c# 过滤字符串中的重复字符

public class OperationString
    {
        /// <summary>
        /// 过滤字符串中的重复字符
        /// </summary>
        /// <param name="str">要过滤的字符串</param>
        /// <returns>返回过滤后的字符串</returns>
        public string FilterRepetitionChar(string sourceStr)
        {
            string returnStr = string.Empty;
            string[] strList = sourceStr.Split(‘,‘);
            Hashtable ht = new Hashtable();
            foreach (string strChar in strList)
            {
                if (!ht.ContainsKey(strChar))
                {
                    ht.Add(strChar, strChar);//这里让ht的key和value值相等，不影响下面的程序
                    returnStr += strChar + ",";//字符以逗号分隔
                }
            }
            returnStr = returnStr.Trim(‘,‘);//去掉最后一个逗号
            return returnStr;
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            string sourceString = "a,b,c,d,a,b";
            string newString = string.Empty;

            OperationString os = new OperationString();
            newString = os.FilterRepetitionChar(sourceString);

            Console.WriteLine("过滤前的字符串：" + sourceString);
            Console.WriteLine("----------------------------------");
            Console.WriteLine("过滤后的字符串：" + newString);

            Console.ReadKey();
        }
    }