ValidateCode.cs验证码时设置缓存的使用

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Drawing;
namespace SmartWaterSys.Web
{
public partial class ValidateCode : System.Web.UI.Page
{
private void Page_Load(object sender, System.EventArgs e)
{
string checkCode = GetRandomCode();
Session["CheckCode"] = checkCode;
SetPageNoCache();
CreateImage(checkCode);
}

/// <summary>
/// 设置页面不被缓存
/// </summary>
private void SetPageNoCache()
{
Response.Buffer = true;
Response.ExpiresAbsolute = System.DateTime.Now.AddSeconds(-);
Response.Expires = ;
Response.CacheControl = "no-cache";
Response.AppendHeader("Pragma", "No-Cache");
}

private string CreateRandomCode(int codeCount)
{
string allChar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,i,J,K,M,N,P,Q,R,S,T,U,W,X,Y,Z";
string[] allCharArray = allChar.Split(',');
string randomCode = "";
int temp = -;

Random rand = new Random();
for (int i = ; i < codeCount; i++)
{
if (temp != -)
{
rand = new Random(i * temp * ((int)DateTime.Now.Ticks));
}
int t = rand.Next();
if (temp == t)
{
return CreateRandomCode(codeCount);//性能问题
}
temp = t;
randomCode += allCharArray[t];
}
return randomCode;
}
private string GetRandomCode(int CodeCount)
{
string allChar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,i,J,K,M,N,P,Q,R,S,T,U,W,X,Y,Z";
string[] allCharArray = allChar.Split(',');
string RandomCode = "";
int temp = -;

Random rand = new Random();
for (int i = ; i < CodeCount; i++)
{
if (temp != -)
{
rand = new Random(temp * i * ((int)DateTime.Now.Ticks));
}

int t = rand.Next();

while (temp == t)
{
t = rand.Next();
}

temp = t;
RandomCode += allCharArray[t];
}

return RandomCode;
}
private void CreateImage(string checkCode)
{
int iwidth = (int)(checkCode.Length * );
System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, );
Graphics g = Graphics.FromImage(image);
Font f = new System.Drawing.Font("Arial ", );//, System.Drawing.FontStyle.Bold);
Brush b = new System.Drawing.SolidBrush(Color.Black);
Brush r = new System.Drawing.SolidBrush(Color.FromArgb(, , ));

//g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue),0,0,image.Width, image.Height);
// g.Clear(Color.AliceBlue);//背景色
g.Clear(System.Drawing.ColorTranslator.FromHtml("#99C1CB"));//背景色

char[] ch = checkCode.ToCharArray();
for (int i = ; i < ch.Length; i++)
{
if (ch[i] >= '' && ch[i] <= '')
{
//数字用红色显示
g.DrawString(ch[i].ToString(), f, r,  + (i * ), );
}
else
{ //字母用黑色显示
g.DrawString(ch[i].ToString(), f, b,  + (i * ), );
}
}

//for循环用来生成一些随机的水平线
// Pen blackPen = new Pen(Color.Black, 0);
// Random rand = new Random();
// for (int i=0;i<5;i++)
// {
// int y = rand.Next(image.Height);
// g.DrawLine(blackPen,0,y,image.Width,y);
// }

System.IO.MemoryStream ms = new System.IO.MemoryStream();
image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
//history back 不重复
Response.Cache.SetNoStore();//这一句
Response.ClearContent();
Response.ContentType = "image/Jpeg";
Response.BinaryWrite(ms.ToArray());
g.Dispose();
image.Dispose();
}
}
}

百度了一下，应该是为了解决获取验证码前后一致的问题，因为缓存会使数据保存，而验证码恰好不需要保存而是需要更新，所以取消缓存。

因为你每次动态生成的验证码的文件名都是一样的，比如是：img.jpg 那么系统就缓存了，下次再调用这个页面的时候ie会认为存在这个图片不会更新。导致验证码不对。