I have a public property called Items, It's a List. I want to tell when it's been altered. How can I do this?
For example, if Items.Add is called, I want to be able to then call UpdateInnerList.
How can I do this?
+2
A:
How about creating a List subclass and overriding the Add method?
void Main()
{
var x=new MySpecialList<string>();
x.Add("hello");
}
class MySpecialList<T>:List<T>
{
public new void Add(T item)
{
//special action here
Console.WriteLine("added "+item);
base.Add(item);
}
}
spender
2010-07-22 16:18:38
And if it's stuffed into the base `List<T>` as a variable, this `.Add` will never be called.
Marc
2010-07-22 16:24:12
"stuffed into the base..."? I don't understand what you mean.
spender
2010-07-22 16:26:58
`List<int> items = new MySpecialList<int>(); items.Add(5);` There is no console output written. It's the bane of using `new` to shadow the base class's method instead of, for example, implementing `IList<T>` instead of what you're suggesting.
Marc
2010-07-22 16:28:25
My only caveat about using `ObservableCollection` is that if you need to action the items that are changing, the `.Clear()` method raises the changed event but has no items in the arguments, thus, you do not know what was removed. This is obviously only relevant if you care.
Marc
2010-07-22 16:21:53
@Mark, in the case of Clear() I would suspect that a Reset would be the action in the CollectionChanged event. In other cases like Add, Remove I think that there are members on the NotifyCollectionChangedEventArgs that can be used? http://msdn.microsoft.com/en-us/library/system.collections.specialized.notifycollectionchangedeventargs.action.aspx
Wil P
2010-07-22 16:27:33
Yes, ObservableCollection is only available 3.5 or higher, but the question is tagged for .Net 3.5.
sixlettervariables
2010-07-22 16:29:41
@Wil P, I wish. When you call `.Clear()`, the event is raised (as you say of type Reset) and both `NewItems` and `OldItems` are null. I believe the `NotifyCollectionChangedEventArgs` actually throw if you attempt to circumvent that behavior.
Marc
2010-07-22 16:31:30
@Wil P, just reread your comment, yes, the items affected are present for all events on the event arguments _except_ the Reset. This is important to those that wish to action those items there were cleared away.
Marc
2010-07-22 16:34:20
Could you provide a code sample? I'm having difficulty getting this to work. Are you sure this will work for WinForms? My research shows this is part of the WPF
Malfist
2010-07-22 16:35:56
+2
A:
It supports a CollectionChanged event which should be what you need.
Shaun Bowe
2010-07-22 16:20:18