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I have a page which performs a long processing (write some pdf somewhere) so I have the following:

My ArchiveDoc.aspx page: (the page has a Label (called progress) and Async="true" in page directive)

delegate void archiveFuncDelegate(int idTurnover, string serverMapPath);
delegate void UpdateUIDelegate(string Text);

protected void Page_Load(object sender, EventArgs e)
{
    Page.AddOnPreRenderCompleteAsync(BeginTask, EndTask);
    this.progress.Text = "0";
}

private void UpdateUI(string Text)
{
    this.progress.Text = Text;
}

private IAsyncResult BeginTask(object sender, EventArgs e, AsyncCallback cb, object state)
{

    // invoke
    archiveFuncDelegate del = asyncArchivageFacture;
    IAsyncResult result = del.BeginInvoke(cb,null); 
    return result;
}

private void EndTask(IAsyncResult ar)
{
    if (!ar.IsCompleted)
    {
        progress.Text = "Error";
    }
    else
    {
        progress.Text = "OK";
    }
}

#region Archive (thread)

// long processing task (exemple)
public void asyncArchivageFacture()
{
    // exemple
    int total = 101;
    int progress = 0;

    for(int i=0; i<total; i++)
    {
        progress = i*100 / total;
        updateUi.Invoke(progress + "%");
        Thread.Sleep(2000);
     }

}

#endregion

When I call my page, the thread is called (asyncArchivageFacture() as well as the UpdateUI()) but the web page never gets updated : I got the loading symbol in the browser with a blank page, and once the thread is completed, the page finally display 'OK'. It doesn't display the progress (10%.. 20%..)

Any idea what's wrong? Thanks!

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  • 1
    HTTP is stateless. You would need an ajax component to be polling for the progress. Or, look into SignalR. Commented Mar 15, 2013 at 10:35

2 Answers 2

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This is because the response(html) is sent to the client only after the EndTask method is called. Whatever changes you are making to the progress.Text is on the server side where you are changing only a server side variable.

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5 Comments

so, you are saying I should start another thread from the asyncArchivageFacture() so that asyncArchivageFacture() returns instantly?
As David Tryon has pointed out, since http is stateless, you cant keep sending the result back to the client. The client(browser/mobile) requests this page and expects a response(html in this case). Till that happens(or error occurs), the Tcp connection is kept alive by the client. You can show progress in the client using Ajax calls by repeating asking the server about the progress. In short, the answer to your question is No. You cant achieve it by starting another thread. You should use repeated Ajax calls
I don't understand, if the page wait for the EndTask() (i.e the thread) to complete, then where's the async aspect ? it's blocking and not asynchronous at all then..
This Async is for thread reuse and does not release connections. i.e. When an async request comes to the web application, a thread is allotted to serve the request, this thread need not be the same as the thread which executes the EndTask method. Till the EndTask, the tcp connection is open but the thread which started processing the request is allotted to a different task(probably a short lived synchronous task which will execute faster) while this long async task is probably waiting on IO to complete(again it should be ensured that you have Non-blocking calls throughout the pipeline).
In summary, for a synchronous request, a particular thread is allotted to serve the entire request from start to end. But for an asynchronous request, the method can return immediately thereby releasing the thread for other tasks. Also, make sure to understand the distinction between the terminologies such as synchronous/asynchronous and blocking/non-blocking.
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I ended up using a simple

Thread t = new Thread(..)

without delaring the page async=true.

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