Killing Torcher r/badcode

using System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
using System.Text;

namespace DeDupe // Submitted to r/badcode by Killing Torcher
{
    class Program
    {
        static void Main(string[] args)
        {
            string deduped;
            Dedupe(Console.ReadLine(), out deduped);
            Console.WriteLine(deduped);
            Console.Read();
        }


        public static void Dedupe(string str, out string target)
        {
            UniqueLetterMatrix ULM = new UniqueLetterMatrix(str);
            StringBuilder sb = new StringBuilder();
            foreach (Letter L in ULM.getVal(true))
            {
                sb.Append(L.letterValue.ToString());
            }
            target = sb.ToString();
        }

        public class UniqueLetterMatrix
        {
            public List<Letter> Letters = new List<Letter>();
            public UniqueLetterMatrix(string value)
            {
                foreach (char c in value)
                {
                    string LetterString = c.ToString();
                    LetterValidator LV = new LetterValidator();
                    LV.LoadString(LetterString);
                    if (string.IsNullOrEmpty(LV.LoadedString))
                    {
                        throw (new MatrixInitializationError("Could not load letter " + LetterString));
                    }
                }
                foreach (char c in value)
                {
                    string realStr = c.ToString();
                    bool seen = false;
                    foreach (Letter L in Letters)
                    {
                        if (L.cmp(new Letter(realStr))) seen = true;
                    }
                    if (!seen) Letters.Add(new FirstSeenLetter(realStr));
                    if (seen) Letters.Add(new MultiSeenLetter(realStr));
                }

            }
            public List<Letter> getVal(bool unique=false)
            {
                List<Letter> temp = new List<Letter>();
                Letter last = null;
                foreach (Letter L in Letters) {
                    if (unique && (L.GetType() == typeof(FirstSeenLetter) || (last != null && !last.cmp(L)))) temp.Add(L);
                    else if(!unique) temp.Add(L);
                    last = L;
                }
                return temp;
            }
        }
        public class MultiSeenLetter : Letter
        {
            public MultiSeenLetter(string letter) : base(letter) { }
            public override string GetValueProper()
            {
                return "";
            }
        }

        public class FirstSeenLetter : Letter
        {
            public FirstSeenLetter(string letter) : base(letter) { }
            public override string GetValueProper()
            {
                return base.GetValueProper();
            }
        }

        public class Letter
        {
            public string letterValue {
                get
                {
                    return _letter;
                }
                set {
                    _letter = value;
                }
            }
            private string _letter;

            public Letter(string letter)
            {
                LetterValidator LV = new LetterValidator();
                LV.LoadString(letter);
                LV.Validate();
                if (string.IsNullOrEmpty(LV.LoadedString)) throw (new LetterValidationError("Failed to validate letter in Letter(string Letter) constructor!"));
                letterValue = LV.LoadedString;
            }
            public virtual string GetValueProper()
            {
                return letterValue.ToString();
            }
            public bool cmp(Letter other)
            {
                return other.letterValue == this.letterValue;
            }
        }

        public class LetterValidator
        {
            public string LoadedString
            {
                get { return _loaded; }
                set { _loaded = value; }
            }
            private string _loaded;
            public void LoadString(string str)
            {
                LoadedString = str;
            }
            public void Validate()
            {
                if (LoadedString.ToLower() != "a" &&
                    LoadedString.ToLower() != "b" &&
                    LoadedString.ToLower() != "c" &&
                    LoadedString.ToLower() != "d" &&
                    LoadedString.ToLower() != "e" &&
                    LoadedString.ToLower() != "f" &&
                    LoadedString.ToLower() != "g" &&
                    LoadedString.ToLower() != "h" &&
                    LoadedString.ToLower() != "i" &&
                    LoadedString.ToLower() != "j" &&
                    LoadedString.ToLower() != "k" &&
                    LoadedString.ToLower() != "l" &&
                    LoadedString.ToLower() != "m" &&
                    LoadedString.ToLower() != "n" &&
                    LoadedString.ToLower() != "o" &&
                    LoadedString.ToLower() != "p" &&
                    LoadedString.ToLower() != "q" &&
                    LoadedString.ToLower() != "r" &&
                    LoadedString.ToLower() != "s" &&
                    LoadedString.ToLower() != "t" &&
                    LoadedString.ToLower() != "u" &&
                    LoadedString.ToLower() != "v" &&
                    LoadedString.ToLower() != "w" &&
                    LoadedString.ToLower() != "x" &&
                    LoadedString.ToLower() != "y" &&
                    LoadedString.ToLower() != "z")
                {
                    LoadedString = "";
                }
            }
        }

        public class LetterValidationError : Exception
        {
            public LetterValidationError(string tx) : base("Validation error: " + tx) { }
        }
        public class MatrixInitializationError : Exception
        {
            public MatrixInitializationError(string tx) : base("Matrix Initialization error: " + tx) { }
        }

    }
}