EventHandler is always null?

Question

I want to create a event and subscribe is on another ViewModel. The event handler is always getting null on the first ViewModel. In the first Viewmodel I declared Event and raised as follows

  public event EventHandler EditSearchChanged;

and raised as

     if (EditSearchChanged != null)
        {
            EditSearchChanged(this, null);
        }

In the second Viewmodel,I have declared a property of first Viewmodel.

   private EditTileViewModel editTileVM;

    public EditTileViewModel EditTileVM
    {
        get
        {
            return editTileVM ?? (editTileVM = new EditTileViewModel());
        }
        set
        {
            editTileVM = value;
            RaisePropertyChanged();
        }
    }

and subscribe the event as follows

EditTileVM.EditSearchChanged += EditTileVM_EditSearchChanged;

  private void EditTileVM_EditSearchChanged(object sender, EventArgs e)
    {
        this.EditTileVM = (sender as EditTileViewModel);
    }

Debugger Result

Is it possible that the line 'EditTileVM.EditSearchChanged += EditTileVM_EditSearchChanged;' is never executed, and therefore never set? Maybe supply some more code? — mnemonic, Apr 04 '16 at 12:19
can you place a debugger at `EditTileVM.EditSearchChanged += EditTileVM_EditSearchChanged` and check if it is fired ? — Rohit, Apr 04 '16 at 12:29

StepUp · Answer 1 · 2016-04-04T13:33:32.947

It happens as you create another new instance of ViewModel in the following property:

private EditTileViewModel editTileVM;
public EditTileViewModel EditTileVM
{
    get
    {
        return editTileVM ?? (editTileVM = new EditTileViewModel());
    }
    set
    {
        editTileVM = value;
        RaisePropertyChanged();
    }
}

So there are two instances of EditViewModel.

I suggest you to use EventAggregator pattern between two viewModels from Prism framework:

// Subscribe
eventAggregator.GetEvent<CloseAppliactionMessage>().Subscribe(ExitMethod);

// Broadcast
eventAggregator.GetEvent<CloseAppliactionMessage>().Publish();

Please, see a very good tutorial of Rachel Lim about simplified Event Aggregator pattern.

Or use MVVM Light messenger:

//Subscribe
Messenger.Default.Register<CloseAppliactionMessage>(ExitMethod);

// Broadcast
Messenger.Default.Send<CloseAppliactionMessage

@subminer feel free to ask any question. If you feel that my reply helps to you, then you can mark my reply as an answer to simplify future search of other people. Please, read this http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work — StepUp, Apr 05 '16 at 08:40

EventHandler is always null?

1 Answers1