Question

I have the presumably common problem of having elements that I wish to place in 2 (or more) lists. However sometimes I want to find an element that could be in one of the lists. Now there is more than one way of doing this eg using linq or appending, but all seem to involve the unnecessary creation of an extra list containing all the elements of the separate lists and hence waste processing time.

So I was considering creating my own generic FindinLists class which would take 2 lists as its constructor parameters would provide a Find() and an Exists() methods. The Find and Exists methods would only need to search the second or subsequent lists if the item was not found in the first list. The FindInLists class could be instantiated in the getter of a ( no setter)property. A second constructor for the FindInLists class could take an array of lists as its parameter.

Is this useful or is there already a way to search multiple lists without incurring the wasteful overhead of the creation of a super list?

Answer 1

I believe IEnumerable<T>.Concat() is what you need. It doesn’t create an extra list, it only iterates through the given pair of collections when queried

Concat() uses deferred execution, so at the time it’s called it only creates an iterator which stores the reference to both concatenated IEnumerables. At the time the resulting collection is enumerated, it iterates through first and then through the second.
Here’s the decompiled code for the iterator – no rocket science going on there:

private static IEnumerable<TSource> ConcatIterator<TSource>(IEnumerable<TSource> first, IEnumerable<TSource> second)
{
    foreach (TSource iteratorVariable0 in first)
    {
        yield return iteratorVariable0;
    }
    foreach (TSource iteratorVariable1 in second)
    {
        yield return iteratorVariable1;
    }
}

When looking to the docs for Concat(), I’ve stumbled across another alternative I didn’t know – SelectMany. Given a collection of collections it allows you to work with the children of all parent collections at once like this:

IEnumerable<string> concatenated = new[] { firstColl, secondColl }
    .SelectMany(item => item);