Question

I think both are same, but why are both used in the same method? I think there is a minor difference. Here is some code to show the difference between the two:

private void LoadItemListing()
{
    _items = new ObservableCollection<SalesItemListingViewModel>();

    foreach (ItemListing x in _sales.Items)
    {
        SalesItemListingViewModel itemListing = new SalesItemListingViewModel(x);
        _items.Add(itemListing);
        _itemAmountSum += itemListing.Amount;

        itemListing.PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(itemListing_PropertyChanged);
    }

    _items.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(_items_CollectionChanged);
}

And for itemListing_PropertyChanged:

void itemListing_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
    if (e.PropertyName == "Amount")
    {
        ItemAmountSum = 0;
        foreach (SalesItemListingViewModel x in Items)
            ItemAmountSum += x.Amount;
    }
}

And this code for _items_CollectionChanged:

void _items_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
    if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
    {
        SalesItemListingViewModel newItemListingViewModel = e.NewItems[0] as SalesItemListingViewModel;
        newItemListingViewModel.PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(itemListing_PropertyChanged);
    }
    else if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Remove)
    {
        ItemAmountSum = 0;
        foreach (SalesItemListingViewModel x in Items)
            ItemAmountSum += x.Amount;
    }

    RaisePropertyChanged("Items");
}

I think there is a difference, but I am not sure. Can somebody please explain any difference?

Answer 1

The PropertyChanged signals that the value of a property has changed. The CollectionChanged event signals that the content of a collection has changed (not the collection itself: it’s still the same collection instance, but elements have been added/removed/replaced).