c#生成不重复的字符串

using System;

namespace DotNet.Utilities
{
    public class RandomOperate
    {
        // 一：随机生成不重复数字字符串  
        private int rep = 0;
        public string GenerateCheckCodeNum(int codeCount)
        {
            string str = string.Empty;
            long num2 = DateTime.Now.Ticks + this.rep;
            this.rep++;
            Random random = new Random(((int)(((ulong)num2) & 0xffffffffL)) | ((int)(num2 >> this.rep)));
            for (int i = 0; i < codeCount; i++)
            {
                int num = random.Next();
                str = str + ((char)(0x30 + ((ushort)(num % 10)))).ToString();
            }
            return str;
        }

        //方法二：随机生成字符串（数字和字母混和）
        public string GenerateCheckCode(int codeCount)
        {
            string str = string.Empty;
            long num2 = DateTime.Now.Ticks + this.rep;
            this.rep++;
            Random random = new Random(((int)(((ulong)num2) & 0xffffffffL)) | ((int)(num2 >> this.rep)));
            for (int i = 0; i < codeCount; i++)
            {
                char ch;
                int num = random.Next();
                if ((num % 2) == 0)
                {
                    ch = (char)(0x30 + ((ushort)(num % 10)));
                }
                else
                {
                    ch = (char)(0x41 + ((ushort)(num % 0x1a)));
                }
                str = str + ch.ToString();
            }
            return str;
        }

        #region

        /// 

        /// 从字符串里随机得到，规定个数的字符串.
        /// 
        /// 
        /// 
        /// 
        private string GetRandomCode(string allChar, int CodeCount)
        {
            //string allChar = "1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,i,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z"; 
            string[] allCharArray = allChar.Split(',');
            string RandomCode = "";
            int temp = -1;
            Random rand = new Random();
            for (int i = 0; i < CodeCount; i++)
            {
                if (temp != -1)
                {
                    rand = new Random(temp * i * ((int)DateTime.Now.Ticks));
                }

                int t = rand.Next(allCharArray.Length - 1);

                while (temp == t)
                {
                    t = rand.Next(allCharArray.Length - 1);
                }

                temp = t;
                RandomCode += allCharArray[t];
            }
            return RandomCode;
        }

        #endregion
    }
}