Collection disappearing when adding items into another collection via a for loop | Tech Off

mVPstar
I'm white because I smelt an onion.

Alright...

I'm trying to iterate through a list, and for each item I'm iterating through another list. Based on certain conditions, I want to add the item to another collection. The problem is, whenever I add it, the initial collection empties out.

Here's a code snippet of what I'm doing:

for (int i = _tempMedia.Count - 1; i >= 0; i--) // Looping through initial collection

{

T mediaItem = _tempMedia[i];

    for(int j = mediaItem.MetaTags.Count - 1; j>=0; j--) //Looping through a List<string> in the current item of initial collection

    {

        string tag = mediaItem.MetaTags[j];

        if (_currentGallery.HasAlbum(tag))

        {

        _currentGallery.Albums[tag].Add(mediaItem); //Add item to second collection

        }
    }
}

Basically, after the item gets added the first time, collection _tempMedia empties out completely, thereby throwin an "Index out of range" error on the next iteration. I had foreach loops before but kept getting "Collection was modified; enumeration operation may not execute." errors. I replaced them with for loops after googling for a solution. Google says I should use the lock keyword, but I have no idea how to use it/if it'll actually work.

What should I do to retain the original collection so that I can iterate through all items and not just the first?

Thanks!

EDIT: SOLVED: http://channel9.msdn.com/forums/TechOff/418221-Collection-disappearing-when-adding-items-into-another-collection-via-a-for-loop/?CommentID=418327

staceyw
Before C# there was darkness...

Debug the collection length before and after the "T mediaItem = tempMedia[i];" line to see if for some reason the collection is popping and causing the 1 off error. Why are you enumerating in reverse instead of forward? Some logic requirement?

figuerres
???

what kind of collection's are they? is this a standard .net type ?

some collections only support forward iteration this may be one of them.

JChung2006

Don't remove items from a collection while iterating through it.

evildictaitor
How could you use the adjective "indescribable" truthfully?

In reply to JChung2006

#Jul 28th @ 9:15 PM

Simple hack to avoid the "don't modify a collection while using it":

Change code of the form:

ICollection<T> collection;

// wrong:
foreach(T f in collection){
modify(f, collection);
}

// right
foreach(var f in new List<T>(collection)) {
modify(f, collection);
}

note that this iterates over the initial state of the collection, so elements added to the collection during the loop won't get changed, and elements removed from the collection before their "turn" in the loop won't get removed from the iteration space.

If your collection supports it, a better way is

foreach(var f in collection.ToArray()){
modify(f, collection);
}

which is slightly faster.

mVPstar
I'm white because I smelt an onion.

In reply to staceyw

#Jul 28th @ 6:24 PM

staceyw said:

Debug the collection length before and after the "T mediaItem = tempMedia[i];" line to see if for some reason the collection is popping and causing the 1 off error.

First run of the loop, mediaItem gets a value and has that value until the next run. On this next run, the Index out of bounds exception occurs right at that statement.

staceyw said:
Why are you enumerating in reverse instead of forward? Some logic requirement?

Paranoia.

I couldn't figure out why my collection was disappearing, so when I googled for solutions to the problem, I copied down one solution exactly as it was written (just to be safe). That solution happened to have enumeration going backwards. When I had it forwards, it made no difference, so I just left it.

mVPstar
I'm white because I smelt an onion.

In reply to figuerres

#Jul 28th @ 7:54 PM

figuerres said:

what kind of collection's are they? is this a standard .net type ?

First one is a List<T>, second is a List<string>.

figuerres said:
some collections only support forward iteration this may be one of them.

I'll change it to forward iteration to see if it makes a difference. There's no reason for it be backwards anyway (see above post).

mVPstar
I'm white because I smelt an onion.

In reply to JChung2006

#Jul 28th @ 9:15 PM

JChung2006 said:

Don't remove items from a collection while iterating through it.

Except I'm having another collection add items from the enumerated collection. I'm not removing (unless calling the add method actually removes the items from the initial collection).

mVPstar
I'm white because I smelt an onion.

In reply to evildictaitor

#Jul 29th @ 12:53 AM

evildictator said:

If your collection supports it, a better way is

foreach(var f in collection.ToArray()){
modify(f, collection);
}