CODEWITHSUNDEEP
c#winformspaintdoublebuffered
WildCrustacean
WildCrustacean

Reputation: 5966

Disable OnPaintBackground without subclassing Panel?

Is there any way to disable the erasing of a panel without subclassing Panel and overriding OnPaintBackground?

I am trying to achieve a double buffering effect without subclassing Panel. I understand that this may be a weird thing to try to do, but I'd at least like to know if I can or not. The following code sample illustrates this:

public partial class Form1 : Form
{
    private Bitmap m_image;

    public Form1()
    {
        InitializeComponent();

        panel1.Paint += new PaintEventHandler(panel1_Paint);
        panel1.MouseMove += new MouseEventHandler(panel1_MouseMove);

        m_image = new Bitmap(panel1.Width, panel1.Height);
    }

    void panel1_MouseMove(object sender, MouseEventArgs e)
    {
        using (Graphics g = Graphics.FromImage(m_image))
        {
            g.FillEllipse(Brushes.Black, new Rectangle(e.X, e.Y, 10, 10));
        }
        panel1.Invalidate();
    }

    void panel1_Paint(object sender, PaintEventArgs e)
    {
        e.Graphics.DrawImage(m_image, 0, 0);
    }
}

This causes a flickering, presumably because it is erasing the panel at each paint cycle.

Upvotes: 3

Views: 1027

Answers (3)

sjh2014
sjh2014

Reputation: 21

Use reflection to the set the protected DoubleBuffered property:

System.Reflection.PropertyInfo aProp =
         typeof(System.Windows.Forms.Control).GetProperty(
               "DoubleBuffered",
               System.Reflection.BindingFlags.NonPublic |
               System.Reflection.BindingFlags.Instance);

aProp.SetValue(panel1, true, null);

You can also make it more efficient by only invalidating the changed area (this is almost flicker-free even without the double-buffering):

void panel1_MouseMove(object sender, MouseEventArgs e)
{
    Rectangle r = new Rectangle(e.X, e.Y, 10, 10);
    using (Graphics g = Graphics.FromImage(m_image))
    {
        g.FillEllipse(Brushes.Black, r);
    }
    panel1.Invalidate(r);
}

Upvotes: 0

Faruk
Faruk

Reputation: 5831

Just add:

panel1.BackgroundImage = m_image;
//on panel1_Paint() function.

Simple isn't it?

Upvotes: 0

Hans Passant
Hans Passant

Reputation: 941625

You can hack OnPaintBackground() or you can hack WndProc(). Either requires deriving your own class. It's trivial, I just don't see why you'd avoid it. The long distance shot is SetWindowsHookEx() with a WH_CALLWNDPROC hook, too silly really.

Upvotes: 4

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