Encoder.cs

using System.Collections.Generic;
using System.Linq;
using static System.String;

namespace Soundex
{
    public class Encoder
    {
        private const int MaxEncodedLength = 4;
        private const string InvalidDigit = "*";

        class EncodingResult
        {
            public EncodingResult(string curr, string prev)
            {
                Curr = curr;
                Prev = prev;
            }

            public string Curr { get; }
            public string Prev { get; }
            public bool IsValidEncoding => Curr != InvalidDigit;
        }

        private readonly Dictionary<char, int> _digitValues = new Dictionary<char, int>{
            {'b', 1}, {'f', 1}, {'p', 1}, {'v', 1},
            {'c', 2}, {'g', 2}, {'j', 2}, {'k', 2}, {'q', 2},
            {'d', 3}, {'t', 3},
            {'l', 4},
            {'m', 5}, {'n', 5},
            {'r', 6}
        };


        private string ConvertCharcterToNumber(char c)
        {
            c = char.ToLower(c);
            return !_digitValues.ContainsKey(c) ? InvalidDigit : _digitValues[c].ToString();
        }

        private EncodingResult EncodeCharacter(string word, char ch, int index)
        {
            if (index == 0)
            {
                return new EncodingResult(char.ToUpper(ch).ToString(), InvalidDigit);
            }

            return new EncodingResult(ConvertCharcterToNumber(ch), ConvertCharcterToNumber(word[index - 1]));
        }

        public string Encode(string word)
        {
            if (IsNullOrEmpty(word))
            {
                return Empty;
            }

            return word
                .Select((ch, index) => EncodeCharacter(word, ch, index))
                .Where((encodedChar, index) => 
                            encodedChar.IsValidEncoding && encodedChar.Curr != encodedChar.Prev)
                .Select(arg => arg.Curr)
                .Concat(Enumerable.Repeat("0", MaxEncodedLength))
                .Take(MaxEncodedLength)
                .Aggregate((i, j) => i + j);
        }
    }
}

Soundex algorithm using LINQ. Used to test LINQ debuggin with OzCode