C# : Blocking a function call until condition met.

Question

I am developing a C# Winforms application, part of the application will be uploading files to a webserver using AsyncUpload (using it,due to the need to use a porgress callback) , In the C# program

i got a simple for loop that calls The Uploading function

 for(int i=0;i < 10 ; i++)
{
  Uploadfun();
}

And the fun does some magic:

Uploadfun()
  { 
  // Logic comes here

   // webClient.UploadFileAsync runs a 2nd thread to perform upload .. 
   webClient.UploadFileAsync(uri, "PUT", fileNameOnHD);  

 }

And a callback that gets called when the Async upload is done

Upload_Completed_callback()
{
  //Callback event
}

Edit

The logic sequence:

1. Fun gets called (from loop)
2. Fun logic is executed and done..
3. Goes back to for loop
4. Callback will be called eventually, when UploadFileAsync (which is running some logic in another thread) will end

The problem is on the 3rd point, when the execution moves back to the for loop, i need to block the loop from continuing until the callback get called.

Answer 1

In C# methods block by default, so you shouldn't need to do anything. I'm assuming that for some reason you are calling a non-blocking method that starts a background task / thread / whatever and gives you a callback when it's done. You want to call this asynchonous method in a synchronous manner.

You can call fun from inside the callback. Something along these lines (pseudo-code):

int n;

callFunTenTimes()
{
    n = 0;
    fun(n);
}

callback()
{
    ++n;
    if (n < 10)
       fun(n);
    else
       print("done");
}

This is similar to continuation passing style.

An advantage to this method is that you can also make your method asynchronous without adding any extra threads, locks, or extra logic - you just supply a callback function which your client can subscribe to. It works well in an event-driven environment.

Answer 2

So if I understand correctly, you want to call UploadFileAsync then block until the async call has hit your callback. If so, I'd use AutoResetEvent i.e

private readonly AutoResetEvent _signal = new AutoResetEvent(false); 

fun()
  { 
  // Logic comes here

   // runs a 2nd thread to perform upload .. calling "callback()" when done
   webClient.UploadFileAsync(uri, "PUT", fileNameOnHD);  

   _signal.WaitOne();   // wait for the async call to complete and hit the callback     
 }



callback()
 {
   //Callback event
   _signal.Set(); // signal that the async upload completed
 }

Using AutoResetEvent means that the state gets automatically reset after Set has been called and a waiting thread receives the signal via WaitOne

Answer 3

The problem is here:

for(int i=0;i < 10 ; i++)
{
  fun(); <-- if we block until this function finishes here, we stop the UI thread
}

What you're doing is sequential. And if you can't afford to block the UI thread, move the loop off the UI thread:

volatile downloadComplete;

void DownloadUpdates()
{
    ThreadPool.QueueUserWorkItem(state =>
        for(int i = 0; i < 10; i++)
        {
            downloadComplete = false;
            webClient.UploadFileAsync(uri, "PUT", fileNameOnHD);
            while(!downloadComplete) { Thread.Sleep(1); }
        });
}

Upload_Completed_callback()
{
    downloadComplete = true;
}

Now you can block the execution of the loop without halting your UI thread, and you also get the benefit of progress indicators from the webclient class.

Answer 4

Zebrabox does have it right using a WaitHandle. While Juliet's solution does work, the thread performing the spin-wait will consume a significant amount of processor in proportion to the WaitHandle, which would essentially be sitting idle.

Answer 5

Hi Medi: How did you get your probelm solved? I have the same problem, but couln't make it work. In my async function, it also does some UI updates. I think that is the problem. If I remove the UI update, it works. From my reading, you also have UI updates in your async function, right?