Windows Form Paints Repeatedly In XP, But Not In Vista

Question

I am trying to solve an issue in my application. I am developing the application in Vista and it works fine there, but when I take it to XP, the form becomes sluggish and unresponsive. When I watch the windows messages using breakpoints, I find that in XP the form is repeatedly painted about once every second (even though it does not really need to); however, the same test on Vista does not show this repetitive painting.

Any ideas as to what might be causing this?

Answer 1

Thanks for everyone's input. The problem has been solved now, with help from Subversion! The sluggishness of the window was a relatively new problem, so I decided to look back in time in my source code using Subversion and discovered that I recently applied the WS_EX_COMPOSITED style flag to the form to reduce flickering in Vista.

protected override CreateParams CreateParams
{
    get
    {
        CreateParams result = base.CreateParams;
        result.ExStyle |= 0x02000000; // WS_EX_COMPOSITED
        return result;
    }
}

When I commented it out, the form was responsive in XP again and the repetitive WM_PAINT messages were gone. So, the solution was to only apply WS_EX_COMPOSITED in Vista or later.

protected override CreateParams CreateParams
{
    get
    {
        CreateParams result = base.CreateParams;
        if (Environment.OSVersion.Platform == PlatformID.Win32NT 
            && Environment.OSVersion.Version.Major >= 6)
        {
            result.ExStyle |= 0x02000000; // WS_EX_COMPOSITED
        }

        return result;
    }
}

Now everything works great!