Parallelism in .NET – Part 20, Using Task with Existing APIs
- by Reed
Although the Task class provides a huge amount of flexibility for handling asynchronous actions, the .NET Framework still contains a large number of APIs that are based on the previous asynchronous programming model. While Task and Task<T> provide a much nicer syntax as well as extending the flexibility, allowing features such as continuations based on multiple tasks, the existing APIs don’t directly support this workflow.
There is a method in the TaskFactory class which can be used to adapt the existing APIs to the new Task class: TaskFactory.FromAsync. This method provides a way to convert from the BeginOperation/EndOperation method pair syntax common through .NET Framework directly to a Task<T> containing the results of the operation in the task’s Result parameter.
While this method does exist, it unfortunately comes at a cost – the method overloads are far from simple to decipher, and the resulting code is not always as easily understood as newer code based directly on the Task class. For example, a single call to handle WebRequest.BeginGetResponse/EndGetReponse, one of the easiest “pairs” of methods to use, looks like the following:
var task = Task.Factory.FromAsync<WebResponse>(
request.BeginGetResponse,
request.EndGetResponse,
null);
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }
The compiler is unfortunately unable to infer the correct type, and, as a result, the WebReponse must be explicitly mentioned in the method call. As a result, I typically recommend wrapping this into an extension method to ease use. For example, I would place the above in an extension method like:
public static class WebRequestExtensions
{
public static Task<WebResponse> GetReponseAsync(this WebRequest request)
{
return Task.Factory.FromAsync<WebResponse>(
request.BeginGetResponse,
request.EndGetResponse,
null);
}
}
This dramatically simplifies usage. For example, if we wanted to asynchronously check to see if this blog supported XHTML 1.0, and report that in a text box to the user, we could do:
var webRequest = WebRequest.Create("http://www.reedcopsey.com");
webRequest.GetReponseAsync().ContinueWith(t =>
{
using (var sr = new StreamReader(t.Result.GetResponseStream()))
{
string str = sr.ReadLine();;
this.textBox1.Text = string.Format("Page at {0} supports XHTML 1.0: {1}",
t.Result.ResponseUri,
str.Contains("XHTML 1.0"));
}
}, TaskScheduler.FromCurrentSynchronizationContext());
By using a continuation with a TaskScheduler based on the current synchronization context, we can keep this request asynchronous, check based on the first line of the response string, and report the results back on our UI directly.
