命名空间:

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

        private void Page_Load(object sender, System.EventArgs e)
        {
            // 在此处放置用户代码以初始化页面
            string checkCode = CreateRandomCode(4);
            Session["CheckCode"] = checkCode;
            CreateImage(checkCode);
        }
        private string CreateRandomCode(int codeCount)
        {

            // 函数功能:产生数字和字符混合的随机字符串
            string allChar = "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,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[] 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(35);
                if(temp == t)
                {
                    return CreateRandomCode(codeCount);
                }
                temp = t;
                randomCode += allCharArray[t];
            }
            return randomCode;
        }


        private void CreateImage(string checkCode)
        {

            // 生成图象验证码函数
            int iwidth = (int)(checkCode.Length * 11.5);
            System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 20);
            Graphics g = Graphics.FromImage(image);
            Font f = new System.Drawing.Font("Arial", 10, System.Drawing.FontStyle.Bold);
            Brush b = new System.Drawing.SolidBrush(Color.Azure);//字母白色
            //g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue),0,0,image.Width, image.Height);
            g.Clear(Color.Gray);//背景灰色
            g.DrawString(checkCode, f, b, 3, 3);

            Pen blackPen = new Pen(Color.Black, 0);
            Random rand = new Random();


            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
            Response.ClearContent();
            Response.ContentType = "image/Jpeg";
            Response.BinaryWrite(ms.ToArray());
            g.Dispose();
            image.Dispose();

        }