PhonewordTranslator.cs

using System.Runtime.CompilerServices;
using System.Text;

namespace Core
{
	public static class PhonewordTranslator
	{
		public static string ToNumber(string raw)
        {
			if (string.IsNullOrWhiteSpace(raw))
				return null;

			raw = raw.ToUpperInvariant();

			var newNumber = new StringBuilder();
			foreach (var c in raw)
            {
				if (" -0123456789".Contains(c))
					newNumber.Append(c);
				else
                {
					var result = TranslateToNumber(c);
					if (result != null)
						newNumber.Append(result);
					else
						return null;
                }
            }
			return newNumber.ToString();
        }

		static bool Contains(this string keyString, char c)
        {
			return keyString.IndexOf(c) >= 0;
        }

		static readonly string[] digits =
		{
			"ABC", "DEF", "GHI", "JKL", "MNO", "PQRS", "TUV", "WXYZ"
		};

		static int? TranslateToNumber(char c)
        {
			for (int i = 0; i < digits.Length; i++)
            {
				if (digits[i].Contains(c))
					return 2 + i;
            }
			return null;
        }
	}
}