Odd Binding behavior in WPF

Question

Hello, I will try and explain this as concise as possible. I have 2 objects, the first which we will call object A that has an Id property and the second we will call object B, which has a ParentId property. The obvious relationship is that object B's ParentId is set to an object A's Id property. I am using the MVVM pattern, so on the viewmodel I have 2 ObservableCollections, one containing objects A the other objects B. On construction of the viewmodel, I create and fill the ObservableCollection<'A'> named ListItems. My xaml is simple,

<StackPanel>
    <ListBox IsSynchronizedWithCurrentItem="True" ItemsSource="{Binding ListItems}">
    </ListBox>

    <ComboBox SelectedValuePath="ParentId" SelectedValue="{Binding Path=ListItems/Id, Mode=OneWay}" ItemsSource="{Binding ComboItems}">
    </ComboBox>

    <Button Click="Button_Click" Content="Push Me"/>            
</StackPanel>

As you can see the combobox's SelectedValue is bound to the ListItems current item's Id property. So essentially the listbox and combobox are in a master details.

If you press the button, it will fill the ObservableCollection<'B'> name ComboItems, which in turn populates the combobox. Now here is where the oddity begins. When I start the program, if the only thing I do is press the button, and then afterwords select an item in the listbox, the combobox will properly select an item due to the SelectedValue binding. But if I start the program and first select an item in the listbox and then press the button, the current combobox item will not change with the current listbox item. The binding appears to be forever broken. Does anyone know why this happens?

Ps. If I set the ItemsSource on the combobox before I set the SelectedValue/SelectedValuePath, the master/detail binding will never work. I know there is order to xaml, but that seems a little fragile. So if anyone has input on that also, I am all ears.

Thanks, Nate

EDIT -

When binding SelectedValue, it is very fragile. If the binding is working, i.e. have not selected anything in the listbox and then filled the combobox, if you choose an item in the combobox, the binding will break. After much time wasted with this, I chose to bind SelectedItem. This binding does not break in any of the conditions I have previously specified. I would however take any answers as to why SelectedValue binding is so ridiculous. Thanks again to all that have answered or will answer.

Answer 1

Yeah this is a problem we stumble upon quite a lot.

The problem is that after the ItemsSource property gets a new value, the SelectedValue binding will be cleared. Sucks, and until today we have not found a proper solution.

Here are a few workarounds:

• Reset the SelectedValue binding in code, as soon as the new ItemsSource has been set. You can do this in a converter, or somewhere you'll know which will replace the ItemsSource binding (like the DataContextChanged event).

• Instead of using the Binding on ItemsSource, try using a CollectionViewSource and a Filter. Put all your items in the CollectionViewSource object and filter the items when your combobox changes value.

• Manually get your item the old fashion way when your listbox throws a SelectionChanged event.

Mind you, all solutions are not the prettiest in the book. I would go for option 2, its the cleanest IMO ;)

Hope this helps!