利用HttpHandler实现验证码 及 当你希望从ashx或HttpHandler里访问你的Session时,你必须实现IReadOnlySessionState接口.

输出图片并保存验证码在Session里面。

注意：
必须继承System.Web.SessionState.IRequiresSessionState接口，才能实现Session读写！

 System.Web.SessionState的一些接口

 IReadOnlySessionState 指定目标 HTTP 处理程序只需要具有对会话状态值的读访问权限。这是一个标记接口，没有任何方法。
 IRequiresSessionState 指定目标 HTTP 处理程序需要对会话状态值具有读写访问权。这是一个标记接口，没有任何方法。

ValidateCode.ashx---------------------------------------------------
<%@ WebHandler Language="C#" Class="ValidateCode" %>

using System;
using System.Web;
using System.Web.SessionState;

/// <summary>
/// 验证码程序
/// </summary>
public class ValidateCode : IHttpHandler, IRequiresSessionState
{
    /// <summary>
    /// 入口
    /// </summary>
    /// <param name="context"></param>
    public void ProcessRequest(HttpContext context)
    {
        string checkCode = CreateRandomCode(4);
        context.Session["Code"] = checkCode;
        CreateImage(checkCode, context);
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

    /// <summary>
    /// 产生验证码
    /// </summary>
    /// <param name="codeCount"></param>
    /// <returns></returns>
    private string CreateRandomCode(int codeCount)
    {
        //验证码中的出现的字符，避免了一些容易混淆的字符。
        string allChar = "3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,J,K,M,N,P,Q,R,S,T,U,W,X,Y";
        string[] allCharArray = allChar.Split(',');
        string randomCode = "";
        int temp = -1;

        Random rand = new Random();
        for (int i = 0; i < codeCount; i++)
        {
            if (temp != -1)
            {
                rand = new Random(i * temp * ((int)DateTime.Now.Ticks));
            }
            int t = rand.Next(allCharArray.Length);
            if (temp == t)
            {
                return CreateRandomCode(codeCount);
            }
            temp = t;
            randomCode += allCharArray[t];
        }
        return randomCode;
    }

    /// <summary>
    /// 创建图片
    /// </summary>
    /// <param name="checkCode"></param>
    /// <param name="context"></param>
    private void CreateImage(string checkCode, HttpContext context)
    {
        int iwidth = (int)(checkCode.Length * 12);
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 20);
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image);
        g.Clear(System.Drawing.Color.White);
        //定义颜色
        System.Drawing.Color[] c = { System.Drawing.Color.DimGray, System.Drawing.Color.DimGray, System.Drawing.Color.DimGray };
        //定义字体           
        string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
        Random rand = new Random();
        //随机输出噪点
        for (int i = 0; i < 50; i++)
        {
            int x = rand.Next(image.Width);
            int y = rand.Next(image.Height);
            g.DrawRectangle(new System.Drawing.Pen(System.Drawing.Color.LightGray, 0), x, y, 1, 1);
        }

        //输出不同字体和颜色的验证码字符

        for (int i = 0; i < checkCode.Length; i++)
        {
            int cindex = rand.Next(3);
            int findex = rand.Next(1);

            System.Drawing.Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
            System.Drawing.Brush b = new System.Drawing.SolidBrush(c[cindex]);
            int ii = 4;
            if ((i + 1) % 2 == 0)
            {
                ii = 2;
            }
            g.DrawString(checkCode.Substring(i, 1), f, b, 3 + (i * 10), ii);
        }
        //画一个边框

        g.DrawRectangle(new System.Drawing.Pen(System.Drawing.Color.DarkGray, 0), 0, 0, image.Width - 1, image.Height - 1);

        //输出到浏览器
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
        context.Response.ClearContent();
        context.Response.ContentType = "image/Jpeg";
        context.Response.BinaryWrite(ms.ToArray());
        g.Dispose();
        image.Dispose();
    }

} 

页面
<img src="ashx文件地址">

判断验证码是否正确
Session["Code"].ToString()==输入的字符

＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝

当你希望从ashx或HttpHandler里访问你的Session时,你必须实现IReadOnlySessionState接口.     

当你希望从ashx或HttpHandler里访问你的Session时,你必须实现IReadOnlySessionState接口.

代码:

using System;
using System.Web;
using System.Web.SessionState;

public class DownloadHandler : IHttpHandler, IReadOnlySessionState
{
   public bool IsReusable { get { return true; } }
  
   public void ProcessRequest(HttpContext ctx)
   {
       ctx.Response.Write(ctx.Session["fred"]);
   }
}