asp.net (C #) Generate verification code click to refresh

First build a page to generate the verification code checkcode.aspx:
The CheckCode.aspx.cs code is as follows
To add a reference:
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; Special attention
Response.Cache.SetExpires (DateTime.Now.AddMilliseconds (-1));//Special attention
Response.Cache.SetCacheability (Httpcacheability.nocache)/special attention
Response.appendheader ("Pragma", "No-cache"); Special attention
String vnum = Makevalidatecode ();
session["Vnum"] = vnum;//Obtain the verification code for later verification
Validatecode (Vnum);
}
public void Validatecode (string vnum)
{
Validateimage = new Bitmap (A, PIXELFORMAT.FORMAT24BPPRGB);
g = Graphics.fromimage (validateimage);
G.fillrectangle (New LinearGradientBrush (0, 0), New Point (a), Color.FromArgb (m, 255, 255, 255), color.fr Omargb (240, 255, 255, 255)), 0, 0, 200, 200); Rectangular box
g.DrawString (vnum, new Font ("Arial", one), New SolidBrush (color.red), New PointF (6, 0));//font/color
G.save ();
MemoryStream ms = new MemoryStream ();
Validateimage. Save (MS, SYSTEM.DRAWING.IMAGING.IMAGEFORMAT.GIF);
Response.clearcontent ();
response.c;
Response.BinaryWrite (Ms. ToArray ());
Response.End ();
}
String Makevalidatecode ()
{
Char[] s = new char[] {' 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 ', ' P ', ' Q ', ' I ', ' S ', ' T ', ' U ', ' V ', ' W ', ' X ', ' Y ', ' Z '};//enumerated array
String num = "";
Random r = new Random ();
for (int i = 0; i < 5; i++)
{
num + + s[r.next (0, S.length)]. ToString ();
}
return num;
}
}

The HTML of the page that displays the captcha is then added
Before <script type= "Text/javascript" >
function Reloadcode () {
document.getElementById ("Yzm"). src = "checkcode.aspx";
}
</script>