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