Adler

public partial class Form1 : Form
    {
            [DllImport("Gdi32.dll", EntryPoint = "CreateRoundRectRgn")]
            private static extern IntPtr CreateRoundRectRgn
            (
                int nLeftRect,     // x-coordinate of upper-left corner
                int nTopRect,      // y-coordinate of upper-left corner
                int nRightRect,    // x-coordinate of lower-right corner
                int nBottomRect,   // y-coordinate of lower-right corner
                int nWidthEllipse, // width of ellipse
                int nHeightEllipse // height of ellipse
            );
            public Form1()
        {
            InitializeComponent();
            this.FormBorderStyle = FormBorderStyle.None;
            Region = System.Drawing.Region.FromHrgn(CreateRoundRectRgn(0, 0, Width, Height, 20, 20));
        }
    }