PropertyChangedEventHandler 和 NotifyCollectionChangedEventHa

我认为两者都是一样的，但为什么两者都以相同的方法使用？我认为有一个细微的区别。下面是一些代码来显示两者之间的区别：

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);
}

对于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;
    }
}

而这段代码_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");
}

我认为有区别，但我不确定。有人可以解释一下任何区别吗？