public static IEnumerable<string> ReadLines(string fileName)
        {
            using (TextReader reader = File.OpenText(fileName))
            {
                string line;
                if ((line = reader.ReadLine()) != null)
                {
                    yield return line;
                }
            }
        }
        static void Main()
        {
            foreach (string line in Iterator.ReadLines(""))
            {
                Console.WriteLine(line);
            }
        }

       public static IEnumerable<T> where<T>(IEnumerable<T> source, Predicate<T> predicate)
        {
            if (source == null || predicate == null)
            {
                throw new ArgumentNullException();
            }
            return WhereImplemeter(source, predicate);
        }
       private static IEnumerable<T> WhereImplemeter<T>(IEnumerable<T> source, Predicate<T> predicate)
        {
            foreach (T item in source)
            {
                if (predicate(item))
                {
                    yield return item;
                }
            }
        }
        static void Main()
        {
            IEnumerable<string> lines = File.ReadAllLines(@"your file name");
            Predicate<string> predicate = delegate(string line)
            {
                return line.StartsWith("using");
            };
            foreach (string str in where(lines, predicate))
            {
                Console.WriteLine(str);
            }

        }