using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var filtroA = "A";

        var filtroB = "B";

        var filtroC = string.Empty;

        var results = new List<string>() {"A", "B", "C"};

        Console.WriteLine("Teste");

​

            var resultado = !results.Any()

                ? results

                : new List<string>().

                    Concat(results.Where(x => !string.IsNullOrEmpty(filtroA) && x == filtroA).DefaultIfEmpty().ToList()).

                    Concat(results.Where(x => !string.IsNullOrEmpty(filtroB) && x == filtroB).ToList()).

                    Concat(results.Where(x => !string.IsNullOrEmpty(filtroC) && x == filtroC).ToList());

​

        Console.WriteLine(string.Join(",", resultado));

    }

}