using System.IO;
using System.Linq;

namespace ConsoleApplication1
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var file1 = "file1";
            var file2 = "file2";
            var file3 = "file3";
            File.WriteAllLines(file3, File.ReadAllLines(file1).Except(File.ReadAllLines(file2)));
        }
    }
}