Question

Often times I need a collection of non-sequential objects with numeric identifiers. I like using the KeyedCollection for this, but I think there’s a serious drawback. If you use an int for the key, you can no longer access members of the collection by their index (collection[index] is now really collection[key]). Is this a serious enough problem to avoid using the int as the key? What would a preferable alternative be? (maybe int.ToString()?)

I’ve done this before without any major problems, but recently I hit a nasty snag where XML serialization against a KeyedCollection does not work if the key is an int, due to a bug in .NET.

Answer 1

Basically you need to decide if users of the class are likely to be confused by the fact that they can’t, for example, do:

for(int i=0; i=< myCollection.Count; i++) {     ... myCollection[i] ... } 

though they can of course use foreach, or use a cast:

for(int i=0; i=< myCollection.Count; i++) {     ... ((Collection<MyType>)myCollection)[i] ... } 

It’s not an easy decision, as it can easily lead to heisenbugs. I decided to allow it in one of my apps, where access from users of the class was almost exclusively by key.

I’m not sure I’d do so for a shared class library though: in general I’d avoid exposing a KeyedCollection in a public API: instead I would expose IList<T> in a public API, and consumers of the API who need keyed access can define their own internal KeyedCollection with a constructor that takes an IEnumerable<TItem> and populates the collection with it. This means you can easily build a new KeyedCollection from a list retrieved from an API.

Regarding serialization, there is also a performance problem that I reported to Microsoft Connect: the KeyedCollection maintains an internal dictionary as well as a list, and serializes both – it is sufficient to serialize the list as the dictionary can easily be recreated on deserialization.

For this reason as well as the XmlSerialization bug, I’d recommend you avoid serializing a KeyedCollection – instead only serialize the KeyedCollection.Items list.

I don’t like the suggestion of wrapping your int key in another type. It seems to me wrong to add complexity simply so that a type can be used as an item in a KeyedCollection. I’d use a string key (ToString) rather than doing this – this is rather like the VB6 Collection class.

FWIW, I asked the same question some time ago on the MSDN forums. There is a response from a member of the FxCop team, but no conclusive guidelines.