protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);
    Rectangle Rect = new Rectangle(0, 0, this.Width, this.Height);
    GraphicsPath GraphPath = new GraphicsPath();
    GraphPath.AddArc(Rect.X, Rect.Y, 50, 50, 180, 90);
    GraphPath.AddArc(Rect.X + Rect.Width - 50, Rect.Y, 50, 50, 270, 90);
    GraphPath.AddArc(Rect.X + Rect.Width - 50, Rect.Y + Rect.Height - 50, 50, 50, 0, 90);
    GraphPath.AddArc(Rect.X, Rect.Y + Rect.Height - 50, 50, 50, 90, 90);
    this.Region = new Region(GraphPath);
}