c# - Await multiple async Task while setting max running task at a time

Question

So I just started to try and understand async, Task, lambda and so on, and I am unable to get it to work like I want. With the code below I want for it to lock btnDoWebRequest, do a unknow number of WebRequests as a Task and once all the Task are done unlock btnDoWebRequest. However I only want a max of 3 or whatever number I set of Tasks running at one time, which I got partly from Have a set of Tasks with only X running at a time.

But after trying and modifying my code in multiple ways, it will always immediately jump back and reenabled btnDoWebRequest. Of course VS is warning me about needing awaits, currently at ".ContinueWith((task)" and at the async in "await Task.WhenAll(requestInfoList .Select(async i =>", but can't seem to work where or how to put in the needed awaits. Of course as I'm still learning there is a good chance I am going at this all wrong and the whole thing needs to be reworked. So any help would be greatly appreciated.

Thanks

    private SemaphoreSlim maxThread = new SemaphoreSlim(3);
    private void btnDoWebRequest_Click(object sender, EventArgs e)
    {
        btnDoWebRequest.Enabled = false;
        Task.Factory.StartNew(async () => await DoWebRequest()).Wait();
        btnDoWebRequest.Enabled = true;
    }

    private async Task DoWebRequest()
    {
        List<requestInfo> requestInfoList = new List<requestInfo>();
        for (int i = 0; dataRequestInfo.RowCount - 1 > i; i++)
        {
            requestInfoList.Add((requestInfo)dataRequestInfo.Rows[i].Tag);
        }
        await Task.WhenAll(requestInfoList .Select(async i => 
        {
            maxThread.Wait();
            Task.Factory.StartNew(() =>
            {
                var task = Global.webRequestWork(i);
            }, TaskCreationOptions.LongRunning).ContinueWith((task) => maxThread.Release());
        }));
    }

Answer 1

First, don't use Task.Factory.StartNew by default. In fact, this should be avoided in async code. If you need to execute code on a background thread, then use Task.Run.

In your case, there's no need to use Task.Run (or Task.Factory.StartNew).

Start at the lowest level and work your way up. You already have an asynchronous web-requesting method, which I'll rename to WebRequestAsync to follow the Task-based Asynchronous Programming naming guidelines.

Next, throttle it by using the asynchronous APIs on SemaphoreSlim:

await maxThread.WaitAsync();
try
{
  await Global.WebRequestWorkAsync(i);
}
finally
{
  maxThread.Release();
}

Do that for each request info (note that no background thread is required):

private async Task DoWebRequestsAsync()
{
  List<requestInfo> requestInfoList = new List<requestInfo>();
  for (int i = 0; dataRequestInfo.RowCount - 1 > i; i++)
  {
    requestInfoList.Add((requestInfo)dataRequestInfo.Rows[i].Tag);
  }
  await Task.WhenAll(requestInfoList.Select(async i => 
  {
    await maxThread.WaitAsync();
    try
    {
      await Global.WebRequestWorkAsync(i);
    }
    finally
    {
      maxThread.Release();
    }
  }));
}

Finally, call this from your UI (again, no background thread is required):

private async void btnDoWebRequest_Click(object sender, EventArgs e)
{
  btnDoWebRequest.Enabled = false;
  await DoWebRequestsAsync();
  btnDoWebRequest.Enabled = true;
}

In summary, only use Task.Run when you need to; do not use Task.Factory.StartNew, and do not use Wait (use await instead). I have an async intro on my blog with more information.