C# 4 and .NET 4

The Thread Class ❘ 495
static void TakesAWhileCompleted(IAsyncResult ar)
{
if (ar == null) throw new ArgumentNullException("ar");
TakesAWhileDelegate d1 = ar.AsyncState as TakesAWhileDelegate;
Trace.Assert(d1 != null, "Invalid object type");
}
int result = d1.EndInvoke(ar);
Console.WriteLine("result: {0}", result);
With a callback method, you need to pay attention to the fact that this method is
invoked from the thread of the delegate and not from the main thread.
Instead of defi ning a separate method and passing it to the BeginInvoke() method, Lambda expressions can be
used. The parameter ar is of type IAsyncResult . With the implementation, there is no need to assign a value to
the last parameter of the BeginInvoke() method because the Lambda expression can directly access variable d1
that is in the outer scope. However, the implementation block of the Lambda expression is still invoked from the
thread of the delegate, which might not be clear immediately when defi ning the method this way.
static void Main()
{
TakesAWhileDelegate d1 = TakesAWhile;
d1.BeginInvoke(1, 3000,
ar =>
{
int result = d1.EndInvoke(ar);
Console.WriteLine(“result: {0}”, result);
},
null);
for (int i = 0; i < 100; i++)
{
Console.Write(“.”);
Thread.Sleep(50);
}
}
The programming model and all of these options with asynchronous delegates — polling, wait
handles, and asynchronous callbacks — are not only available with delegates. The same programming
model — this is the asynchronous pattern — can be found in various places in the .NET Framework.
For example, you can send an HTTP Web request asynchronously with the BeginGetResponse()
method of the HttpWebRequest class. You can send an asynchronous request to the database with
the BeginExecuteReader() of the SqlCommand class. The parameters are similar to those of the
BeginInvoke() class of the delegate, and you can use the same mechanisms to get the result.
HttpWebRequest is covered in Chapter 24, “ Networking, ” and SqlCommand is
discussed in Chapter 30, “ Core ADO.NET.”
Instead of using the delegate for creating threads, you can create threads with the Thread class, which is
covered in the next section.
The Thread Class
With the Thread class, you can create and control threads. The code here is a very simple example of
creating and starting a new thread. The constructor of the Thread class is overloaded to accept a delegate
parameter of type ThreadStart or ParameterizedThreadStart . The ThreadStart delegate defi nes a
www.it-ebooks.info