private readonly char[] constant = { 
  '0','1','2','3','4','5','6','7','8','9', 
  'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z', 
  'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};//一个由数字和大小写英文字母组成的字符数组
 
  protected void Page_Load(object sender, EventArgs e)
  {
   Bitmap bitmap = new Bitmap(100, 25);//创建一个位图，宽100，高25，就是我所说的第一部分，矩形背景
   Graphics g = Graphics.FromImage(bitmap);//创建画布
   g.Clear(Color.YellowGreen);//为画布填充黄绿色
 
   Font font1 = new Font("Arial", 15);//设置字体类型和大小 
   Brush brush = new SolidBrush(Color.Blue);//设置画刷颜色
   Pen myPen = new Pen(Color.Blue, 5);//创建画笔对象
 
   StringBuilder random = new StringBuilder(5); //创建可变字符串对象，用于存放随机生成的验证码
   Random rd = new Random();//创建一个随机数生成器对象
   for (int i = 0; i < random.Capacity; i++)
   {
   random.Append(constant[rd.Next(62)]);//生成一个随机字符加到random里
   }
 
   g.DrawString(random.ToString(), font1, brush, 10, 5);//在画布上画出字符串
 
   System.IO.MemoryStream ms = new System.IO.MemoryStream();//创建数据流MemoryStream
   bitmap.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);//指定图像的输出格式为gif
 
   Response.ClearContent();
   Response.ContentType = "image/Gif";
   Response.BinaryWrite(ms.ToArray());//输出二进制数据流
  }

private readonly char[] constant = { 
  '0','1','2','3','4','5','6','7','8','9', 
  'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z', 
  'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};//一个由数字和大小写英文字母组成的字符数组
 
  protected void Page_Load(object sender, EventArgs e)
  {
   Bitmap bitmap = new Bitmap(100, 25);//创建一个位图，宽100，高25，就是我所说的第一部分，矩形背景
   Graphics g = Graphics.FromImage(bitmap);//创建画布
   g.Clear(Color.YellowGreen);//为画布填充黄绿色
 
   Font font1 = new Font("Arial", 15);//设置字体类型和大小 
   float angle = 60;//旋转的一个基础角度
   float length = 0;//显示字符的基础位置，往后看
   Brush brush = new SolidBrush(Color.Blue);//设置画刷颜色
   Pen myPen = new Pen(Color.Blue, 5);//创建画笔对象
 
   StringBuilder random = new StringBuilder(5); //创建可变字符串对象，用于存放随机生成的验证码
   Random rd = new Random();//创建一个随机数生成器对象
   for (int i = 0; i < random.Capacity; i++)
   {
    random.Append(constant[rd.Next(62)]);//生成一个随机字符加到random里
 
    g.ResetTransform();//将画布重置矩阵
    SizeF size = g.MeasureString(random[random.Length - 1].ToString(), font1);//得到新生成字符的尺寸
    g.TranslateTransform(length + size.Width / 2, size.Height / 2);//选择此次旋转的中心位置
    g.RotateTransform((float)rd.NextDouble() * angle * 2 - angle);//进行随机角度旋转
    g.DrawString(random[random.Length - 1].ToString(), font1, brush, new PointF(-size.Width / 2, -size.Height / 2));//注意，这里不是前一个例子，一次性把5个字符全部画出来，而是一个一个画
    length += size.Width;//保证下次画字符的位置不会覆盖前一次的字符
   }
 
   System.IO.MemoryStream ms = new System.IO.MemoryStream();//创建数据流MemoryStream
   bitmap.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);//指定图像的输出格式为gif
 
   Response.ClearContent();
   Response.ContentType = "image/Gif";
   Response.BinaryWrite(ms.ToArray());//输出二进制数据流
  }