asp.net下生成英文字符数字验证码的代码

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));
}
}