Question

I use C#, VisualStudio 2010 and I created a custom UserControl for a Windows Forms application. They don’t have much behavior apart from displaying themselves and allowing themselves to be dragged elsewhere. However they are circular in shape and I cannot display them correctly when they overlap on the corners.

Here is my code for painting them on the screen:

  public void Circle_Paint(object sender, PaintEventArgs e)
  {
     var g = e.Graphics;

     g.FillEllipse(brushForOuterCircle, 0, 0, Width, Height);
     g.FillEllipse(brushForInnerCircle, lineWidth, lineWidth, Width - 2*lineWidth, Height - 2*lineWidth);

     if(!textLocation.HasValue)
     {
        SizeF m = g.MeasureString(text, textFont);
        textLocation = new PointF((float)((Width - m.Width)/2.0), (float)((Height - m.Height)/2.0));
     }
     g.DrawString(text, textFont, brushForText, textLocation.Value);
  }

Here is an example of incorrect display, the southeast part of the AB circle does not display because CD overrides that area.

How should I prevent that, is there a way to tell the UserControl “you are transparent by default; any portion I don’t draw onto should remain so” ?

Answer 1

Try to:

setup on your user control first WS_EX_COMPOSITED extended style, by overriding

protected override CreateParams CreateParams
{
   get
   {
     CreateParams cp = base.CreateParams;
     cp.ExStyle |= 0x00000020; // add this
     return cp;
   }
}

after do not paint anything in background by

protected override void OnPaintBackground(PaintEventArgs e)
{
   // leave this empty 
}

and finally in Paint draw your shape.

Should work.