四位随机验证码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;

namespace FriendsInfoAdministrators.Web.ASPX
{
    public partial class ValidateNumbers : System.Web.UI.Page
    {
        /// <summary>
        /// 加载时生成验证码
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected void Page_Load(object sender, EventArgs e)
        {
            string strNum = Rndnum();
            Session["Num"] = strNum;//页面间传值，用于验证判断
            ValidateCode(strNum);
        }
        /// <summary>
        /// 建立一个函数Rndnum，用来生成4个随即数字
        /// </summary>
        /// <returns></returns>
        public string Rndnum()
        {
            int j1;
            string StrChoice = "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[] strResult = StrChoice.Split(new Char[] { ',' });//分割字符串函数，并放于strResult数组中                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
            string strReturn = "";
            Random rnd = new Random(unchecked((int)DateTime.Now.Ticks));
            for (int i = 0; i < 4; i++)
            {
                Random rnd1 = new Random(rnd.Next() * unchecked((int)DateTime.Now.Ticks));
                j1 = rnd1.Next(35);
                rnd = new Random(rnd.Next() * unchecked((int)DateTime.Now.Ticks));
                strReturn = strReturn + strResult[j1].ToString();
            }
            return strReturn;
        }
        /// <summary>
        /// 数字转换为位图图像
        /// </summary>
        /// <param name="vnum"></param>
        public void ValidateCode(string vnum)
        {
            MemoryStream ms = new MemoryStream();//内存中分配空间
            double Height = (vnum.Length) * 15;
            int gHeight = Convert.ToInt32(Height);
            Bitmap img = new Bitmap(gHeight, 25);//定义位图，使用了两个参数
            Graphics g = Graphics.FromImage(img);
            g.DrawString(vnum, (new Font("幼圆", 13)), (new SolidBrush(Color.Blue)), 1, 4);//将文本vnum,转换为图片，在指定的位置（X，Y），执行字体和刷子
            //SolidBrush(Color.Blue)用蓝色
            img.Save(ms, ImageFormat.Png);//以PNG格式保存
            Response.ClearContent();
            Response.ContentType = "image/Bmp";//浏览器显示图片，否则浏览器将下载提示保存该图
            Response.BinaryWrite(ms.ToArray());//输出图片
            //以下函数及时删除对象，释放资源
            g.Dispose();
            img.Dispose();
            Response.End();

        }
    }
}