Question

private void UserControl1_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.DrawEllipse(Pens.Black, new Rectangle(-200, -500, this.Width + 400, this.Height + 420));
}

Paste the above code into a usercontrol. Drop the usercontrol onto a form and anchor it to all 4 points.

In the designer (Under Visual Studio 2010) it renders perfectly (even as you resize). Run it and try and resize the form and the ellipse becomes skewed.

Here are two examples both after resizes the first while running, the second in the designer.

Obviously the behavior in the designer can’t always be assumed to be the same (Although it would be nice) but my understanding is that the code above is completely legal. Am I wrong?

Answer 1

Stecya has posted the “fix”, but omitted an explanation detailing why that works or why you’re seeing a discrepancy between the behavior in the designer and in the running application. In my mind, that makes the answer only minimally useful, so I thought I’d weigh in with an attempt at an explanation.

You already know that calling the Invalidate method is the solution. What this does is tell Windows that the entire surface area of the control needs to be redrawn the next time the window gets painted. Simple enough, but why don’t you have to do this in the designer?

The answer lies in the fact that you’re running the program with the Windows Aero theme enabled. Aero uses an entirely new window manager called the Desktop Window Manager (or DWM for short) based on composition. Each window is double-buffered, meaning that its graphics are rendered off-screen into a temporary bitmap, and only then blitted to the screen. This allows all sorts of cool effects and fancy transitions that users seem to like nowadays.

But, of course, it means that the sections that have already been drawn aren’t erased and redrawn unless you explicitly instruct Windows that it needs to do so. That’s not a problem for the form inside the designer, because there, Aero’s DWM composition is not enabled. When the window gets resized, it is automatically redrawn, and your swoosh looks smooth and correct.

Outside of the designer, with Aero composition enabled, only the newly-exposed portions of your control are redrawn (the rest are still there in the buffer), so the shape is wrong. Part of the old shape is still there, and part of the new shape was just drawn in. Calling Invalidate tells Windows “the graphic surface of this control has changed; forget everything you thought you knew about it and redraw it from scratch next time”. So Windows dutifully obeys, discarding that portion from its off-screen buffer and redraws it in from scratch, producing a correctly-rendered, nicely-smoothed path.

You can effect the same change in another somewhat more elegant way: Tell the control that it needs to redraw itself every time that it is resized. Insert the following code into the control’s constructor method:

public MyUserControl()
{
    // Force the control to redraw itself each time it is resized
    this.SetStyle(ControlStyles.ResizeRedraw);
}