c#中的observablecollection的CollectionChanged事件

Question

如何更好地编写此代码:

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;
            }
        }
    }
}

Answer 1

1. 当e.Action == NotifyCollectionChangedAction.Remove你需要迭代e.OldItems而不是e.NewItems.
2. 当e.Action == NotifyCollectionChangedAction.Replace您需要迭代e.OldItems以从旧项中删除事件处理程序时,您需要迭代e.NewItems以将事件处理程序添加到新项.

3. 像这样重构代码:

   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;
           }
       }
   }
   

这是有效的,因为所有的Promotion类都实现了INotifyPropertyChanged.