Progressbar value not accurate?

Sonhospa

Hi all, I have a strange effect in a little test application which contains a ProgressBar control (set to 'continuous'), together with a list box and a label. All three of them are fed with values from a loop on a different thread (using a delegate) and updated within the same Sub (member of the main form). Just a very simple example from Micrsoft. A funny effect now is that the progressbar seems to be updated with a delay: While the other controls show the value returned from the loop, the progressBar shows an approx. 10% lower value - i.e. when label and listbox show the 50% value, the progressbar stands at 40%, and when the loop is finished (label and listbox 100%) the progressbar still is at 90%. Just to mention it: The code updates the progressbar FIRST with the percentage (with Statusbar.Refresh), and after that updates the other controls. In the application that I plan I didn't want to show the values in labels etc, so the progressbar should show progress more accurate. Compared with the other controls, it also seems to start a little bit delayed... :confused: Are there any known issues with the accuracy of progress bar? Or am I dealing with something like 'race conditions'? Thank you, Mick

Dave Kreskowiak

Without seeing your code, it's impossible to tell you what you did wrong.

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Dave Kreskowiak

Sonhospa

Hi Dave, it's basically the code from the Microsoft sample which I had linked to ('Control.Invoke Method'), just with an additional progressbar. But your answer sounds like it's not normal that the control lacks accuracy... The minor changes I made are in the following method:

Public Sub AddListItemMethod()
    Dim myItem As String
    Dim max As Integer = 50
    Dim i As Integer
    Dim percent As Integer

    For i = 1 To max
        myItem = "MyListItem" + i.ToString()
        percent = CInt(i / max \* 100)
        ProgressBar1.Value = percent
        StatusStrip1.Refresh()
        myListBox.Items.Add(myItem)
        myListBox.Update()
        myLabel.Text = String.Format("{0} entries ({1}%) added.", CStr(i), percent)
        myLabel.Update()
        Thread.Sleep(30)
    Next i
End Sub

I also reduced the Sleep time of the thread - that's why I asked if 'race conditions' might be involved. It seems as if the delay between label/listbox and progressBar is getting bigger with a lower value for 'Thread.Sleep' - very little delay with 800, much bigger delay with 20 or 30.

Dave Kreskowiak

It doesn't work because you're hogging the UI thread with this code. The Thread.Sleep doesn't help. What I mean by "hogging" is that you're not letting the UI thread process any messages from Windows, like WM_PAINT, that tells individual controls to update themselves. For a "quick fix" you can replace the line that says myLabel.Update() with Application.DoEvents(). This will process all pending messages in the application message pump, including drawing messages. This is NOT the correct way to do this! This is a hack to get your example code working! The correct way to do this would be to move the long running code (you simulated this with the Thread.Sleep call) to a background thread and Invoke calls to update the UI controls on the UI thread.

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Dave Kreskowiak