以逗号分隔的字符串，剔除重复的字符

                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(',');