Question

A common task I have to do for a site I work on is the following:

• Download data from some third-party API
• Process the data in some fashion
• Display the results on the page

I was initially using WebClient.DownloadStringAsync and doing my processing on the result. However I was finding that DownloadStringAsync was not respecting the AsyncTimeout parameter, which I sort of expected once I did a little reading about how this works.

I ended up adapting the code from the example on how to use PageAsyncTask to use DownloadString() there – please note, it’s the synchronous version. This is probably okay, because the task is now asynchronous. The tasks now properly time out and I can get the data by PreRender() time – and I can easily genericize this and put it on any page I need this functionality.

However I’m just worried it’s not ‘clean’. The page isn’t notified when the task is done like the DownloadStringAsync method would do – I just have to scoop the results (stored in a field in the class) up at the end in my PreRender event.

Is there any way to get the Webclient’s Async methods to work with RegisterPageTask, or is a helper class the best I can do?

Notes: No MVC – this is vanilla asp.net 4.0.

Answer 1

If you want an event handler on your Page called when the async task completes, you need only hook one up. To expand on the MSDN “how to” article you linked:

1. Modify the “SlowTask” class to include an event, like – public event EventHandler Finished;
2. Call that EventHandler in the “OnEnd” method, like – if (Finished != null)
{
Finished(this, EventArgs.Empty);
}
3. Register an event handler in your page for SlowTask.Finished, like – mytask.Finished += new EventHandler(mytask_Finished);

Regarding ExecuteRegisteredAsyncTasks() being a blocking call, that’s based only on my experience. It’s not documented explicitly as such in the MSDN – http://msdn.microsoft.com/en-us/library/system.web.ui.page.executeregisteredasynctasks.aspx

That said, it wouldn’t be all that practical for it be anything BUT a blocking call, given that it doesn’t return a WaitHandle or similar. If it didn’t block the pipeline, the Page would render and be returned to the client before the async task(s) completed, making it a little difficult to get the results of the task back to the client.