public static IEnumerable<T> Where<T>(this IEnumerable<T> source, Func<T, bool> predicate)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (predicate == null) throw new ArgumentNullException(nameof(predicate));

    return Iterator();

    IEnumerable<T> Iterator()
    {
        foreach (T item in source)
        {
            if (predicate(item))
            {
                yield return item;
            }
        }
    }
}