On a winform, is there a way to draw a rectangle like this (without top) using the DrawRectangle method or a workaround?
You can try using the SetClip
method:
private void DrawTopless(Graphics g, Rectangle r) {
g.SetClip(new Rectangle(r.Left, r.Top, r.Width + 1, 10), CombineMode.Exclude);
g.DrawRectangle(Pens.Red, r);
g.ResetClip();
}