using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var numericList = new List<string>{"100", "--", "-0.98", "N/A", "0.00", "-888"};

        var stringList = new List<string> {"Smith", "--", "Peter", "", "Jim", "Ken", "NA"};

        Console.WriteLine(String.Join(", ", numericList.OrderBy(v => v, new NumberTextComparer())));

        Console.WriteLine(String.Join(", ", stringList.OrderBy(v => v, new NumberTextComparer())));

    }

    public class NumberTextComparer : IComparer<string>

    {

        public int Compare(string s1, string s2)

        {

            double number1, number2;

            var isS1Numeric = double.TryParse(s1, out number1);

            var isS2Numeric = double.TryParse(s2, out number2);

​

            if (isS1Numeric && isS2Numeric)

            {

                if (number1 > number2) return 1;

                if (number1 < number2) return -1;

                return 0;

            }

            if (isS1Numeric)

                return 1;

            if (isS2Numeric)

                return -1;

​

            bool s1StartsWithLetter = char.IsLetter(s1.FirstOrDefault());

            bool s2StartsWithLetter = char.IsLetter(s2.FirstOrDefault());

​

            if (s1StartsWithLetter == s2StartsWithLetter)

                return String.Compare(s1, s2, StringComparison.OrdinalIgnoreCase);

            if (s1StartsWithLetter)

                return -1;

            return 1;

        }

    }

}