Srting 字符串去掉重複的數據

string a="1,2,2,3,4,5,6,6,7,7,7,8";

如果過濾字符串相同的數據，顯示爲：1,2,3,4,5,6,7,8

String str = "1,2,2,3,4,5,6,6,7,7,7,8";
                HashSet<String> db = new HashSet<String>(str.Split(','));
                String s = String.Join(",", db);

string str = "8912341253789";
    str=new string(str.ToCharArray().Distinct().ToArray());
    Console.WriteLine(str); //89123457