C# Cookbook

15.6. Being Notified of the Completionof an Asynchronous Delegate

Problem

You need a way of receiving notification from an asynchronously invoked delegate that it has finished. However, it must be more flexible than the notification schemes in the previous two recipes (Recipe 15.4 and Recipe 15.5). 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:

using System;
using System.Threading;

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

        AsyncInvoke method1 = new AsyncInvoke(TestAsyncInvoke.Method1);
        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)
    {
        AsyncResult asyncResult = (AsyncResult)iresult;
        AsyncInvoke method1 = (AsyncInvoke)asyncResult.AsyncDelegate;

        int retVal = method1.EndInvoke(asyncResult);
        Console.WriteLine("retVal (Callback): " + retVal);
    }
}

This callback delegate will call the DelegateCallback method on the thread the method was ultimately invoked ...