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);             }         }