Double Buffered Control Base

What it is for

A control that draws a lot in OnPaint, or hosts many child controls, tends to flash as each piece reaches the screen. This class switches on the two Windows mechanisms that stop that, so a derived control gets them without having to remember either. It adds no drawing, properties or events of its own.

The two mechanisms

• OptimizedDoubleBuffer, set in the constructor: the control's own painting goes to an off-screen buffer and the finished picture is copied to the screen in one step.
• The WS_EX_COMPOSITED extended window style, added in CreateParams: the window and all its descendants are painted bottom-to-top off-screen, so a container full of children does not flash as each child paints in turn. Double buffering alone cannot do that, because it covers only one control's own drawing.

using System.Drawing;
using System.Windows.Forms;
using Binarymission.WinForms.Controls.BaseControls;

// A progress gauge that redraws many times a second without flicker.
public class UploadGauge : DoubleBufferedControlBase
{
    private int percentComplete;

    public int PercentComplete
    {
        get { return percentComplete; }
        set { percentComplete = value; Invalidate(); }
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        e.Graphics.Clear(BackColor);
        int width = ClientSize.Width * percentComplete / 100;
        e.Graphics.FillRectangle(Brushes.SeaGreen, 0, 0, width, ClientSize.Height);
        TextRenderer.DrawText(e.Graphics, percentComplete + "%", Font, ClientRectangle, ForeColor);
    }
}