asp.net で英語の文字と数字の検証コードを生成するコード

using System; 
using System.Data; 
using System.Configuration; 
using System.Web; 
using System.Web.Security; 
using System.Web.UI; 
using System.Web.UI.WebControls; 
using System.Web.UI.WebControls.WebParts; 
using System.Web.UI.HtmlControls; 
using System.Drawing; 
using System.Drawing.Drawing2D; 

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

Bitmap bmp = new Bitmap(50, 25); 
Graphics g = Graphics.FromImage(bmp); 
SolidBrush sb = new SolidBrush(getColor()); 
g.DrawString(CheckNumber(), new Font("宋体", 16), sb, 0, 0); 
bmp.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Gif); 

} 
public static string CheckNumber() 
{ 
string checkcode = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; 
int len = 4; 
string tmpstr = ""; 
int iRandNum; 
Random rnd = new Random(); 
for (int i = 0; i < len; i++) 
{ 
iRandNum = rnd.Next(checkcode.Length); 
tmpstr += checkcode[iRandNum]; 
} 
return tmpstr; 
} 
private Color getColor() 
{ 
Random r = new Random(); 
return Color.FromArgb(r.Next(256), r.Next(256), r.Next(256)); 
} 
}