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By Root 5763 0

class has a fixed signature, and it can hold references only to methods that match that signature. A delegate is equivalent to a type-safe function pointer or a callback. As a result, an AsyncCallback object is just the code that executes when the asynchronous handler has completed its job.

The AsyncCallback delegate has the following signature:

public delegate void AsyncCallback(IAsyncResult ar);

It uses the IAsyncResult interface to obtain the status of the asynchronous operation. To illustrate the plumbing of asynchronous handlers, I’ll show you what the HTTP runtime does when it deals with asynchronous handlers. The HTTP runtime invokes the BeginProcessRequest method as illustrated here:

// Sets an internal member of the HttpContext class with

// the current instance of the asynchronous handler

context.AsyncAppHandler = asyncHandler;

// Invokes the BeginProcessRequest method on the asynchronous HTTP handler

asyncHandler.BeginProcessRequest(context, OnCompletionCallback, context);

The context argument is the current instance of the HttpContext class and represents the context of the request. A reference to the HTTP context is also passed as the custom data sent to the handler to process the request. The extraData parameter in the BeginProcessRequest signature is used to represent the status of the asynchronous operation. The BeginProcessRequest method returns an object of type HttpAsyncResult—a class that implements the IAsyncResult interface. The IAsyncResult interface contains a property named AsyncState that is set with the extraData value—in this case, the HTTP context.

The OnCompletionCallback method is an internal method. It gets automatically triggered when the asynchronous processing of the request terminates. The following listing illustrates the pseudocode of the HttpRuntime private method:

// The method must have the signature of an AsyncCallback delegate

private void OnHandlerCompletion(IAsyncResult ar)

{

// The ar parameter is an instance of HttpAsyncResult

HttpContext context = (HttpContext) ar.AsyncState;

// Retrieves the instance of the asynchronous HTTP handler

// and completes the request

IHttpAsyncHandler asyncHandler = context.AsyncAppHandler;

asyncHandler.EndProcessRequest(ar);

// Finalizes the request as usual

...

}

The completion handler retrieves the HTTP context of the request through the AsyncState property of the IAsyncResult object it gets from the system. As mentioned, the actual object passed is an instance of the HttpAsyncResult class—in any case, it is the return value of the BeginProcessRequest method. The completion routine extracts the reference to the asynchronous handler from the context and issues a call to the EndProcessRequest method:

void EndProcessRequest(IAsyncResult result);

The EndProcessRequest method takes the IAsyncResult object returned by the call to BeginProcessRequest. As implemented in the HttpApplication class, the EndProcessRequest method does nothing special and is limited to throwing an exception if an error occurred.

Implementing Asynchronous Handlers

Asynchronous handlers essentially serve one particular scenario—a scenario in which the generation of the markup is subject to lengthy operations, such as time-consuming database stored procedures or calls to Web services. In these situations, the ASP.NET thread in charge of the request is stuck waiting for the operation to complete. Because threads are valuable resources, lengthy tasks that keep threads occupied for too long are potentially the perfect scalability killer. However, asynchronous handlers are here to help.

The idea is that the request begins on a thread-pool thread, but that thread is released as soon as the operation begins. In BeginProcessRequest, you typically create your own thread and start the lengthy operation. BeginProcessRequest doesn’t wait for the operation to complete; therefore, the thread is returned to the pool immediately.

There are a lot of tricky details that this bird’s-eye description just omitted. In the first place, you should strive