How does ObservableCollection.Add work?

Question

I was trying to implement a specialized collection that works like ObservableCollection to encapsulate some more mechanisms in it, to do that i also let my collection inherit from Collection and i also implement the same interfaces.

I just do not get though how one actually implements the whole collection-changed-logic, for example Collection<T>.Add is not being overridden (it is not even marked as virtual), so how does the ObservableCollection fire the CollectionChanged event if items were added using that method?

score 10 · Accepted Answer · answered Feb 16 '11 at 22:11

10

To answer your specific question, Collection<T>.Add calls the InsertItem virtual method (after checking that the collection is not read-only). ObservableCollection<T> indeed overrides this method to do the insert and raise the relevant change notifications.

answered Feb 16 '11 at 22:11

Ani

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Ah, so that's how that works, thank you once again! (How do you even know that?) – H.B. Feb 16 '11 at 22:14
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@H.B., have you heard of [Reflector](http://www.red-gate.com/products/dotnet-development/reflector/)? – Darin Dimitrov Feb 16 '11 at 22:16
@Darin I have but they told me it costs money now and they will hunt me down if I leave a copy on my system http://tirania.org/blog/archive/2011/Feb-04.html – Aaron McIver Feb 16 '11 at 22:18
@H.B., nobody forces you to use V7. But I must say that it's a crappy decision from RedGate to make the best .NET tool ever being paid. – Darin Dimitrov Feb 16 '11 at 22:19
@Darin: All of the versions are time-bombed, so our free ride is going to come to an end soon no matter what. – Ani Feb 16 '11 at 22:20
@Ani, yeah, that really sucks badly. Will have to crack it. Never gonna pay 35 bucks for it. – Darin Dimitrov Feb 16 '11 at 22:21
I heard of reflector but i was quite unaware of its uses and i never used it myself so far. Thank you for the link, maybe i shall give the free version a spin some time. – H.B. Feb 16 '11 at 22:22
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@H.B.: Don't waste any time on Reflector; ILSpy is the future. – Rick Sladkey Feb 16 '11 at 22:36
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@Darin Dimitrov *"Best .NET tool"* *"Will have to crack it. Never gonna pay 35 bucks for it."* ...Your concept of value is completely baffling to me. – Dan J Feb 16 '11 at 23:01
Reflector 6.8.2.5 is the version I use. Free and without time limits. – AMissico Feb 06 '12 at 17:18

score 8 · Answer 2 · answered Feb 16 '11 at 22:14

8

It does so by calling InsertItem which is overridden and can be seen upon decompilation

protected override void InsertItem(int index, T item)
{
    this.CheckReentrancy();
    base.InsertItem(index, item);
    this.OnPropertyChanged("Count");
    this.OnPropertyChanged("Item[]");
    this.OnCollectionChanged(NotifyCollectionChangedAction.Add, item, index);
}

answered Feb 16 '11 at 22:14

Aaron McIver

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Thank you, too, Ani was faster though so i handed this one to him/her. – H.B. Feb 16 '11 at 22:25

score 0 · Answer 3 · answered Feb 16 '11 at 22:12

Remember, the key is not in overriding the base Collection methods, it's in the fact that you will be implementing the ICollection interface. And frankly, rather than inheriting from a Collection class, I would suggest instead creating an adapter class that takes a ICollection in the constructor and your methods will just delegate to the inner collection and raise the appropriate events.

How does ObservableCollection.Add work?

3 Answers3