C# Tasks with Progress Reporting and Return Value Not Reporting Progress

Question

I am trying to run a task in a method and have the method return a value. The reason I am using a task is because I need progress reporting.

This was my original strategy

   private async void CallLongRunningTask()
   {
       string fileName = "TestFN";
       Progress<string> progress = new Progress<string>();
       progress.ProgressChanged += (_, newText) => lbl.Text=newText;
       bool retVal = await Task.Run(() => LongRunningTask(fileName, progress));
       Task.WaitAll();
       // do some other stuff    

   }

   public bool LongRunningTask(string fn, IProgress<string> progress)
   {
       // long running task with progress report
         while (i < 100000000)
        {
            i++;
            if (i % 10000 == 0)
            {
                progress.Report("Processing "+i.ToString());
            }
        }
        return true;
   }

As expected, it blew through the WaitAll, went back to the prior method.

So, I tried this:

got rid of async in the signature

bool retVal = Task.Run(() => LongRunningTask(fileName, progress));
bool retVal = Task.Run(async () => LongRunningTask(fileName, progress));

Both instances, I lost my progress.

So I tried to change my calling program:

   Task.Run(() => CallLongRunningTask()).Wait();

That broke my progress reporting because my progress reporting updates a label on the form so there was some kind of cross thread contamination.

I have read and tried all the relevant similar problems/solutions. This seems so simple but clearly I am missing something. The entire Task() and async thing seems to be on a level of abstraction that I can't seem to grasp no matter how many times I read Stephen Cleary's book.

Any thoughts or ideas or help in any way, would be greatly appreciated!

Answer 1

There is no need for the WaitAll as already mentioned.

private async Task CallLongRunningTask() {
    string fileName = "TestFN";
    Progress<string> progress = new Progress<string>();
    progress.ProgressChanged += (_, newText) => lbl.Text=newText;

    //waiting here for long running task to finish
    bool retVal = await Task.Run(() => LongRunningTask(fileName, progress));

    // do some other stuff after waiting
}

async-await spreads all the way up. You need to await everything including the calling method.

await CallLongRunningTask();

and so on.

If calling from an event handler, that is the only exception where async void can be used

private async void OnSomeHandler(object sender, EventArgs args) {
    //...

    await CallLongRunningTask();

    //...
}

Reference Async/Await - Best Practices in Asynchronous Programming