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I have two collections of strings: CollectionA is a StringCollection property of an object stored in the system, while CollectionB is a List generated at runtime. CollectionA needs to be updated to match CollectionB if there are any differences. So I devised what I expected to be a simple LINQ method to perform the removal.

var strDifferences = CollectionA.Where(foo => !CollectionB.Contains(foo)); foreach (var strVar in strDifferences) { CollectionA.Remove(strVar); }

But I am getting a "Collection was modified; enumeration operation may not execute" error on strDifferences... even though it is a separate enumerable from the collection being modified! I originally devised this explicitly to evade this error, as my first implementation would produce it (as I was enumerating across CollectionA and just removing when !CollectionB.Contains(str)). Can anyone shed some insight into why this enumeration is failing?

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The two are not completely separate. Where does not make a separate copy of the collection. It internally keeps a reference to the original collection and fetches elements from it as you request them.

You can solve your problem by adding ToList() to force Where to iterate through the collection immediately.

var strDifferences = CollectionA .Where(foo => !CollectionB.Contains(foo)) .ToList();
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Wouldn't ToArray() be better here? There is no need to use List.
@svick That's something I've wondered about for quite some time, but that'd be a question I'd ask on another day. Provided it hasn't already been asked, of course -chuckle-
@GraceNote catch that here: stackoverflow.com/questions/1105990/…
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Where passes back IEnumerable<T> and then you are using that in your foreach (var strVar in strDifferences)

You are then trying to remove it from the collection that created the IEnumerable<T>. You haven't created a new list, it's referencing CollectionA to pull the next item from, so you can't edit CollectionA.

You can do this also:

var strDifferences = CollectionA.Where (foo => CollectionB.Contains(foo)).ToList(); CollectionA = strDifferences; //or instead of reassigning CollectionA CollectionA.Clear(); CollectionA.AddRange(strDifferences);

Since you are removing the ones that aren't in CollectionB. Just look for the ones that are, create a list and assign that list to the CollectionA variable.

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Ah... I'd love to use your alternative (however identical in effect it is), but unfortunately CollectionA, it turns out, is a read-only property, so I can't set it. You do give a much more in-depth explanation, though, so +1 for that.
@ccornet if can't set it you could clear it and add the items back in.
I thought about it long and hard. I decided to still go with the removal approach. I don't wish to modify CollectionA if no change is necessary, building a list of what needs to be removed makes this very quick and simple to know (just see if strDifferences.Count > 0). If I build an array of the goal, I'd still have to check if CollectionA has something that needs to be removed.
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Try modifying the first line a bit:

var strDifferences = CollectionA.Where(foo => !CollectionB.Contains(foo)).ToList();

I think LINQ is using lazy execution of your query. The call to ToList will force execution of the query prior to enumerating.

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Yes, and make sure not to put it over two lines i..e. var strDifferencesTemp = CollectionA.Where(foo => !CollectionB.Contains(foo)); var strDifferences = strDifferencesTemp.ToList(); if you do so, the collection can be modified by some other thread between these two lines.
@Markus if multi-threading, it doesn't make any difference even you put it in one line. You'd better consider immutablelist in that case.

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