That isn't going to work. Your code creates a string, then overwrites the current value repeatedly, leaving only the final "117" value.
Instead, look at using a List or an Array to hold multiple strings:
string[] arrayOfStrings = new string[] { "128", "124", "122", "121", "120", "", "117" };
Array.Sort(arrayOfStrings);
List<string> listOfStrings = new List<string>();
listOfStrings.Add("128");
listOfStrings.Add("124");
listOfStrings.Add("122");
listOfStrings.Add("121");
listOfStrings.Add("120");
listOfStrings.Add("");
listOfStrings.Add("117");
listOfStrings.Sort();
The list is more flexible, as you don't need to know the number of entries at any time - you can just add more.