asp.net(c#)生成验证码 点击可刷新

using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;

public partial class Default2 : System.Web.UI.Page
{
    private Bitmap validateimage;
    private Graphics g;
    protected void Page_Load(object sender, EventArgs e)
    {
        Response.BufferOutput = true;  //特别注意
        Response.Cache.SetExpires(DateTime.Now.AddMilliseconds(-1));//特别注意
        Response.Cache.SetCacheability(HttpCacheability.NoCache);//特别注意
        Response.AppendHeader("  Pragma", "No-Cache"); //特别注意
        string VNum = MakeValidateCode();
        Session["VNum"] = VNum;//取得验证码，以便后来验证
        ValidateCode(VNum);
    }
    public void ValidateCode(string VNum)
    {
        validateimage = new Bitmap(60, 20, PixelFormat.Format24bppRgb);
        g = Graphics.FromImage(validateimage);
        g.FillRectangle(new LinearGradientBrush(new Point(0, 0), new Point(110, 20), Color.FromArgb(240, 255, 255, 255), Color.FromArgb(240, 255, 255, 255)), 0, 0, 200, 200); //矩形框
        g.DrawString(VNum, new Font("arial", 11), new SolidBrush(Color.Red), new PointF(6, 0));//字体/颜色
        g.Save();
        MemoryStream ms = new MemoryStream();
        validateimage.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
        Response.ClearContent();
        Response.C;