第一种：

using System.Drawing;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        CreateCheckCodeImage(GenerateCheckCode());
    }

    private string GenerateCheckCode()
    {
        //创建整型型变量
        int number;
        //创建字符型变量
        char code;
        //创建字符串变量并初始化为空
        string checkCode = String.Empty;
        //创建Random对象
        Random random = new Random();
        //使用For循环生成4个数字
        for (int i = 0; i < 4; i++)
        {
            //生成一个随机数
            number = random.Next();
            //将数字转换成为字符型
           code = (char)(‘0‘ + (char)(number % 10));        

            checkCode += code.ToString();
        }
        //将生成的随机数添加到Cookies中
        Response.Cookies.Add(new HttpCookie("CheckCode", checkCode));
        //返回字符串
        return checkCode;
    }

    private void CreateCheckCodeImage(string checkCode)
    {
        //判断字符串不等于空和null
        if (checkCode == null || checkCode.Trim() == String.Empty)
            return;
       //创建一个位图对象
        System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length * 12.5)), 22);
        //创建Graphics对象
        Graphics g = Graphics.FromImage(image);

        try
        {
            //生成随机生成器
            Random random = new Random();

            //清空图片背景色
            g.Clear(Color.White);

            //画图片的背景噪音线
            for (int i = 0; i < 2; i++)
            {
                int x1 = random.Next(image.Width);
                int x2 = random.Next(image.Width);
                int y1 = random.Next(image.Height);
                int y2 = random.Next(image.Height);

                g.DrawLine(new Pen(Color.Black), x1, y1, x2, y2);
            }

            Font font = new System.Drawing.Font("Arial", 12, (System.Drawing.FontStyle.Bold));
            System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
            g.DrawString(checkCode, font, brush, 2, 2);

            //画图片的前景噪音点
            for (int i = 0; i < 100; i++)
            {
                int x = random.Next(image.Width);
                int y = random.Next(image.Height);

                image.SetPixel(x, y, Color.FromArgb(random.Next()));
            }

            //画图片的边框线
            g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

            //将图片输出到页面上
            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
            Response.ClearContent();
            Response.ContentType = "image/Gif";
            Response.BinaryWrite(ms.ToArray());
        }
        finally
        {
            g.Dispose();
            image.Dispose();
        }
    }
}

第二种：

protected void Page_Load(object sender, EventArgs e)
    {
        if(!IsPostBack)
        {
          Label1.Text =RandomNum(4);//产生验证码
        }
    }
    public string RandomNum(int n)
    {

         string strchar="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[] VcArray=strchar.Split(‘,‘);
         string VNum="";
         int temp=-1;//记录上次随机数值，尽量避免产生几个一样的随机数
                     //采用一个简单的算法以保证生成随机数的不同
         Random rand=new Random();
         for(int i=1;i<n+1;i++)
         {
             if(temp!=-1)
             {
                 rand=new Random(i * temp *(int)DateTime.Now.Ticks);
             }
             int t=rand.Next(61);
             if(temp!=-1&&temp==t)
             {
                 return RandomNum(n);
             }
             temp=t;
             VNum+=VcArray[t];
         }
        return VNum;   //返回生成的随机数
    }

 第三种：

using System.Drawing;
using System.Drawing.Imaging;

public partial class _Default : System.Web.UI.Page
{
     protected void Page_Load(object sender, EventArgs e)
    {
        Session["code"] = RandNum(4);
        ValidateCode(Session["code"].ToString(), 40, 20, "黑体", 10, "#FFFFFF");
    }
    private string RandNum(int VcodeNum)
    {
        string Vchar = "0,1,2,3,4,5,6,7,8,9";
        string[] VcArray = Vchar.Split(‘,‘);//拆分成数组
        string VNum = "";
        int temp = -1;//记录上次随机数值，尽量避避免生产几个一样的随机数

        Random rand = new Random();
        //采用一个简单的算法以保证生成随机数的不同
        for (int i = 0; i < VcodeNum; i++)
        {
            if (temp != -1)
            {
                rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));
            }

            int t = rand.Next(VcArray.Length - 1);
            if (temp != -1 && temp == t)
            {
                return RandNum(VcodeNum);

            }
            temp = t;
            VNum += VcArray[t];
        }
        return VNum;
    }

    private void ValidateCode(string VNum, int w, int h, string font, int fontSize, string bgColor)
    {
        Bitmap Img = new Bitmap(w, h);//生成图像的实例
        Graphics g = Graphics.FromImage(Img);//从Img对象生成新的Graphics对象
        g.Clear(ColorTranslator.FromHtml(bgColor));//背景颜色
        Font f = new Font(font, fontSize);//生成Font类的实例
        SolidBrush s = new SolidBrush(Color.Black);//生成笔刷类的实例
        g.DrawString(VNum, f, s, 3, 3);//将VNum写入图片中
        Img.Save(Response.OutputStream, ImageFormat.Jpeg);//将此图像以Jpeg图像文件的格式保存到流中
        Response.ContentType = "image/Jpeg";
        //回收资源
        g.Dispose();
        Img.Dispose();
        Response.End();
    }   
   
}