c# .Net随机生成字符串代码

/// <summary>
        /// 随机生成字符串
        /// </summary>
        /// <param name="operationtype">数字字母组合：numberletter 数字：number  大小写组合字母：allletter 大写字母：upletter 小写字母：lowletter</param>
        /// <param name="length">位数</param>
        /// <returns></returns>
        public static string charm(string operationtype, int length)
        {
            string chars = "0123456789abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz";
            switch (operationtype)
            {
                case "numberletter"://全字符模式
                    chars = "0123456789abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz";
                    break;
                case "number"://数字模式
                    chars = "0123456789";
                    break;
                case "allletter"://大小写字母模式
                    chars = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz";
                    break;
                case "upletter"://大写字母模式
                    chars = "abcdefghijklmnopqrstuvwxyz";
                    break;
                case "lowletter"://小写字母模式
                    chars = "abcdefghijklmnopqrstuvwxyz";
                    break;             
            }
            random randrom = new random((int)datetime.now.ticks);

            string str = "";
            for (int i = 0; i < length; i++)
            {
                str += chars[randrom.next(chars.length)];
            }          
            return str;
        }

生成随机字符串，字符组合等...