Skip to main content

Task based Asynchronous pattern, Async & Await and .NET 4.5

One of the key features in .Net 4.5 is to write asynchronous programs much easier. So if I was to write asynchronous programs in .Net 2.0/3.5, I would either follow the event based model or the callback based model. For an example, a synchronous method that does intensive work (say the DoWork()) can be made asynchronous by using the following patterns

1) Implementing the IAsyncResult pattern. in this implementation, 2 methods are exposed for the DoWork() synchronous method, the BeginDoWork() and the EndDoWork() method. The user will call the BeingDoWork() passing in the required parameters and a callback of the delegate type AsyncCallback(IAsyncResult). The BeginDoWork() will spawn a new thread a return control back to the user. Once work is completed in the spawned method, as a last step, it will call the inform the AsyncResult implementation, which in turns will call the EndDoWork() (which is the callback that was passed in to the BeginDoWork()).

2) Implementing the event pattern. Here a BeginDoWork() is implemented but an event is exposed, so that the user can subscribe to that event. The event is invoked by the implementation once the method completes.

.Net 4.0 introduced Tasks, the Task object exposes a continuation feature by which it becomes much more easier to write asynchronous programs. Using this feature, i can assign some work to the task to be executed in a separate thread and then subscribe a continuation to executed once the primary task is done.
       Task<int> task = new Task<int>(() =>
{/*Do intensive work here*/ return 100; });
task.ContinueWith(t => { Console.WriteLine("Result is -- " + t.Result); });
task.Start();
In .Net 4.5 this becomes much easier with the async and await keywords. So the same program can be written like this...
     private async static void InvokeAsync()
{
int val = await WorkAsync();
//This is your continuation code...
Console.WriteLine("Result is -- " + val);
}
The async keyword tells the compiler that this method is an asynchronous method. Once the compiler sees the await method, it subscribes the rest of the code i.e the consoling (in this case) and any other logic as a continuation to the Task object returned by the WorkAsync(), control is given to the main thread.
The WorkAsync method creates a Task object that encapsulate the work that needs to be done and returns it. The await keyword works on the Task object returned by the WorkAsync()

The WorkAsync() method would be like this...
     private static Task<int> WorkAsync()
{
//Do intensive work here...
Task<int> task = new Task<int>(() =>
{/*Do stuff here*/ return 1000; });
task.Start();
return task;
}
Essentially, the await keyword is used at the point where your are doing some work in a separate thread, this is usually through a Task class, the rest of the method after the await call is the callback method. It is interesting to note that you can use multiple await calls inside a async meth
Practically, you would not need to implement the WorkAsync() method, as most of the asynchronous version of APIs also support a XXXAsync() in .net 4.5, that can be directly used with the await keyword. For an example, WCF proxy's would now expose Task based asynchronous method so it can work with the await keyword.

Comments

  1. Any idea on how this works with the asp.net page lifecycle? does the continuation code get executed before PreRenderComplete like all other async tasks?

    ReplyDelete
  2. Nandun, PageAsyncTask is a way to execute asynchronous code an return the processing thread to the asp.net worker pool. You can do almost do the same thing with Tasks, you can create a task that calls the web service and subscribe a continuation method for the callback. You can start the task and then run it on the ThreadPool...this would be similar to what you do with PageAsyncTasks...now in in you Page pre render you can do a Task.WaitAll() to wait untill all the threads complete......

    The other approach is to use the PageAsyncTask and plug in the Task into it. PageAsyncTask works on the IasyncResult pattern, Task implement the same pattern, so you can integrate this easily. Let me know if you need more information

    ReplyDelete

Post a Comment

Popular posts from this blog

Hosting WCF services on IIS or Windows Services?

There came one of those questions from the client whether to use II7 hosting or windows service hosting for WCF services. I tried recollecting a few points and thought of writing it down. WCF applications can be hosted in 2 main ways - In a Windows service - On IIS 7 and above When WCF was first released, IIS 6 did not support hosting WCF applications that support Non-HTTP communication like Net.TCP or Net.MSMQ and developers had to rely on hosting these services on Windows Services. With the release of IIS 7, it was possible to deploy these Non-Http based applications also on IIS 7. Following are the benefits of using IIS 7 to host WCF applications Less development effort Hosting on Windows service, mandates the creating of a Windows service installer project on windows service and writing code to instantiate the service, whereas the service could just be hosted on IIS by creating an application on IIS, no further development is needed, just the service implementa

The maximum nametable character count quota (16384) has been exceeded

Some of our services were growing and the other day it hit the quote, I could not update the service references, nor was I able to run the WCFTest client. An error is diplayed saying " The maximum nametable character count quota (16384) has been exceeded " The problem was with the mex endpoint, where the XML that was sent was too much for the client to handle, this can be fixed by do the following. Just paste the lines below within the configuration section of the devenve.exe.config and the svcutil.exe.config files found at the locations C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE , C:\Program Files\Microsoft SDKs\Windows\v6.0A\bin Restart IIS and you are done. The detailed error that you get is the following : Error: Cannot obtain Metadata from net.tcp://localhost:8731/ Services/SecurityManager/mex If this is a Windows (R) Communication Foundation service to which you have access, please check that you have enabled metadata publishing at the specified address. F

ASP.NEt 2.0 Viewstate and good practices

View state is one of the most important features of ASP.NET because it enables stateful programming over a stateless protocol such as HTTP. Used without strict criteria, though, the view state can easily become a burden for pages. Since view state is packed with the page, it increases size of HTTP response and request. Fortunately the overall size of the __VIEWSTATE hidden field (in ASP.NET 2.0) in most cases is as small as half the size of the corresponding field in ASP.NET 1.x. The content of the _VIEWSTATE field (in client side) represent the state of the page when it was last processed on the server. Although sent to the client, the view state doesn't contain any information that should be consumed by the client. In ASP.NET 1.x, if you disable view state of controls, some of them are unable to raise events hence control become unusable. When we bind data to a grid, server encodes and put whole grid in to view state, which will increase size of view state (proportional to the