These are the two options I managed to build, not having the luxury of working with var
type, nor with additional variables on the line:
string f = "aS.".Substring(0, "aS.".IndexOf("S"));
Console.WriteLine(f);
string s = "aS.".Split("S".ToCharArray(),StringSplitOptions.RemoveEmptyEntries)[0];
Console.WriteLine(s);
This is what it gets: