ObservableCollection: raising event when updating values

Question

"Occurs when an item is added, removed, changed, moved, or the entire list is refreshed." This is written in MSDN, talking about CollectionChanged event... but.. what does it mean with "changed"??

I have an observableCollection. It's populated from db and it is binded in my view (i'm working with mvvm pattern).

So, I think: "well, from my view I edit my fields, binded with the observableCollection". With the debug, I see the fields updated, but... the event is not fired... why?

What does the msdn mean with "changed"? and, if this way is wrong, how can I trigger my action when the value of the observablecollection are updated in the way I explained before?

possible duplicate of [ObservableCollection not updating](http://stackoverflow.com/questions/7941559/observablecollection-not-updating) — paparazzo, Sep 22 '14 at 15:31

score 1 · Answer 1 · answered Sep 22 '14 at 15:08

1

In this context changed means replacing the item at a certain index with another item, not mutating an existing item. Mutating an item in the collection is not a change in the collection, nor does it have a way of observing such changes in the general case.

answered Sep 22 '14 at 15:08

Servy

202,030
26
332
449

and so, in your opinion, how can I implement this control? – Piero Alberto Sep 22 '14 at 15:14
@PieroAlberto If you want to know when the objects in the collection are mutated you need to put objects in the collection that notify you when they are mutated. `ObservableCollection` won't do it for you. – Servy Sep 22 '14 at 15:16
it is a collection made of entity framework edmx class..how can I do it? – Piero Alberto Sep 22 '14 at 15:44
@PieroAlberto Sounds like you just need to spend some time reading through tutorials on the topic. – Servy Sep 22 '14 at 15:46
i know it... any links about this? – Piero Alberto Sep 22 '14 at 15:46
@PieroAlberto Google is full of links on the topic. – Servy Sep 22 '14 at 15:47

score 0 · Accepted Answer · answered Sep 22 '14 at 15:16

You're confusing the INotifyCollectionChanged interface with the INotifyPropertyChanged interface. It sounds like you want to know when any property of any item in the collection changes. In order to make that happen, you will need to implement the INotifyPropertyChanged interface in your data type class and attach a handler to the INotifyPropertyChanged.PropertyChanged event:

item.PropertyChanged += Item_PropertyChanged;

private void Item_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    // The 'e.PropertyName' property changed
}

You can improve this further by extending the ObservableCollection<YourDataType> class and adding this code into that class:

public new void Add(T item)
{
    item.PropertyChanged += Item_PropertyChanged;
    base.Add(item);
}

...

public new bool Remove(T item)
{
    if (item == null) return false;
    item.PropertyChanged -= Item_PropertyChanged;
    return base.Remove(item);
}

There are also other methods that you should override in this way, but once you have, you can just let this class do all of the work for you... even better if you put it into some kind of generic base class. If you make that class also implement the INotifyPropertyChanged interface, then you can also 'forward' all property changes for convenience:

private void Item_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    NotifyPropertyChanged(e.PropertyName);
}

ObservableCollection: raising event when updating values

2 Answers2