WinForms: force GUI update from UI Thread

Question

In WinForms, how do I force an immediate UI update from UI thread?

What I'm doing is roughly:

label.Text = "Please Wait..."
try 
{
    SomewhatLongRunningOperation(); 
}
catch(Exception e)
{
    label.Text = "Error: " + e.Message;
    return;
}
label.Text = "Success!";

Label text does not get set to "Please Wait..." before the operation.

I solved this using another thread for the operation, but it gets hairy and I'd like to simplify the code.

Answer 1

Call Application.DoEvents() after setting the label, but you should do all the work in a separate thread instead, so the user may close the window.

Answer 2

Call label.Invalidate and then label.Update() - usually the update only happens after you exit the current function but calling Update forces it to update at that specific place in code. From MSDN:

The Invalidate method governs what gets painted or repainted. The Update method governs when the painting or repainting occurs. If you use the Invalidate and Update methods together rather than calling Refresh, what gets repainted depends on which overload of Invalidate you use. The Update method just forces the control to be painted immediately, but the Invalidate method governs what gets painted when you call the Update method.

Answer 3

It's very tempting to want to "fix" this and force a UI update, but the best fix is to do this on a background thread and not tie up the UI thread, so that it can still respond to events.

Answer 4

Try calling label.Invalidate()

http://msdn.microsoft.com/en-us/library/system.windows.forms.control.invalidate%28VS.80%29.aspx

Answer 5

Update() - Causes the control to redraw the invalidate regions within its client are.

label1.Text = "Started...";
label1.Update();
for (int i = 1; i <= 100000; i++){
  for (int j = 1; j <= 600; j++){}
}
label1.Text = "Stopped.";

Answer 6

you can try this

using System.Windows.Forms; // u need this to include.

MethodInvoker updateIt = delegate
                {
                    this.label1.Text = "Started...";
                };
this.label1.BeginInvoke(updateIt);

See if it works.

Answer 7

At first I wondered why the OP hadn't already marked one of the responses as the answer, but after trying it myself and still have it not work, I dug a little deeper and found there's much more to this issue then I'd first supposed.

A better understanding can be gained by reading from a similar question: http://stackoverflow.com/questions/2341731/why-wont-control-update-refresh-mid-process

Lastly, for the record, I was able to get my label to update by doing the following:

private void SetStatus(string status) {
            lblStatus.Text = status;
            lblStatus.Invalidate();
            lblStatus.Update();
            lblStatus.Refresh();
            Application.DoEvents();
        }

Though from what I understand this is far from an elegant and correct approach to doing it. It's a hack that may or may not work depending upon how busy the thread is.