protected void Page_Load(object sender, EventArgs e)
{
// 生成验证码
string checkCode = RandLetter(4);
// 把新的验证码保存到Session中
Session["CheckCode"] = checkCode;
// 输入验证码
CreateImages(checkCode);
}

/// <summary>
/// 生成验证图片
/// </summary>
/// <param name="checkCode">验证字符</param>
private void CreateImages(string checkCode)
{
int iwidth = (int)(checkCode.Length * 13);
System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 23);
Graphics g = Graphics.FromImage(image);
g.Clear(Color.White);
//定义颜色
Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
//定义字体
string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
Random rand = new Random();
//随机输出噪点
for (int i = 0; i < 50; i++)
{
int x = rand.Next(image.Width);
int y = rand.Next(image.Height);
g.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);
}

//输出不同字体和颜色的验证码字符
for (int i = 0; i < checkCode.Length; i++)
{
int cindex = rand.Next(7);
int findex = rand.Next(5);

Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
Brush b = new System.Drawing.SolidBrush(c[cindex]);
int ii = 4;
if ((i + 1) % 2 == 0)
{
ii = 2;
}
g.DrawString(checkCode.Substring(i, 1), f, b, 3 + (i * 12), ii);
}
//画一个边框
g.DrawRectangle(new Pen(Color.Black, 0), 0, 0, image.Width - 1, image.Height - 1);

//输出到浏览器
System.IO.MemoryStream ms = new System.IO.MemoryStream();
image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
HttpContext.Current.Response.ClearContent();
//Response.ClearContent();
HttpContext.Current.Response.ContentType = "image/Jpeg";
HttpContext.Current.Response.BinaryWrite(ms.ToArray());
g.Dispose();
image.Dispose();
}

/// <summary>
/// 生成随机的字母
/// </summary>
/// <param name="count">生成字母的个数</param>
/// <returns>生成的验证码</returns>
private string RandLetter(int count)
{
string allLetters = "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[] allChar = allLetters.Split(‘,‘);
string checkCode = "";
int temp = -1; //记录上次随机数值，尽量避免生产几个一样的随机数

//采用一个简单的算法以保证生成随机数的不同
Random rand = new Random();
for (int i = 1; i < count + 1; i++)
{
if (temp != -1)
{
rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));
}
int t = rand.Next(allChar.Length);
temp = t;
checkCode += allChar[t];
}
return checkCode;
}