Asynchronous Pages

ASP.NET pages are served by an HTTP handler like an instance of the Page class. Each request takes up a thread in the ASP.NET thread pool and releases it only when the request completes. What if a frequently requested page starts an external and particularly lengthy task? The risk is that the ASP.NET process is idle but has no free threads in the pool to serve incoming requests for other pages. This happens mostly because HTTP handlers, including page classes, work synchronously. To alleviate this issue, ASP.NET has supported asynchronous handlers since version 1.0 through the IHTTPAsyncHandler interface. Starting with ASP.NET 2.0, creating asynchronous pages was made easier thanks to specific support from the framework.

Two aspects characterize an asynchronous ASP.NET page: a tailor-made attribute on the @Page directive, and one or more tasks registered for asynchronous execution. The asynchronous task can be registered in either of two ways. You can define a Begin/End pair of asynchronous handlers for the PreRenderComplete event or create a PageAsyncTask object to represent an asynchronous task. This is generally done in the Page_Load event, but any time is fine provided that it happens before the PreRender event fires.

In both cases, the asynchronous task is started automatically when the page has progressed to a well-known point. Let’s dig out more details.

Note

An ASP.NET asynchronous page is still a class that derives from Page. There are no special base classes to inherit for building asynchronous pages.

The Async Attribute

The new Async attribute on the @Page directive accepts a Boolean value to enable or disable asynchronous processing. The default value is false.

<%@ Page Async="true" ... %>

The Async attribute is merely a message for the page parser. When used, the page parser implements the IHttpAsyncHandler interface in the dynamically generated class for the .aspx resource. The Async attribute enables the page to register asynchronous handlers for the PreRenderComplete event. No additional code is executed at run time as a result of the attribute.

Let’s consider a request for a TestAsync.aspx page marked with the Async directive attribute. The dynamically created class, named ASP.TestAsync_aspx, is declared as follows:

public class TestAsync_aspx : TestAsync, IHttpHandler, IHttpAsyncHandler

{

...

}

TestAsync is the code file class and inherits from Page or a class that in turn inherits from Page. IHttpAsyncHandler is the canonical interface that has been used for serving resources asynchronously since ASP.NET 1.0.

The AddOnPreRenderCompleteAsync Method

The AddOnPreRenderCompleteAsync method adds an asynchronous event handler for the page’s PreRenderComplete event. An asynchronous event handler consists of a Begin/End pair of event handler methods, as shown here:

AddOnPreRenderCompleteAsync (

new BeginEventHandler(BeginTask),

new EndEventHandler(EndTask)

);

The call can be simplified as follows:

AddOnPreRenderCompleteAsync(BeginTask, EndTask);

BeginEventHandler and EndEventHandler are delegates defined as follows:

IAsyncResult BeginEventHandler(

object sender,

EventArgs e,

AsyncCallback cb,

object state)

void EndEventHandler(

IAsyncResult ar)

In the code file, you place a call to AddOnPreRenderCompleteAsync as soon as you can, and always earlier than the PreRender event can occur. A good place is usually