SLaks SLaks - 1 month ago 14
C# Question

Double buffered ListBox

I have a CheckedListBox (WinForms) control (which inherits from ListBox; googling shows that the problem is with ListBox) that is anchored to all four sides of its form. When the form is resized, the ListBox has an ugly flicker. I tried inheriting CheckedListBox and setting

DoubleBuffered
to
true
in the ctor (this technique works with other controls, including ListView and DataGridView), but it had no effect.

I tried adding the
WS_EX_COMPOSITED
style to
CreateParams
, and this helped, but makes the form resize mush more slowly.

Is there any other way to prevent this flickering?

Answer

I was having similar issues albeit with an owner drawn listbox. My solution was to use BufferedGraphics objects. Your mileage may vary with this solution if your list isn't owner drawn, but maybe it will give you some inspiration.

I found that TextRenderer had difficulties rendering to the correct location unless I suppled TextFormatFlags.PreserveGraphicsTranslateTransform. The alternative to this was to use P/Invoke to call BitBlt to directly copy pixels between the graphics contexts. I chose this as the lesser of two evils.

/// <summary>
/// This class is a double-buffered ListBox for owner drawing.
/// The double-buffering is accomplished by creating a custom,
/// off-screen buffer during painting.
/// </summary>
public sealed class DoubleBufferedListBox : ListBox
{
    #region Method Overrides
    /// <summary>
    /// Override OnTemplateListDrawItem to supply an off-screen buffer to event
    /// handlers.
    /// </summary>
    protected override void OnDrawItem(DrawItemEventArgs e)
    {
        BufferedGraphicsContext currentContext = BufferedGraphicsManager.Current;

        Rectangle newBounds = new Rectangle(0, 0, e.Bounds.Width, e.Bounds.Height);
        using (BufferedGraphics bufferedGraphics = currentContext.Allocate(e.Graphics, newBounds))
        {
            DrawItemEventArgs newArgs = new DrawItemEventArgs(
                bufferedGraphics.Graphics, e.Font, newBounds, e.Index, e.State, e.ForeColor, e.BackColor);

            // Supply the real OnTemplateListDrawItem with the off-screen graphics context
            base.OnDrawItem(newArgs);

            // Wrapper around BitBlt
            GDI.CopyGraphics(e.Graphics, e.Bounds, bufferedGraphics.Graphics, new Point(0, 0));
        }
    }
    #endregion
}