Foreach while adding items to the looping collection

Question

This is the situation:

I'm browsing through some code and I wondered if the following statement takes a reference of the selected collection or a copy with which it replaces the original object when the foreach loop finishes. If the first, will it take the new found pages and join them in the loop?

foreach(Page page in Pages)
{
    page.AddRange(RetrieveSubPages(page.Id));
}

Edit: I'm sorry, I made a typo.

It should be this:

foreach(Page page in pages)
{
    pages.AddRange(RetrieveSubPages(page.Id));
}

What i tried to say is that if i add some objects to the enumerating collection, will it join those objects in the foreach?

Answer 1

foreach uses an enumerator. The collection over which you loop using foreach, has to implement IEnumerable (or IEnumerable<T>). Then, foreach calls the GetEnumerator method of that collection, and uses the Enumerator to traverse the collection.

Answer 2

In most cases, foreach works against the live collection (no explicit clone), and if you try to change the collection while enumerating it, then the enumerator breaks with an exception. So if you are adding to Pages, expect problems.

Answer 3

It looks like the code doesn't modify the Pages collection, but the content of the objects in the Page objects in the Pages collection. The Page type having at least collection like method.

In general each collection implements iteration in a way suitable for itself, and generally becomes unmodifiable while iterating, but one could implelment a collection which iterates by taking a snapshot of itself.

There is no mechanism to detect exit from a loop which would allow action to be taken at that point (consider how this would interact with exceptions, break and return in the body of the loop).

Answer 4

I'm pretty sure you'll get an exception thrown complaining that the underlying collection was modified

Answer 5

You are not modifying the collection you are enumerating, therefore you won't have any problems with this code.

It is also irrelevant, if an clone of the collection is being enumerated, because the objects contained by both, collection and clone, are still the same (reference equals).

Answer 6

I think the safest way is this:

Array<Page> newpages = new Array<Page>();    
foreach(Page page in pages)
{
    newpages.AddRange(RetrieveSubPages(page.Id));
}

pages.AddRange(newpages);

You'd have to extend this a bit if you wanted to recurse into the subpages.

Answer 7

In response to you question, it does not make a copy.

It creates an enumerator and iterates through the collection. If the collection is changed while this enumeration is happening, in the foreach itself, or asynchronously, you will get an exception:

An unhandled exception of type 'System.InvalidOperationException' occurred in mscorlib.dll

Additional information: Collection was modified; enumeration operation may not execute.

You can, use a temporary collection and join the two afterwards, or just not use an enumerator.

    for (int i = 0; i < pages.Count; i++)
    {
        test.AddRange(RetrieveSubPages(pages[i].Id));
    }