CollectionChanged event of observablecollection in c# [closed]

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How can write this code better:

void CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
    if (sender is ObservableCollection<PromotionPurchaseAmount>)
    {
        if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            foreach (PromotionPurchaseAmount item in e.NewItems)
            {
                //Removed items
                item.PropertyChanged -= EntityViewModelPropertyChanged;
            }
        }
        else if (e.Action == NotifyCollectionChangedAction.Add)
        {
            foreach (PromotionPurchaseAmount item in e.NewItems)
            {
                //Added items
                item.PropertyChanged += EntityViewModelPropertyChanged;
            }
        }
    }
    else if (sender is ObservableCollection<PromotionItemPricing>)
    {
        if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            foreach (PromotionItemPricing item in e.NewItems)
            {
                //Removed items
                item.PropertyChanged -= EntityViewModelPropertyChanged;
            }
        }
        else if (e.Action == NotifyCollectionChangedAction.Add)
        {
            foreach (PromotionItemPricing item in e.NewItems)
            {
                //Added items
                item.PropertyChanged += EntityViewModelPropertyChanged;
            }
        }
    }
    else if (sender is ObservableCollection<PromotionItem>)
    {
        if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            foreach (PromotionItem item in e.NewItems)
            {
                //Removed items
                item.PropertyChanged -= EntityViewModelPropertyChanged;
            }
        }
        else if (e.Action == NotifyCollectionChangedAction.Add)
        {
            foreach (PromotionItem item in e.NewItems)
            {
                //Added items
                item.PropertyChanged += EntityViewModelPropertyChanged;
            }
        }
    }
}

---

1. When e.Action == NotifyCollectionChangedAction.Remove you need to iterate over e.OldItems instead of e.NewItems.
2. When e.Action == NotifyCollectionChangedAction.Replace you need to iterate over e.OldItems to remove the event handler from the old items and you need to iterate over e.NewItems to add the event handler to the new items.

3. Refactor the code like this:

   if (sender is ObservableCollection<PromotionPurchaseAmount> || 
       sender is ObservableCollection<PromotionItemPricing> || 
       sender is ObservableCollection<PromotionItem>)
   {
       if (e.Action == NotifyCollectionChangedAction.Remove ||
           e.Action == NotifyCollectionChangedAction.Replace)
       {
   
           foreach (INotifyPropertyChanged item in e.OldItems)
           {
               //Removed items
               item.PropertyChanged -= EntityViewModelPropertyChanged;
           }
       }
       if (e.Action == NotifyCollectionChangedAction.Add ||
           e.Action == NotifyCollectionChangedAction.Replace)
       {
           foreach (INotifyPropertyChanged item in e.NewItems)
           {
               //Added items
               item.PropertyChanged += EntityViewModelPropertyChanged;
           }
       }
   }
   

This works, because all of your Promotion classes implement INotifyPropertyChanged.