Keep UI thread responsive when running long task in windows forms

Question

I am trying to read a large text file into a TextBox and keep the ui responsive when a file is dragged to the textbox.

Not works as expected, the windows forms is fr

Answer 1

If you need to keep the UI responsive, just give it the time to breath.
Reading one line of text is so fast that you are (a)waiting almost nothing, while updating the UI takes longer. Inserting even a very little delay lets the UI update.

Using Async/Await (SynchronizationContext is captured by await)

public Form1()
{
   InitializeComponent();
   values.DragDrop += new DragEventHandler(this.OnDrop);
   values.DragEnter += new DragEventHandler(this.OnDragEnter);
}

public async void OnDrop(object sender, DragEventArgs e)
{
   string dropped = ((string[])e.Data.GetData(DataFormats.FileDrop))[0];
   if (dropped.Contains(".csv") || dropped.Contains(".txt")) {
      try {
         string line = string.Empty;
         using (var reader = new StreamReader(dropped)) {
            while (reader.Peek() >= 0) {
               line = await reader.ReadLineAsync();
               values.AppendText(line.Replace(";", " ") + "\r\n");
               await Task.Delay(10);
            }
         }
      }
      catch (Exception) {
         //Do something here
      }
   }
}

private void OnDragEnter(object sender, DragEventArgs e)
{
   e.Effect = e.Data.GetDataPresent(DataFormats.FileDrop, false) 
            ? DragDropEffects.Copy 
            : DragDropEffects.None;
}

TPL using Task.Factory
TPL executes Tasks through a TaskScheduler.
A TaskScheduler may be used to queue tasks to a SynchronizationContext.

TaskScheduler _Scheduler = TaskScheduler.FromCurrentSynchronizationContext();

//No async here
public void OnDrop(object sender, DragEventArgs e)
{
   string dropped = ((string[])e.Data.GetData(DataFormats.FileDrop))[0];
   if (dropped.Contains(".csv") || dropped.Contains(".txt")) {
      Task.Factory.StartNew(() => {
         string line = string.Empty;
         int x = 0;
         try {
            using (var reader = new StreamReader(dropped)) {
               while (reader.Peek() >= 0) {
                  line += (reader.ReadLine().Replace(";", " ")) + "\r\n";
                  ++x;
                  //Update the UI after reading 20 lines
                  if (x >= 20) {
                     //Update the UI or report progress 
                     Task UpdateUI = Task.Factory.StartNew(() => {
                        try {
                           values.AppendText(line);
                        }
                        catch (Exception) {
                           //An exception is raised if the form is closed
                        }
                     }, CancellationToken.None, TaskCreationOptions.PreferFairness, _Scheduler);
                     UpdateUI.Wait();
                     x = 0;
                  }
               }
            }
         }
         catch (Exception) {
            //Do something here
         }
      });
   }
}