实现效果：
知识运用：
Graphics对象的DrawLine方法
public void DrawLine (Pen pen, Point pt1,Point pt2)
public void DrawLine (Pen pen, int x1,int y1,int x2,int y2)
实现代码：
private void Form1_MouseDown(object sender, MouseEventArgs e)
{
Graphics g = this.CreateGraphics();
g.DrawLine(new Pen(Color.Black,2),new Point(0,e.Y),new Point(this.Width,e.Y));
g.DrawLine(new Pen(Color.Black, 2), new Point(e.X, 0), new Point(e.X,this.Height));
}
原文：https://www.cnblogs.com/feiyucha/p/10274393.html