asp.net验证码

using System.Drawing;
using System.Drawing.Drawing2D;
类：
/// <summary>
/// 生成验证码
/// </summary>
private void DrowCode()
{
Response.Cache.SetNoStore();
System.Drawing.Bitmap image = new System.Drawing.Bitmap(77, 20);
Graphics g = Graphics.fromImage(image);
Color[] borders = new Color[10] { Color.AliceBlue, Color.Aqua, Color.Black, Color.Brown, Color.DarkRed, Color.SkyBlue, Color.Silver, Color.Tan, Color.Violet, Color.SpringGreen };
try
{
Random random = new Random();
g.Clear(Color.White);
//画图片的背景噪音线
for (int i = 0; i < 2; i++)
{
int x1 = random.Next(image.Width);
int x2 = random.Next(image.Width);
int y1 = random.Next(image.Height);
int y2 = random.Next(image.Height);
g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);
}
string str = "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";
string[] codes = str.Split(',');
string serial = string.Empty;
for (int i = 0; i < 6; i++) // i 控制验证码字符个数
{
serial += codes[random.Next(codes.Length)];
}
Font font = new System.Drawing.Font("Verdana", 11, System.Drawing.FontStyle.Bold);
System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
g.DrawString(serial, font, brush, random.Next(image.Width - 77), random.Next(image.Height - 20));
//画图片的前景噪音点
for (int i = 0; i < 50; i++)
{
int x = random.Next(image.Width);
int y = random.Next(image.Height);
image.SetPixel(x, y,