Is it guaranteed that all the WPF bindings will be functional when OnStartup() is called?

Question

I am working on an application that uses WPF/C# with MVVM. I have one particular ObservableCollection<> which is bound to ListBox.

Scenario 1: When the application is running, I modify this ObservableCollection<> and ListBox is populated as expected.

Scenario 2: With new requirements I have to fill this ListBox by default with some init values. So I have added a method for initializing it in my ViewModel. I call this method in OnStartup() after initializing View & ViewModel. DataContext is also set properly. In this scenario values are updated in the ObservableCollection<>, Unfortunately they are not reflected in the ListBox.

Just to verify if anything is wrong with the OnStartup(), I added the same method call in a callback on ContentRendered from the View, instead of OnStartup() it did work fine.

So my question, When exactly MVVM guarantees that all the bindings are setup correctly?

Edit: One more observation, if I pop a message/dialog ListBox is populated as expected. It calls ContentRendered callback. And then it populates correctly.

I am almost convinced that ContentRendered should be the function that guarantees the bindings.

Answer 1

I can't say with certainty that this is your issue, without seeing the change to the code, but it sounds like you may now have an initial value of null for the property to which you are binding.

If you subsequently set that property to an ObservableCollection<T>, there is no automatic change notification.

One option would be to raise a notification that the property value changed (from null to something), and then let the ObservableCollection<T> handle change notification from there.

The better solution would be to initialize the property with an empty ObservableCollection<T> from the get-go, then your initialization to default values will mean adding those values and change notification should happen as you expect.

You might get the same problem by starting with a non-null collection, then setting the property to a new instance of a collection, without raising a notification, but I'm not 100 percent certain of that.

If this is not your issue, then I'll be happy to take another look.