WPF Getting Control null reference during InitializeComponent

Question

So my InitializeComponent method call in the Window's constructor is running through the XML and adding the controls and plugging them into their events.

So when a property of one of the controls changes it calls the Method that subscribes to the event. The method references a control that has not yet been built.

Why does this happen in this order here? It worked in WinForms because the events were not fired until later, after all the controls were created. Is there a way to force this in WPF?

The other solutions I see are

• I need to subscribe to the events after initialization. This is a pain in the deriere.

• I need to check for null whenever i deal with a control. Also a pain in the deriere, with the addition of being pain-staking.

Any help?

Answer 1

Are any of the controls using two-way databinding? I've run into this issue where I've had textboxes bound to a properties on a ViewModel. The ViewModel initialization was triggering INotifyPropertyChanged up to the bound control, which in turn caused the textbox's TextChanged event to fire. My short-term workaround was to move the event subscription to the window Loaded event, but like you state, that's kind of a pain. I need to refactor the code to change the order in how my objects get initialized, so that the WPF Views (i.e. windows and user controls) are created before the ViewModels. Then I'll be able to move the event handler registration back into XAML.

Answer 2

You should be able to check the IsInitialized or IsLoaded properties on your Window to verify it has finished initializing/loading. Otherwise, you would need to check for null or add your event subscriptions in your code behind (after InitializeComponent).

Also, you may be able to adjust how you access the elements. For example, if you have something like:

<ListBox x:Name="listBox" SelectionChanged="OnListBoxSelectionChanged" />

Then in your code behind you can get the list box several ways:

private void OnListBoxSelectionChanged(object sender, SelectionChangedEventArgs e) {
    ListBox lb = this.listBox; // May be null
    ListBox lb = sender as ListBox; // Should never be null
    ListBox lb = e.Source as ListBox; // Same as sender in this case
    ListBox lb = e.OriginalSource as ListBox; // Always the element that started the event (if handler is not attached directly to ListBox).
    // ... Do Something ...
}

Answer 3

It was the Checked event on a radiobutton. When i removed Checked="true" from the xaml the issue went away. (though it is checked when the Window starts). Not sure what's going on here, but at least I didn't have to change anything major to fix it... yet.