Code
using System;
using System.Collections.Generic;
using System.Text;
using System.Web;

using System.Web.SessionState;
using System.Web.UI;
using System.Drawing;

namespace ControlValidater
{
public class CValidater:IHttpHandler,IRequiresSessionState
{
HttpResponse response = null;
HttpSessionState sessionState = null;

public void ProcessRequest(HttpContext context)
{
response = context.Response;
sessionState = context.Session;

this.GenImg(this.GenCode(4));
}

bool System.Web.IHttpHandler.IsReusable
{
get { return true; }
}

//产生随机字符串
private string GenCode(int num)
{
string[] source ={"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 code = "";
Random rd = new Random();
for (int i = 0; i < num; i++)
{
code += source[rd.Next(0, source.Length)];
}
return code;
}

//生成图片
private void GenImg(string code)
{
Bitmap myPalette = new Bitmap(60, 20);//定义一个画板

Graphics gh = Graphics.FromImage(myPalette);//在画板上定义绘图的实例

Rectangle rc = new Rectangle(0, 0, 60, 20);//定义一个矩形

gh.FillRectangle(new SolidBrush(Color.Silver), rc);//填充矩形

gh.DrawString(code, new Font("宋体", 16), new SolidBrush(Color.Gray), rc);//在矩形内画出字符串

myPalette.Save(response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);//将图片显示出来

sessionState["ValidateCode"] = code;//将字符串保存到Session中，以便需要时进行验证

gh.Dispose();
myPalette.Dispose();
}
}
}

步骤二(使用方法)
直接引用编译后的dll文件
在web.config的web节点,加入元素,
例如

Code

当然,如果直接把类库作为一个普通类,放在使用的Website的App_Data目录的话,就不需要配置了:)

步骤三(使用方法)
在需要使用验证码的页面,扔一个image标记,然后把他的url指向"类名.aspx"
例如
Code

到此配置完成,是不是很简单呢:)
哎...
还是把最后一个步骤代码贴出来吧.
Code
protected void Button1_Click(object sender, EventArgs e)
{
string strIn = this.TextBox1.Text;
string strSess = Session["ValidateCode"].ToString();
if (strIn == strSess)
Button1.Text = "ok";
else
Button1.Text = "error";
}
需要注意的几个地方:)
一,记得要实现IRequiresSessionState接口,虽然她只是一个标记,没有她,Session状态无法使用.
二,HttpSessionState的对象是通过上下文获得的.
三,配制config的时候,不要忘记path="CValidater.aspx",如果path="*.aspx",那你所有的页面都将被CValidater处理,这不是我们所要的.

最后
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