Question

I have a class MyCol, which inherits from ObservableCollection(Of T). It overrides InsertItem method in such manner:

Public Event PreInsertItem As EventHandler(Of EventArgs)

Protected Overridable Sub OnPreInsertItem(e As EventAtgs)
    RaiseEvent PreInsertItem(Me, e)
End Sub

Protected Overrides Sub InsertItem(index As Integer, item As T)
    OnPreInsertItem(EventArgs.Empty)

    MyBase.InsertItem(index, item)
End Sub

As you can see I have added an event which is raised every time an item is added to MyCol collection.

Next I create another class MyColSubClass, which inherits from MyCol, and also overrides InsertItem method:

Public Overrides Sub InsertItem(index as Integer, item as T)
    OnPreInsertItem(EventArgs.Empty)

    ' some additional code goes here

    MyBase.InsertItem(index, item)
End Sub

THE QUESTION:

Now, when I use an instance of MyColSubClass and add an item, PreInsertItem event is raised twice: first in MyColSubClass and than in MyCol.

What design pattern should I use to make PreInsertItem event raise only once: in MyColSubClass?

N.B.

Classes and events, shown in examples are simplified from real life application, but assume they show exact structure of application. Raising event in last inherited class is a must.

Answer 1

If you know for sure that the base class will raise the event, it’s useless to do so in the derived class.

Just change your override to :

Public Overrides Sub InsertItem(index as Integer, item as T)
    ' some additional code goes here

    MyBase.InsertItem(index, item)
End Sub

And that should be fine.

But if you change your derived method, and stop calling MyBase.InsertItem(…), you should raise the event in your override to ensure it gets raised :

Public Overrides Sub InsertItem(index as Integer, item as T)
    ' some additional code goes here

    OnPreInsertItem(EventArgs.Empty)

    ' insert your item and do whatever...
End Sub

EDIT

If you need to change the way the event is raised, but want to make sure that it only gets raised once, simply override the OnPreInsertItem method in your derived class :

Protected Overrides Sub OnPreInsertItem(e as EventArgs)
    ' Do wahetever you need here, change e, add info, whatever...
    ' ...
    ' Then raise the event (or call MyBase.OnPreInsertItem, as you like)
    RaiseEvent PreInsertItem(Me, e)
End Sub

Public Overrides Sub InsertItem(index as Integer, item as T)
    ' some additional code goes here

    ' This will work only if MyBase.InsertItem calls OnPreInsertItem. 
    ' Otherwise, you have to handle the insertion and raise the event
    ' yourself without calling the base method.
    MyBase.InsertItem(index, item)
End Sub

Since OnPreInsertItem is overridable, the version from your derived class will get called when you insert an item in your derived class.

Hope that helps 🙂