using System;

using System.IO;

using System.Text;

using System.Collections.Generic;

​

class Transformer {

    readonly StreamReader _input;

    readonly Dictionary<string, char> _str2Char;

    public Transformer(Stream stream, Dictionary<string, char> str2Char) {

        _input = new StreamReader(stream);

        _str2Char = str2Char;

    }

    public Stream Transfrom() {

        var output = new StreamWriter(new MemoryStream());

        int c = -1;

        // Buffer to keep read chars.

        var buffer = new StringBuilder();

        // Read until the end of file (-1)

        while ((c = _input.Read()) != -1) {

            char ch = (char)c;

            // We separate input stream by upper char

            if (char.IsUpper(ch)) {

                if (buffer.Length >= 2) {

                    var str = buffer.ToString();

                    // We have to process "KeyOemcomma" individually

                    bool isSpecialCase = str == "Key" && ch == 'O';

                    if (!isSpecialCase) {

                        WriteToOutput(str, output);

                        buffer.Clear();

                    }

                    buffer.Append(ch);

                }

                else buffer.Append(ch);

            }

            else buffer.Append(ch);

        }

        if (buffer.Length > 0) {

            WriteToOutput(buffer.ToString(), output);

            buffer.Clear();

        }

        output.Flush();

        return output.BaseStream;

    }

    void WriteToOutput(string str, StreamWriter output) {

        // Console.WriteLine(str);

        char c;

        if (_str2Char.TryGetValue(str, out c))

            output.Write(c);

        else 

            output.Write(str);

    }

}

​

public class Program

{

    public static void Main()

    {

        Dictionary<string, char> str2Char = new Dictionary<string, char> {

            {"Return", '\n'},

            {"Space", ' '},

            {"KeyOemcomma", '?'}

        };

        string str = "Hello!ReturnHowSpaceAreSpaceYouKeyOemcomma";

        var input = new MemoryStream(System.Text.Encoding.UTF8.GetBytes(str));

        var t = new Transformer(input, str2Char);

        var output = t.Transfrom();

        // Start reading output stream from the begining

        output.Seek(0, SeekOrigin.Begin);

        var s = new StreamReader(output);

        int c;

        while ((c = s.Read()) != -1) {

            Console.Write((char)c);

        }

    }

}