What's your favorite LINQ to Objects operator which is not built-in? [closed]

Answer 1

Window

Enumerates arrays ("windows") with the length of size containing the most current values.
{ 0, 1, 2, 3 } becomes to { [0, 1], [1, 2], [2, 3] }.

I am using this for example to draw a line graph by connecting two points.

public static IEnumerable Window(
    this IEnumerable source)
{
    return source.Window(2);
}

public static IEnumerable Window(
    this IEnumerable source, int size)
{
    if (size <= 0)
        throw new ArgumentOutOfRangeException("size");

    return source.Skip(size).WindowHelper(size, source.Take(size));
}

private static IEnumerable WindowHelper(
    this IEnumerable source, int size, IEnumerable init)
{
    Queue q = new Queue(init);

    yield return q.ToArray();

    foreach (var value in source)
    {
        q.Dequeue();
        q.Enqueue(value);
        yield return q.ToArray();
    }
}