RoundBorder

public class CustomGraphicsPath
    {
        //Importo dalla api di windows la chiamata alla funzione CreateRoundRectRgn per creare la         GraphicsPath con bordi arrotondati
        [DllImport("Gdi32.dll", EntryPoint = "CreateRoundRectRgn")]
        public static extern IntPtr CreateRoundRectRgn
        (
            int nLeftRect,
            int ntopRect,
            int nRightRect,
            int nBottomRect,
            int nWidthEllipse,
            int nHeightEllipse
            );

    }