using System.Drawing;
using System.Drawing.Imaging;
private void Page_Load(object sender, System.EventArgs e)
  {
   // 在此处放置用户代码以初始化页面
   //RndNum是一个自定义函数
   string VNum=RndNum(4); //这里的数字4代表显示的是4位的验证字符串！
   Session["VNum"]=VNum;
   ValidateCode(VNum);
  }
//生成图像函数
  private void ValidateCode(string VNum)
  {
   int Gheight=(int)(VNum.Length * 11.5);
   //gheight为图片宽度,根据字符长度自动更改图片宽度
   System.Drawing.Bitmap Img = new System.Drawing.Bitmap(Gheight,20);
   Graphics g = Graphics.FromImage(Img);
   g.DrawString(VNum,new System.Drawing.Font("Arial",10),new System.Drawing.SolidBrush(Color.Red),3,3);
   //在矩形内绘制字串（字串，字体，画笔颜色，左上x.左上y）
   System.IO.MemoryStream ms=new System.IO.MemoryStream();
   Img.Save(ms,System.Drawing.Imaging.ImageFormat.Png);
   Response.ClearContent(); //需要输出图象信息 要修改HTTP头
   Response.ContentType="image/Png";
   Response.BinaryWrite(ms.ToArray());
   g.Dispose();
   Img.Dispose();
   Response.End();
  }

//生成随机数函数中从Vchar数组中随机抽取
//字母区分大小写
public string RndNum(int VcodeNum)
  {
   string Vchar = "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,W,X,Y,Z" ;
   
   string[] VcArray = Vchar.Split(',') ;
   string  VNum = "" ;//由于字符串很短，就不用StringBuilder了
   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 < VcodeNum+1 ; i++ )
   {   
    if ( temp != -1)
    {
     rand =new Random(i*temp*unchecked((int)DateTime.Now.Ticks));
    }   
    //int t =  rand.Next(35) ;
    int t=rand.Next(35);
    if (temp != -1 && temp == t)
    {
     return RndNum( VcodeNum );
    }
    temp = t  ;
    VNum += VcArray[t];

   }
   return VNum ;
  }
-----------------------------------end--------------------------------------------

下面是Login.Aspx在在调用提交按钮的响应事件中

private void Page_Load(object sender, System.EventArgs e)
  {
   if(!Page.IsPostBack )
   {
     Session.Clear();
     }
  }

public void doit(object sender, System.EventArgs e)
  {
   if(Page.IsValid)
   {
     
    string VNum;
    VNum=Session["VNum"].ToString();        
    ViewState["VNum"]=VNum;
    if(this.Vcode.Text==ViewState["VNum"].ToString())
    { 

     Hover.Manage.CheckLogin bj=new Hover.Manage.CheckLogin();
     string name=username.Text;
     string password=FormsAuthentication.HashPasswordForStoringInConfigFile(pass.Text.ToString(),"md5");
     if(!obj.checklogin(name,password))
     {
      Response.Redirect("../Error.aspx?action=Errorlogin");
      Response.End();
      return;
     }
     else
     {
      Session.Add("adminname",name);
      Session.Add("adminpass",password);
      Response.Redirect("Default.aspx");
      Response.End();
      return;
     }
     
    }
    else
    {
     Response.Write("");
    }
   }
  }
   如果提交过来的验证字符串正确就验证是否是合法用户！
否则弹出提示返回登陆页!你也可以简化此页!
   if(this.Vcode.Text==ViewState["VNum"].ToString())
    { 
        Response.Write("验证码正确");
    }
   else
    {
        Response.Write("验证码错误!");