.net如何做验证码

发一个我用的，在页当成图片调用就OK了。

cs文件内容如下：
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using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;

public partial class validate : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if(!this.IsPostBack)
{
this.GenImg(this.GetCode(4));
}
}
//产生随机字符串
private string GetCode(int num)
{
string[] source={"1","2","3","4","5","6","7","8","9","A","B","C","D",