这是生成验证码的部分代码，希望有用
[CODE]
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;

/// <summary>
/// CheckCode 的摘要说明
/// </summary>
public class CheckCode
{
public CheckCode()
{
//
// TODO: 在此处添加构造函数逻辑
//
}
public static void DrawImage()
{
CheckCode img = new CheckCode();
HttpContext.Current.Session["CheckCode"] = img.RndNum(4);
img.checkCodes(HttpContext.Current.Session["CheckCode"].ToString());
}
/// <summary>
/// 生成验证图片
/// </summary>
/// <param name="checkCode">验证字符</param>
private void checkCodes(string checkCode)
{
int iwidth = (int)(checkCode.Length * 13);
System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 23);
Graphics g = Graphics.FromImage(image);
g.Clear(Color.White);
//定义颜色
Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
//定义字体
string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
Random rand = new Random();
//随机输出噪点
for (int i = 0; i < 50; i++)
{
int x = rand.Next(image.Width);
int y = rand.Next(image.Height);
g.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);
}

//输出不同字体和颜色的验证码字符
for (int i = 0; i < checkCode.Length; i++)
{
int cindex = rand.Next(7);
int findex = rand.Next(5);

Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
Brush b = new System.Drawing.SolidBrush(c[cindex]);
int ii = 4;
if ((i + 1) % 2 == 0)
{
ii = 2;
}
g.DrawString(checkCode.Substring(i, 1), f, b, 3 + (i * 12), ii);
}
//画一个边框
g.DrawRectangle(new Pen(Color.Black, 0), 0, 0, image.Width - 1, image.Height - 1);

//输出到浏览器
System.IO.MemoryStream ms = new System.IO.MemoryStream();
image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
HttpContext.Current.Response.ClearContent();
//Response.ClearContent();
HttpContext.Current.Response.ContentType = "image/Jpeg";
HttpContext.Current.Response.BinaryWrite(ms.ToArray());
g.Dispose();
image.Dispose();
}

/// <summary>
/// 生成随机的字母
/// </summary>
/// <param name="VcodeNum">生成字母的个数</param>
/// <returns>string</returns>
private string RndNum(int VcodeNum)
{
string Vchar = "0,1,2,3,4,5,6,7,8,9";
string[] VcArray = Vchar.Split(',');
string VNum = ""; //由于字符串很短，就不用StringBuilder了
int temp = -1; //记录上次随机数值，尽量避免生产几个一样的随机数

//采用一个简单的算法以保证生成随机数的不同
Random rand = new Random();
for (int i = 1; i < VcodeNum + 1; i++)
{
if (temp != -1)
{
rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));
}
int t = rand.Next(VcArray.Length);
if (temp != -1 && temp == t)
{
return RndNum(VcodeNum);
}
temp = t;
VNum += VcArray[t];
}
return VNum;
}
}

[/CODE]

像现在的验证码无论它是大写还是小写,都可以用小写来输入,请问2楼这个代码可以这样吗??

请问那应该用什么函数啊??