初步理解OnMouseDown和OnMouseUp

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 using System.Drawing;
 6 using System.Windows.Forms;
 7 
 8 namespace OnMouseDownDemo
 9 {
10     class Program:Form
11     {
12         Point ptBeg, ptEnd;
13 
14         static void Main(string[] args)
15         {
16             Application.Run(new Program());
17         }
18         public Program()
19         {
20             Text = "OnMouseDown Demo";
21             ForeColor = SystemColors.WindowText;
22             BackColor = SystemColors.Window;
23         }
24         protected override void OnMouseDown(MouseEventArgs e)
25         {
26             if (e.Button == MouseButtons.Left)
27             {
28                 ptBeg = ptEnd = new Point(e.X,e.Y);
29 
30                 Graphics grfx = CreateGraphics();
31                 grfx.DrawRectangle(new Pen(Color.Red),e.X,e.Y,30,40);
32                 //Invalidate();
33             }
34         }
35         protected override void OnMouseUp(MouseEventArgs e)
36         {
37             if (e.Button == MouseButtons.Left)
38             {
39                 ptBeg = ptEnd = new Point(e.X, e.Y);
40 
41                 Graphics grfx = CreateGraphics();
42                 grfx.DrawRectangle(new Pen(Color.Red), e.X, e.Y, 60, 80);
43                 //Invalidate();
44             }
45         }
46     }
47 }