using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Tree".MakePlural(2));

        Console.WriteLine("Tree".MakePlural(1));

        Console.WriteLine("Fish".MakePlural(2));

        Console.WriteLine("winery".MakePlural(2));

        Console.WriteLine("shoe".MakePlural(2));

        Console.WriteLine("sample".MakePlural(2));

        Console.WriteLine("kiss".MakePlural(2));

        Console.WriteLine("country".MakePlural(2));

    }

}

​

public static class StringExtensions

{

        /// <summary>

        ///     Makes a term singular or plural based on English language grammar rules.

        ///     Attempts to take into account strings that are already plural and will return the orignal term

        /// </summary>

        /// <param name="term">The term.</param>

        /// <param name="count">The count.</param>

        /// <returns>System.String.</returns>

        /// <remarks>

        ///     Note: this is a general algorithm.

        ///     English is weird. does nto take into account language irregularities or apophonic digraphs

        /// </remarks>

        public static string MakePlural(this string term, int count)

        {

            if (string.IsNullOrEmpty(term)) return "";

            if (term.Length < 2) return term;

            if (count == 1) return term;

            var lastChar = term[term.Length - 1];

            var nextLastChar = term[term.Length - 2];

            switch (lastChar)

            {

                case 's':

                case 'h':

                case 'o':

                {

                    return term + "es";

                }

                case 'y':

                {

                    if ((nextLastChar <= 'e') || (nextLastChar != 'i' && nextLastChar != 'o' && nextLastChar != 'u'))

                    {

                        return term.Substring(0, term.Length - 1) + "ies";

                    }

                    break;

                }

                default:

                    return term + "s";

            }

            return term + "s";

        }   

}