using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        private bool ismousedown = false;//开始画图
        private PointF startposition;
        private PointF nowposition;
        private Image img;
        private Image image;
        private Pen pen;
        private Graphics g;

        public Form1()
        {
            InitializeComponent();
            SetStyle(ControlStyles.OptimizedDoubleBuffer | ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint, true);
            this.UpdateStyles();
            img = new Bitmap(this.Width, this.Height);
            image = new Bitmap(this.Width,this.Height);
            pen = new Pen(Color.Red, 2);
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                ismousedown = true;
                startposition.X = e.X;
                startposition.Y = e.Y;
            }
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                
                if (ismousedown)
                {
                    nowposition.X = e.X;
                    nowposition.Y = e.Y;
                    g = Graphics.FromImage(img);
                    g.Clear(Color.WhiteSmoke);
                    g.DrawImage(image, 0, 0);
                    g.DrawRectangle(pen,Math.Min(nowposition.X,startposition.X),Math.Min(nowposition.Y,startposition.Y),Math.Abs(nowposition.X -startposition.X ),Math.Abs(nowposition.Y-startposition.Y));
                    g.Dispose();
                    Graphics graphcis = this.CreateGraphics();
                    graphcis.DrawImage(img, 0, 0);
                    graphcis.Dispose();
                }              
            }
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            ismousedown = false;
            g = Graphics.FromImage(image);
            g.DrawImage(img, 0, 0);
            g.Dispose();
        }   
    }
}