EMR_ROUNDRECT          = 44,

static extern IntPtr CreateRoundRectRgn(int x1, int y1, int x2, int y2,

        IntPtr _RegionHandle = CreateRoundRectRgn( 10, 10, 60, 60, 10, 10 );

static extern bool RoundRect(IntPtr hdc, int nLeftRect, int nTopRect,

    public static extern bool RoundRect(RECT lpRect, Point point);

    public static extern bool RoundRect(IntPtr hdc, int nLeftRect, int nTopRect,

        DrawRoundRect(e.Graphics);

    public void DrawRoundRect(Graphics gr)

        GDI32.RoundRect(hDC, 0, 0, this.Width, this.Height, 20, 20);

static extern IntPtr CreateRoundRectRgn(int x1, int y1, int x2, int y2,int cx, int cy);

SetWindowRgn (this.Handle, CreateRoundRectRgn ( 0, 0, 562, 257, 20, 20 ),true);