随机生成四个字符的一个方法

using System.Text;

        System.Random rand = new Random();

        int len = rand.Next(4,4);

        char[] chars = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ".ToCharArray();

        System.Text.StringBuilder myStr = new System.Text.StringBuilder();

        for (int iCount = 0; iCount < len; iCount++)
        {

            myStr.Append(chars[rand.Next(chars.Length)]);

        }

        string text = myStr.ToString();

        // 保存验证码到 session 中以便其他模块使用

        this.Session["checkcode"] = text;