Find sequence in IEnumerable using Linq

Question

What is the most efficient way to find a sequence within a IEnumerable using LINQ

I want to be able to create an extension method which allows

Answer 1

I understand this is an old question, but I needed this exact method and I wrote it up like so:

public static int ContainsSubsequence(this IEnumerable elements, IEnumerable subSequence) where T: IEquatable
{
    return ContainsSubsequence(elements, 0, subSequence);
}

private static int ContainsSubsequence(IEnumerable elements, int index, IEnumerable subSequence) where T: IEquatable
{
    // Do we have any elements left?
    bool elementsLeft = elements.Any();

    // Do we have any of the sub-sequence left?
    bool sequenceLeft = subSequence.Any();

    // No elements but sub-sequence not fully matched
    if (!elementsLeft && sequenceLeft)
        return -1; // Nope, didn't match

    // No elements of sub-sequence, which means even if there are
    // more elements, we matched the sub-sequence fully
    if (!sequenceLeft)
        return index - subSequence.Count(); // Matched!

    // If we didn't reach a terminal condition,
    // check the first element of the sub-sequence against the first element
    if (subSequence.First().Equals(e.First()))
        // Yes, it matched - move onto the next. Consume (skip) one element in each
        return ContainsSubsequence(elements.Skip(1), index + 1 subSequence.Skip(1));
    else
        // No, it didn't match. Try the next element, without consuming an element
        // from the sub-sequence
        return ContainsSubsequence(elements.Skip(1), index + 1, subSequence);
}

Updated to not just return if the sub-sequence matched, but where it started in the original sequence.

This is an extension method on IEnumerable, fully lazy, terminates early and is far more linq-ified than the currently up-voted answer. Bewarned, however (as @wai-ha-lee points out) it is recursive and creates a lot of enumerators. Use it where applicable (performance/memory). This was fine for my needs, but YMMV.