we can modify the previous answer to make it efficient...
check it..

string x = "asdfsdaaasasa";
            string output = "";
            int count = 0;
            IEnumerator ie = x.GetEnumerator();
            IEnumerator iee = x.GetEnumerator();
            while(ie.MoveNext())
            {
             if (!output.Contains(ie.Current.ToString()))
               {
                while(iee.MoveNext())
                {
                    if ((char)ie.Current == (char)iee.Current)
                    {
                        count++;
                    }
                }
                iee.Reset();                
                output = output + count + ie.Current;
                count = 0;
               }
            }
            Console.WriteLine(output);