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using System;
using System.IO;
namespace Example
{
    class Program
    {
        private static void StreamReader()
        {
            string Path = Environment.GetFolderPath(Environment.SpecialFolder.Desktop) + @"\Readme.txt";
            string Text = "!!!Cheat!!!" + Environment.NewLine + "http://gall.dcinside.com/board/view/?id=programming&no=642409&page=2" + Environment.NewLine + "http://gall.dcinside.com/board/view/?id=programming&no=642380&page=2";
            File.WriteAllText(Path, Text);
        }
        static void Main(string[] args)
        {
            Console.Write("Enter a letter: ");
            var Letter = Console.ReadLine();
            
            using (var Reader = new StreamReader(Environment.CurrentDirectory + @"\MOCK_DATA.txt"))
            {
                while (!Reader.EndOfStream)
                {
                    var Line = Reader.ReadLine();
                    if (String.IsNullOrEmpty(Line)) continue;
                    if (Line.IndexOf(Letter, StringComparison.CurrentCultureIgnoreCase) >= 0)
                    {
                        Console.WriteLine(Line);
                    }
                }
            }
            Console.ReadLine();
            StreamReader();
        }
    }
}
 
코드 수정 없이 그대로 복붙하시면 됩니다.

^_^
cs