Working with the Framework - Effective C# - page 218

Let's dig a little deeper into this pattern to examine the methods used for
starting and reporting the completion of each download. The finish-
Download method is rather simple, and I'll show it only for completeness:
private static void finishDownload( string url, byte [] bytes)
{
Console .WriteLine( "Read {0} bytes from {1}" ,
bytes.Length, url);
}
StartDownload shows a bit more of the interface provided by the Parallel
Ta s k L i b r a r y. T h e s p e c i fi c t y p e s u s e d h e l p t o s u p p o r t t h e Ta s k i n t e r f a c e .
I'd like to abstract that away, but handling these types will be a bit differ-
ent for each specific task you want to accomplish. In fact, the Parallel Task
Library puts a common interface on top of the many different async pat-
terns that existed in the .NET BCL prior to this version.
private static Task < byte []> startDownload( string url)
{
var tcs = new TaskCompletionSource < byte []>(url);
var wc = new WebClient ();
wc.DownloadDataCompleted += (sender, e) =>
{
if (e.UserState == tcs)
{
if (e.Cancelled)
tcs.TrySetCanceled();
else if (e.Error != null )
tcs.TrySetException(e.Error);
else
tcs.TrySetResult(e.Result);
}
};
wc.DownloadDataAsync( new Uri (url), tcs);
return tcs.Task;
}
This method has a mix of general Task code and code specific to down-
loading data from a URL, so let's go through it very carefully. First, it creates
a TaskCompletionSource object for this task. A TaskCompletionSource
object enables you to separate the creation of a task from its completion.