如何实现验证码功能？

#region 实现一个验证码的类

public class ValidateCode

{

private Bitmap validateimage;

private Graphics g;

public ValidateCode()

{

validateimage = new Bitmap(120, 30, PixelFormat.Format24bppRgb);

g = Graphics.FromImage(validateimage);

}

public void DrawValidateCode(Page e, string i)

{

g.DrawString(i, new Font("黑体",16,FontStyle.Bold),new SolidBrush(Color.White),new PointF(2,4));

g.FillRectangle(new LinearGradientBrush(new Point(0,0), new Point(120,30), Color.FromArgb(0,0,0,0),Color.FromArgb(255,255,255,255)),0,0,120,30); 

//validateimage.Save(e.OutputStream, ImageFormat.Jpeg);

g.Save();

MemoryStream ms=new MemoryStream();

validateimage.Save(ms,System.Drawing.Imaging.ImageFormat.Gif);

e.Response.ClearContent();

e.Response.ContentType="image/gif";

e.Response.BinaryWrite(ms.ToArray());

e.Response.End();

//validateimage.Save(e.MapPath("NumImage/ValidateImage.gif"), ImageFormat.Gif);

//e.End();

}

}

#endregion

private void 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','O','P','Q','R','S','T','U','V','W'

,'X','Y','Z'};

string num = "";

Random r = new Random();

for(int i = 0; i < 5; i++)

{

num += s[r.Next(0, s.Length)].ToString(); 

}

((LoginUserInfo)Session["LoginUserInfo"]).ValidateNum = num;

TextBox3.Text = "";

}