C# winform绘图的问题

我在panel上写的代码(在屏幕上画虚线框,做为选中控件使用)
当画线与panel的子控件重合时就会有线条残留在子控件上.这个问题如何解决?

补充:
protected override void OnPaint(PaintEventArgs e)
{
#region 画选择框
if (isDrawSelectRectangle)
{
using (Pen p = new Pen(Color.Black))
{
//设置虚线笔
p.DashStyle = System.Drawing.Drawing2D.DashStyle.Dot;
//画选择框
//e.Graphics.DrawRectangle(p, selectRectangle);
//screenGraphics获取屏幕并设置为画板,selectRectangle要在屏幕上画的矩形
screenGraphics.DrawRectangle(p,RectangleToScreen(selectRectangle));

}

}
#endregion