Strange GraphicsPath.AddArc() behaviour

Question

I experimented with GraphicPaths and find that funny thing: DrawArc() with same ellipse width and height, but different start angle (0,90,180,270) are different Code:

            GraphicsPath pth = new GraphicsPath();
            pth.AddArc(10, 10, 16, 16, 180, 90);
            pth.AddArc( 40, 10, 16, 16, 270, 90);
            pth.AddArc( 40, 40, 16, 16, 0, 90);
            pth.AddArc( 10, 40, 16, 16, 90, 90);
            e.Graphics.FillPath(new SolidBrush(Color.FromArgb(100, 120, 200)), pth);

Expected:

But painted (only left top arc is correct):

How to fix that?

Answer 1

To create a round rectangle region, I ended up using CreateRoundRectRgn:

[System.Runtime.InteropServices.DllImport("gdi32.dll")]
private static extern IntPtr CreateRoundRectRgn(int nLeftRect, int nTopRect,
    int nRightRect, int nBottomRect, int nWidthEllipse, int nHeightEllipse);

public partial class RoundCornerControl : Control
{
    private int radius = 20;
    [DefaultValue(20)]
    public int Radius
    {
        get { return radius; }
        set { radius = value; this.RecreateRegion(); }
    }
    [System.Runtime.InteropServices.DllImport("gdi32.dll")]
    private static extern IntPtr CreateRoundRectRgn(int nLeftRect, int nTopRect,
        int nRightRect, int nBottomRect, int nWidthEllipse, int nHeightEllipse);
    private void RecreateRegion()
    {
        var bounds = ClientRectangle;
        this.Region = Region.FromHrgn(CreateRoundRectRgn(bounds.Left, bounds.Top,
            bounds.Right, bounds.Bottom, Radius, radius));
        this.Invalidate();
    }
    protected override void OnSizeChanged(EventArgs e)
    {
        base.OnSizeChanged(e);
        this.RecreateRegion();
    }
}