c# .Net隨機生成字符串代碼

/// <summary>
        /// 隨機生成字符串
        /// </summary>
        /// <param name="OperationType">數字字母組合：NUMBERLETTER 數字：NUMBER  大小寫組合字母：ALLLETTER 大寫字母：UPLETTER 小寫字母：LOWLETTER</param>
        /// <param name="Length">位數</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"://數字模式
                    chars = "0123456789";
                    break;
                case "ALLLETTER"://大小寫字母模式
                    chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
                    break;
                case "UPLETTER"://大寫字母模式
                    chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
                    break;
                case "LOWLETTER"://小寫字母模式
                    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;
        }

生成隨機字符串，字符組合等...