先要新建validate aspx验证页面。然后生成验证码控件imagebutton，再生成picture窗体。

代码如下：

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Drawing;
using System.IO;

public partial class Picture : System.Web.UI.Page
{
    Random ran = new Random();
    protected void Page_Load(object sender, EventArgs e)
    {
        string str = getRandomValidate(4);
        Session["CheckCode"] = str;//这一部是Wie了验证码写入Session,进行验证，也可以使用cookie
        getImageValidate(str);  
    }
    //得到随机字符串，长度自定义
    private string getRandomValidate(int len)
    {
        int num;
        int tem;
        string rtuStr="";
        for (int i = 0; i < len;i++ )
        {
            num = ran.Next();
            tem = num % 10 + ‘0‘;//生成数字
            //tem = num % 26 + ‘A‘;//生成字符
            rtuStr += Convert.ToChar(tem).ToString();public partial class Picture : System.Web.UI.Page
{
    Random ran = new Random();
    protected void Page_Load(object sender, EventArgs e)
    {
        string str = getRandomValidate(4);
        Session["CheckCode"] = str;//这一部是Wie了验证码写入Session,进行验证，也可以使用cookie
        getImageValidate(str);  
    }
    //得到随机字符串，长度自定义
    private string getRandomValidate(int len)
    {
        int num;
        int tem;
        string rtuStr="";
        for (int i = 0; i < len;i++ )
        {
            num = ran.Next();
            tem = num % 10 + ‘0‘;//生成数字
            //tem = num % 26 + ‘A‘;//生成字符
            rtuStr += Convert.ToChar(tem).ToString();public partial class Picture : System.Web.UI.Page
{
    Random ran = new Random();
    protected void Page_Load(object sender, EventArgs e)
    {
        string str = getRandomValidate(4);
        Session["CheckCode"] = str;//这一部是Wie了验证码写入Session,进行验证，也可以使用cookie
        getImageValidate(str);  
    }
    //得到随机字符串，长度自定义
    private string getRandomValidate(int len)
    {
        int num;
        int tem;
        string rtuStr="";
        for (int i = 0; i < len;i++ )
        {
            num = ran.Next();
            tem = num % 10 + ‘0‘;//生成数字
            //tem = num % 26 + ‘A‘;//生成字符
            rtuStr += Convert.ToChar(tem).ToString();

}
        return rtuStr;
    }
    //生成图像
    private void getImageValidate(string strValue)
    {
        //string str=oo00;前两个为字母o，后两个数为0
        int width = Convert.ToInt32(strValue.Length*12);
        Bitmap img = new Bitmap(width,23);
        Graphics gfc = Graphics.FromImage(img);
        gfc.Clear(Color.White);
        drawLine(gfc,img);
        //写验证码，要定义Font
        Font font = new Font("arial",12,FontStyle.Bold);
        //Font font = new Font("宋体",12,FontStyle.Bold|FontStyle.Italic);
        System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0,0,img.Width,img.Height),Color.DarkOrchid,Color.Blue,1.5f,true);
        gfc.DrawString(strValue,font, brush ,3,2);
        drawPoint(img);
        gfc.DrawRectangle(new Pen(Color.DarkBlue),0,0,img.Width-1,img.Height-1);
        //将图像添加到页面
        MemoryStream ms = new MemoryStream();
        img.Save(ms,System.Drawing.Imaging.ImageFormat.Gif);

//更改HTTP
        Response.ClearContent();
        Response.ContentType = "image/gif";
        Response.BinaryWrite(ms.ToArray());
        //Dispose
        gfc.Dispose();
        img.Dispose();
        Response.End();

    }

    private void drawLine(Graphics gfc,Bitmap img)
    {
        //选择画10条线，也可以增加，也可以不要线，只要随机杂点就行
        for (int i = 0; i < 10;i++ )
        {
            int x1 = ran.Next(img.Width);
            int y1 = ran.Next(img.Height);
            int x2 = ran.Next(img.Width);
            int y2 = ran.Next(img.Height);
            gfc.DrawLine(new Pen(Color.Silver),x1,y1,x2,y2);//注意画笔要淡，不然看不清
        }

  }

    //private void drawPoint(Bitmap img)
    //{
   
    //}

    private void drawPoint(Bitmap img)
    {
        int col = ran.Next();//在一次的图片中杂点颜色相同
        for (int i = 0; i < 100; i++)
        {
            int x = ran.Next(img.Width);
            int y = ran.Next(img.Height);
            img.SetPixel(x,y,Color.FromArgb(col));
        }
    }
  
}

asp.net如何实现生成验证码的登录界面