以逗號分隔的字符串，剔除重複的字符

                string str = "a,b,b,d,d";

               string[] arr = str.Split(",".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);

               StringBuilder sb = newStringBuilder();

               Hashtable ht = newHashtable();

               foreach (strings in arr)

               {

                   if (!ht.ContainsKey(s))

                   {

                       sb.AppendFormat("{0},", s);

                       ht.Add(s, string.Empty);

                   }

               }

               string resultDay = sb.ToString().TrimEnd(',');