Programming contest example (Palindromes)

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

namespace unideb_mother_bear
{
    // Mother Bear (example)
    // https://www.inf.unideb.hu/progcont/exercises.html?pid=10945

    class Program
    {
        const string EATYOU = "Uh oh...";
        const string NOTEATYOU = "You won't be eaten!";

        static void Main(string[] args)
        {
            char[] unneededChars = new char[] { ',', ' ', '.', '!', '?' };
            string[] lines = ReadFromFile(new StreamReader("in.txt"));
            lines = RemoveCharacters(lines, unneededChars);
            string[] messages = new string[lines.Length];
            for (int i = 0; i < lines.Length; i++)
            {
                bool result = IsPalindrome(lines[i]);
                Console.WriteLine("{0} {1} palindrome!", lines[i], result ? "is" : "is not");
                messages[i] = result ? NOTEATYOU : EATYOU;
            }
            WriteToFile(messages, new StreamWriter("out.txt"));
            Console.WriteLine("...written in file!");
            Console.ReadLine();
        }

        static string[] ReadFromFile(StreamReader sr)
        {
            string content = sr.ReadToEnd();
            content = content.Substring(0, content.IndexOf("DONE"));
            return content.Split(new char[] { '\n', '\r' }, StringSplitOptions.RemoveEmptyEntries);
        }

        static string[] RemoveCharacters(string[] lines, char[] removableChars)
        {
            for (int i = 0; i < lines.Length; i++)
            {
                // Remove upper chars
                lines[i] = lines[i].ToLower();
                List<char> chars = new List<char>();
                // Split the string to characters.
                char[] source = lines[i].ToCharArray();
                for (int j = 0; j < source.Length; j++)
                {
                    for (int k = 0; k < removableChars.Length; k++)
                    {
                        if (source[j] == removableChars[k])
                        {
                            // We are sure that this char is not needed.
                            break;
                        }
                        if (k == removableChars.Length - 1)
                        {
                            // Iteration done, no break happened: we add this char to the list!
                            chars.Add(source[j]);
                        }
                        // We continue iterating.
                    }
                }
                lines[i] = new string(chars.ToArray());
            }
            return lines;
        }

        static bool IsPalindrome(string str)
        {
            for (int i = 0; i < str.Length / 2; i++)
            {
                if (str[i] != str[str.Length - i - 1])
                {
                    return false;
                }
            }
            return true;
        }

        static void WriteToFile(string[] lines, StreamWriter sw)
        {
            if (sw != null)
            {
                foreach (string line in lines)
                {
                    sw.WriteLine(line);
                }
                sw.Close();
            }
            else throw new ArgumentNullException("StreamWriter is null!");
        }
    }
}