C#: Inheritance Event Problem - Event in inherited class is always null.

Question

I have 2 types of objects, Parents, and Children. I have an abstract class for these two objects. There are 2 types of parents, ParentA, ParentB and there are 2 types of children, ChildA, ChildB. These all inherited their corresponding base classes, i.e ParentA:Parent, ParentB:Parent, ChildA:Child, ChildB:Child.

Each parent has a collection of children. Type A Parents can only have Type A children, and Type B parents can only have TypeB children.

To add a child to a parent, I use the method addChild(child) defined in Parent abstract class. This method executes exactly the same for both parent types. In this method, I would like the parent to subcsribe tothe child's events. I use the following code:

internal void addChild(Child child)
{
 //Code I execute to add the child

 rChild.ChildPropertyChanged += ChildPropertyChanged;
}

Now, when I execute the following code:

ParentA parentA = new ParentA();
ChildA childA = new ChildA();

parentA.addChild(childA);

and follow the event in the childA object, I see that after the subcsription code above, the event is still null. Why is this event still null?

Answer 1

Lacking any further code, the only issue I can see with that you may not attaching the event to the correct object. Maybe something like this would fix it:

internal void addChild(Child child)
{        
    //Code I execute to add the child        
    child.ChildPropertyChanged += ChildPropertyChanged;
}

Answer 2

It maybe because that no other objects subscribed to the client event.

In this case the client event is null and therefor the parent event.

Try this:

var childA = new ChildA();
var parentA = new ParentA();

childA .PropertyChanged = new ChildAPropertyChangedEventHandler(PropertyChangedMethod);
parentA.AddChild(childA );

Answer 3

The code example is not really that complete, so it is difficult to guess what the problem id.

But could it be that you have created a new event definition in the ChildA class that hides the original event definition in the Child class?

Answer 4

Thanks everyone, but everything works correctly now. I feel stupid for it, but somehow it eluded me all day yesterday. Indeed, the prolem is the line:

child.ChildPropertyChanged += ChildPropertyChanged;

where ChildPropertyChanged was the event and NOT the method. The correct way, obviously, is something like

child.ChildPropertyChanged += OnChildPropertyChanged;

where OnChildPropertyChanged is a method.

I'm converting my code over from VB, where I'm used to a single line to raise an event :)