MVVM ListBox controlling a Content Control

Question

I've been going round in circles with this for a couple of days, and I'm hoping a WPF guru can see where I'm going wrong.

I'm setting CurrentViewModel in code. The Selected item of my ListBox and the Content of my ContentControl bind correctly. But when changing the selected item in the Listbox via the UI the CurrentViewModel is being set but the Content Control is not being updated.

I'm using a data template to map my Views and View Models.

    <DataTemplate DataType="{x:Type ViewModel:MyViewModel}">
       <View:MyView />
    </DataTemplate>

I have a ListBox which is bound to an observable collection of ViewModels. The Selected Item is bound to the current view model.

 <ListBox ItemsSource="{Binding MyViewModelCollection}" DisplayMemberPath="DisplayName" SelectedItem="{Binding CurrentViewModel, Mode=TwoWay}"/>

I also have a content control that is also bound to the CurrentView Model

 <ContentControl Content="{Binding CurrentViewModel, Mode=TwoWay}"/>

This is the property that they are both bound to

        public MyViewModel CurrentViewModel
    {
        get
        {
            return _currentViewModel;
        }
        set
        {
            if (_currentViewModel== value) return;

            _currentViewModel= value;
            OnPropertyChanged("CurrentViewModel");
        }
    }

I've edited the names for clarity and removed formatting information.

Any help greatly appreciated.

Cheers,

Daniel

EDIT: Came across the link How can I debug WPF bindings?. I set a break point on the Content binding and it does indeed only get called once when the binding is first set.

Answer 1

You should not be setting TwoWay as the mode on your ContentControl:

<ContentControl Content="{Binding CurrentViewModel, Mode=OneWay}"/>

This is because you intend your ContentControl to read the value, but never write it.

---

As an aside, you can also bind the ContentControl to the currently selected item in the collection, rather than to that property by doing this:

<ListBox ItemsSource="{Binding MyViewModelCollection}" 
         DisplayMemberPath="DisplayName" 
         IsSynchronizedWithCurrentItem="True"/>

<ContentControl Content="{Binding MyViewModelCollection/}"/>

The "slash" (/) at the end of the collection indicates the current item selected in the collection and setting that current item property is as simple as setting the IsSynchronizedWithCurrentItem equal to true.

A lot of times I find with this combination, I really don't need the extra property on my view model.

Anyway, I hope this helps.