webform验证码

建一个一般处理程序，即.ashx文件

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

public class show : IHttpHandler ,IRequiresSessionState
{


    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "image/jpeg";//要输出的类型

        //写代码
        //造一张图出来
        Bitmap img = new Bitmap(50, 20);//造空白图
        Graphics gr = Graphics.FromImage(img);//往哪个图上去绘制
        Font font = new Font("宋体", 12, FontStyle.Bold);//设置字体
        SolidBrush brush = new SolidBrush(Color.White);//设置刷子
        gr.FillRectangle(brush, 0, 0, 50, 20);//刷子绘制的形状
        brush.Color = Color.Red;//颜色

        //往图上画文字,先生成四位验证码
        string s = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        string str = "";
        Random rand = new Random();//初始化随机数
        int[] arr = new int[4];
        for (int i = 0; i < 4; i++)
        {
            arr[i] = rand.Next(62);
            int count = 0;
            if (i > 0)
            {
                for (int j = 0; j < i; j++)
                {
                    if (arr[i] == arr[j])
                    {
                        count++;
                    }
                }
            }

            if (count > 0)
            {
                i--;
                continue;
            }
            str += s.Substring(arr[i], 1).ToString();
        }

        context.Session["yanzheng"] = str;//存到session

        gr.DrawString(str, font, brush, 0, 0);//画内容

        img.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
    }
    
 
    public bool IsReusable {
        get {
            return false;
        }
    }

}