WPF DataBinding - bind to code-behind variable which changes referenced object

Question

What I want to do: I have a ListBox bound to an ObservableCollection<MyClass> . When I select an item, I want to display some of its properties in some labels. What are the steps here? I have set the content of labels to {Binding Path=PropertyName} . What should be their DataContext ?

How I was going do it: I'm keeping a private variable in my window's code-behind called MyClass selectedItem , and I would like all bindings to point to that variable and its properties. It just seems more "semantic" to me, to bind to a variable with a clear meaning within the code logic, than to a presentation element's selected item.

This variable can, and will, reference a different MyClass instance at any given time (I'll change it in ListBox SelectionChanged handler). I want the bindings to reflect that - to always point to the properties of the instance currently referenced by selectedItem. Will this just work ? Will the binding always point to the object that the variable points to? Or will it point to the object the variable is referencing at the moment I set up the binding? (the latter would be my guess)

Possible alternatives I can think of: Bind to listbox's SelectedItem instead of the variable referencing it. If this is the way to go, how to make it type-safe? How to ensure the binding will know it's not working with a generic ListBoxItem, but a MyClass instance, so it can see its properties? Is this needed?

Any suggestions, or explanations why I'm being stupid, or alternative approaches from the more experienced? Thanks!

Answer 1

I would recommend using your alternate approach, I think this is the common way to do what you want. If you dont fill your ListBox in code-behind or in your XAML, the SelectedItem-property will hold the MyClass instance and not a ListBoxItem instance.

Then you either take the SelectedItem property of your ListBox as DataContext of your labels or you create a property of Type MyClass in your ViewModel which you bind to the SelectedItem property (Mode=TwoWay). Then you can bind to its properties like so:

<Label Text="{Binding Path=MySelectedItem.Text}"/>

In this case the DataContext of your Labels has to be the ViewModel.