为了便于大家学习，修改vb.net code成为c#
gif.aspx文件修改的内容
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(5);
Session["VNum"] = VNum;
ValidateCode(VNum);

}
private void ValidateCode(string VNum)
{
//生成验证code
System.Drawing.Bitmap img;
System.Drawing.Graphics g;
System.IO.MemoryStream ms;
int gheight = Convert.ToInt32(VNum.Length*11.5);
//gheight为图片宽度,根据字符长度自动更改图片宽度
img = new Bitmap(gheight,20);
g = Graphics.FromImage(img);
//在矩形内绘制字串（字串，字体，画笔颜色，左上x.左上y）
g.DrawString(VNum,new Font("Arial",10),new SolidBrush(System.Drawing.Color.Blue),3,3);
ms = new MemoryStream();
img.Save(ms,ImageFormat.Png);

Response.ClearContent(); //需要输出图象信息 要修改HTTP头
Response.ContentType = "image/gif";
Response.BinaryWrite(ms.ToArray());
g.Dispose();
img.Dispose();
Response.End();
}

//'函数名称:RndNum
//'函数参数:VcodeNum--设定返回随机字符串的位数
//'函数功能:产生数字和字符混合的随机字符串
private 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(",".ToCharArray()); //将字符串生成数组
string VNum = "";
int i;
for(i=1;i<VcodeNum;i++)
{
System.Random ro = new Random();
double decA = ro.NextDouble();
VNum = VNum + VcArray[Convert.ToInt32(35*decA)];
}
// For i=1 to VcodeNum
//Randomize
//VNum=VNum & VcArray(Int(35*Rnd)) '数组一般从0开始读取，所以这里为35*Rnd
//Next
return VNum;

}

ValidateCode.aspx修改内容
private void Page_Load(object sender, System.EventArgs e)
{
// 在此处放置用户代码以初始化页面
string VNum;
if(Session["VNum"]!= null)
{
VNum = Session["VNum"].ToString();
Session.Abandon();
ViewState["VNum"]=VNum;

}


}

private void Button1_Click(object sender, System.EventArgs e)
{
if(txtValidateCode.Text == ViewState["VNum"].ToString() )
{

lblShow.Text="提示:验证通过" ;
}
else
{
lblShow.Text="所填写的验证码与所给的不符";
}
}