C# 4 and .NET 4

494 ❘ ChaPTer 20 threAds, tAsks, And synchrOnizAtiOn
the result is received with the delegate EndInvoke() method. From the UI standpoint the result is similar
to the previous sample; just the wait is done in a different manner.
static void Main()
{
TakesAWhileDelegate d1 = TakesAWhile;
}
IAsyncResult ar = d1.BeginInvoke(1, 3000, null, null);
while (true)
{
Console.Write(".");
if (ar.AsyncWaitHandle.WaitOne(50, false))
{
Console.WriteLine("Can get the result now");
break;
}
}
int result = d1.EndInvoke(ar);
Console.WriteLine("result: {0}", result);
You can read more information about wait handles later in the “ Synchronization ”
section of this chapter.
asynchronous Callback
The third version of waiting for the result from the delegate uses an asynchronous callback. With the third
parameter of BeginInvoke() , you can pass a method that fulfi lls the requirements of the AsyncCallback
delegate. The AsyncCallback delegate defi nes a parameter of IAsnycResult and a void return type. Here,
the address of the method TakesAWhileCompleted is assigned to the third parameter, which fulfi lls the
requirements of the AsyncCallback delegate. For the last parameter, you can pass any object for accessing
it from the callback method. It is useful to pass the delegate instance itself, so the callback method can use it
to get the result of the asynchronous method.
The method TakesAWhileCompleted() is invoked as soon as the delegate TakesAWhileDelegate has
completed its work. There is no need to wait for a result inside the main thread. However, you may not end
the main thread before the work of the delegate threads is fi nished, unless you don ’ t have a problem with
delegate threads stopping when the main thread ends.
static void Main()
{
TakesAWhileDelegate d1 = TakesAWhile;
}
d1.BeginInvoke(1, 3000, TakesAWhileCompleted, d1);
for (int i = 0; i < 100; i++)
{
Console.Write(".");
Thread.Sleep(50);
}
code snippet Async Delegate/Program.cs
The method TakesAWhileCompleted() is defi ned with the parameter and return type specifi ed by the
AsyncCallback delegate. The last parameter passed with the BeginInvoke() method can be read here
by using ar.AsyncState . With the TakesAWhileDelegate , you can invoke the EndInvoke method to get
the result.
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