {0}", Thread.CurrentThread.ManagedThreadId); IAsyncResult asyncResult = method1.BeginInvoke(callBack, method1); // No need to poll or use the WaitOne method here, so return to the calling // method. return; } private static void DelegateCallback(IAsyncResult iresult) { Console.WriteLine("Getting callback on Thread {0}", Thread.CurrentThread.ManagedThreadId); AsyncResult asyncResult = (AsyncResult)iresult; AsyncInvoke method1 = (AsyncInvoke)asyncResult.AsyncDelegate; int retVal = method1.EndInvoke(asyncResult); Console.WriteLine("retVal (Callback): " + ...