Question

Why does the ProgressBar update in theoretically blocked UI thread?

In simple app I have a ProgressBar and a Label. I run a time-consuming method in UI thread which tries to update the ProgressBar and the label. This is not supposed to work, because of blocked UI thread. But the ProgressBar is updating!

Until I do anything on the form and it freezes, the ProgressBar updates (the label does not).

Why is that?

Example code (put a Button, a ProgressBar and a Label on form):

private void button1_Click(object sender, EventArgs e)
{
    while (true)
    {
        progressBar1.Value += 1;
        label1.Text += "1";
        Thread.Sleep(100);
    }
}

ProgressBar is updating, Label is not. My question is not how to make label update aswell, but why ProgressBar is updating. I DO know about Threads, DoEvents, async/await and that is not the answer.

Answer 1

I think it’s hard to answer this completely without disassembling a bit of Windows, which is too much work for me right now.

But basically, when you set .Value on an WinForms ProgressBar control, it does little more than call SendMessage with message 1026 (PBM_SETPOS), which tells the Windows progress bar to set its position.

I would conclude that the Windows progress bar redraws itself synchronously in response to PBM_SETPOS as well as in response to WM_PAINT. Or perhaps it’s running a timer on another thread in order to do the fancy glare animation, and that’s able to redraw the control without waiting for a paint message.

Either way, it’s just Windows internals you’re seeing – and drawing stuff outside WM_PAINT handlers is not that an unusual technique, even though it’s not the textbook way of doing things.

Actually, looking at the docs for PBM_SETPOS ( http://msdn.microsoft.com/en-us/library/bb760844(v=vs.85).aspx ) it’s documented as causing a redraw – I guess this is done deliberately to help the lazy/inexperienced to get blocking progress bar updates to work without all usual hassle of doing it properly.