Length_Conversion.cs

    class Program
    {
        static Dictionary<string,double> conv = new Dictionary<string, double>() { { "mm", 1000 }, { "cm", 100 }, { "miles", 0.000621371192 }, { "inches", 39.3700767 }, { "feet", 3.2808399 }, { "yards", 1.0936133 } };
        static string ConvertLength(string message)
        {
            var arr = message.Split(' ', StringSplitOptions.RemoveEmptyEntries);
            if (arr.Length < 3)
                return "Not enough arguments.";

            var notFound = string.Join("\" and \"", arr.Skip(1).Where(w => !conv.ContainsKey(w)));

            if (!double.TryParse(arr[0], out double val))
                return $"The first argument \"{arr[0]}\" is not a number.";
            else if (notFound.Length != 0)
                return $"I don't understand \"{notFound}\".";
            else
                return $"{val} {arr[1]} is {((val / conv[arr[1]]) * conv[arr[2]]).ToString("N0")} {arr[2]}";
        }
        static void Main(string[] args)
        {
            Console.WriteLine("The options are:\n[{0}].", string.Join(", ", (conv.Select(kvp => kvp.Key))));

            while (true)
            {
                Console.WriteLine("\nNext input: ");
                var input = Console.ReadLine().Trim();
                if (input.ToLower() == "exit") break;
                Console.WriteLine(ConvertLength(input));
            }
        }
    }