Need help with MVVM, Usercontrols and dependency properties

Hi!

Please help me do things the correct way!!

My scenario is as following.

I have a window, that window has two usercontrol embedded. The first user control A contains a button, the second user control B contains a list. There is a bound property on B that keeps track of which item in the list is selected. The button on A opens a new window C which is a preview window for an image contained in the selected list item. I work with MVVM.

Simple enough? Well I have some problems understanding the proper way to do the bindings here. The hierarchy is like this:

UC:A -contains--> UC:B -opens-> Window:C

I have solved this, but it feels very clumpsy. I solved it by:

1) I created a dependency property in the code behind on B that contain SelectedItem from VM in A

 public CoreObjectViewModel  SelectedCoreObject {
            get { return (CoreObjectViewModel)GetValue(SelectedCoreObjectProperty); }
            set { SetValue(SelectedCoreObjectProperty, value); }
        }
        public static readonly DependencyProperty SelectedCoreObjectProperty =
            DependencyProperty.Register("SelectedCoreObject", typeof(CoreObjectViewModel), typeof(SearchBoxView), new PropertyMetadata(null, OnSelectedCoreObjectChanged));
        public static void OnSelectedCoreObjectChanged(DependencyObject Sender, DependencyPropertyChangedEventArgs e) {
            var sender = Sender as SearchBoxView;
            var dc = (SearchBoxViewModel)sender.DataContext;
            dc.SelectedCoreObject = e.NewValue as CoreObjectViewModel;
            sender.DataContext = dc;        
        }

2) With this dependency property I can bind the selected item to my control by this code in A

<m:SearchBoxView SelectedCoreObject="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type UserControl}}, Path=DataContext.SelectedItem}"

Now I have the selected item from the list in A into the user control B. BUT the ´B viewmodel still has no idea about this, so if you look at the first code block, in the method OnSelected...Changed() I pull the datacontext from B (meaning the viewmodel), and manually set the value, so that the viewmodel know about this,

3) Finally I assign the B viewmodel as datacontext for the window C in the command that opens the window.

private void TogglePreviewWindow() {
            if (previewWindow == null) {
                previewWindow = new SearchPreviewWindow(this);
                previewWindow.DataContext = this;
                previewWindow.Show();
            } else {
                previewWindow.Close();
                previewWindow = null;
            }
            RaisePropertyChanged(() => IsPreviewShowing);
        }

My question to you is, is all this code reqally needed to accomplis this task? I find it very difficult to pass data between usercontrols within usercontrols where they all have their own VM. If they all share the VM then it is easy, just bind the same datacontext so they share the same VM.

Granted, the previewwindow has no need for it's own VM, but if that had been the case, how do you suggest that I would bound the selected item to the window image control? A usser control can't have two datacontexts, right?

Also another question, when working with MVVM and dependency properties, what is the correct way to get a dependency property from an UC into the UC's VM? That is where it is useful, and not in the code behind. But I have no other choice but to define the DP in the code behind, if I want to use it in XAML, right? Is it correct to do as I do, to pull the VM from the UC, assign the DP manually, then put back the VM datacontext property?

As you may see, I am a newbie at this...

/S

Henrik