Question

I am creating a quick’n’dirty utility that will enable editing of data read sequentially from a set of files. Here’s a very simplified explanation of what will happen in a single iteration of a loop:

• Read a line from the input stream
• Parse it and use the parsed results to populate form controls
• Allow user editing and await a confirmation button click
• Retrieve the updated form control values and write to the output stream

What I can’t figure out is how to integrate the processing loop with the event-driven UI. For example, how do I suspend operation of the loop while waiting for user input.

I understand that this is possible by launching the loop operation on its own thread and writing code to manage its interaction with the UI thread, but I am wondering if there is a simpler approach that works out of the box.

Thanks for any ideas you may be able to offer.
Tim

Answer 1

I can think of 2 approaches to do that:

• The easiest is probably to use a modal dialog: when your worker thread needs input from the user, display a dialog, which is a blocking operation. Something like that:

  // Worker thread loop
  while(...)
  {
      ...
  
      // prompt user for data (invoke synchronously on UI thread)
      UserData data = (UserData)window.Invoke(PromptUserData);
  
      ...
  }
  
  ...
  
  
  
  UserData PromptUserData()
  {
      UserInputDialog dlg = new UserInputDialog();
      dlg.ShowDialog();
      return dlg.UserData;
  }
  

• The other option, if you don’t want to use a modal dialog, is to use a wait handle to synchronize the worker thread and the UI:

  private readonly AutoResetWaitHandle _userInputWaitHandle = new AutoResetWaitHandle(false);

  ...
  
  // Worker thread loop
  while(...)
  {
      ...
  
      // Setup the UI to allow user input
      window.Invoke(SetupUIForInput);
      // Wait for the input to be validated
      _userInputWaitHandle.WaitOne();
      ...
  }
  
  
  ...
  
  void SetupUIForInput()
  {
      // Enable the UI
      inputForm.Enabled = true;
  
      // Whatever else you need to do...
      ...
  }
  
  
  void buttonOK_Click(object sender, EventArgs e)
  {
      // Signal the worker thread to continue
      _userInputWaitHandle.Set();
  }