How to pass the selectedItem of a listbox to the View Model

This is a running question that I have updated to hopefully be a little more clear. In short what I am trying to accomplish is pass a property from a listbox selected item to the viewmodel so that this property can be used within a new query. In the code below the Listbox inherits databinding from the parent object. The listbox contains data templates (user controls) used to render out detailed results.

The issue I am having is that within the user control I have an expander which when clicked calls a command from the ViewModel. From what I can see the Listbox object is loosing it's data context so in order for the command to be called when the expander is expanded I have to explicitly set the datacontext of the expander. Doing this seems to instantiate a new view model which resets my bound property (SelectedItemsID) to null.

Is there a way to pass the selected item from the view to the viewmodel and prevent the value from being reset to null when a button calls a command from within the templated listbox item?

I realize that both Prism and MVVMLite have workarounds for this but I am not familiar with either framework so I don't know the level of complexity in cutting either of these into my project.

Can this be accomplished outside of Prism or MVVMLite?

original post follows:

Within my project I have a listbox usercontrol which contains a custom data template.

<ListBox x:Name="ResultListBox"
             HorizontalAlignment="Stretch"
             Background="{x:Null}"
             BorderThickness="0"
             HorizontalContentAlignment="Stretch"
             ItemsSource="{Binding SearchResults[0].Results,
                                   Mode=TwoWay}"
             ScrollViewer.HorizontalScr开发者_如何学CollBarVisibility="Disabled"
             SelectionChanged="ResultListBox_SelectionChanged">
        <ListBox.ItemTemplate>

            <DataTemplate>
                <dts:TypeTemplateSelector Content="{Binding}" HorizontalContentAlignment="Stretch">
                    <!--  CFS Template  -->
                    <dts:TypeTemplateSelector.CFSTemplate>
                        <DataTemplate>
                                <qr:srchCFS />
                        </DataTemplate>
                    </dts:TypeTemplateSelector.CFSTemplate>

                    <!--  Person Template  -->
                    <dts:TypeTemplateSelector.PersonTemplate>
                        <DataTemplate>
                                <qr:srchPerson /> 
                        </DataTemplate>
                    </dts:TypeTemplateSelector.PersonTemplate>

                   <!-- removed for brevity -->

            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

SelectionChanged calls the following method from the code behind

private void ResultListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (((ListBox)sender).SelectedItem != null)
        _ViewModel.SelectedItemID = (((ListBox)sender).SelectedItem as QueryResult).ID.ToString();
        this.NotifyPropertyChanged(_ViewModel.SelectedItemID);//binds to VM
    }

Within the ViewModel I have the following property

public string SelectedItemID
    {
        get
        {
            return this._SelectedItemID;
        }
        set
        {
            if (this._SelectedItemID == value) 
                return;
            this._SelectedItemID = value;
        }

    }

the listbox template contains a custom layout with an expander control. The expander control is used to display more details related to the selected item. These details (collection) are created by making a new call to my proxy. To do this with an expander control I used the Expressions InvokeCommandAction

<toolkit:Expander Height="auto"
                          Margin="0,0,-2,0"
                          Foreground="#FFFFC21C"
                          Header="View Details"
                          IsExpanded="False"
                          DataContext="{Binding Source={StaticResource SearchViewModelDataSource}}"
                          Style="{StaticResource DetailExpander}">

            <i:Interaction.Triggers>
                <i:EventTrigger EventName="Expanded">
                    <i:InvokeCommandAction Command="{Binding GetCfsResultCommand}" />
                </i:EventTrigger>
            </i:Interaction.Triggers>

Within the ViewModel the delegate command GetCFSResultCommandExecute which is called is fairly straight forward

private void GetCfsResultCommandExecute(object parameter)
    {
        long IdResult;
        if (long.TryParse(SelectedItemID, out IdResult))
        {
            this.CallForServiceResults = this._DataModel.GetCFSResults(IdResult);}

The issue I am experiencing is when selecting a listbox Item the selectionchanged event fires and the property SelectedItemID is updated with the correct id from the selected item. When I click on the expander the Command is fired but the property SelectedItemID is set to null. I have traced this with Silverlight-Spy and the events are consistent with what you would expect when the expander is clicked the listbox item loses focus, the expander (toggle) gets focus and there is a LeftMouseDownEvent but I cannot see anything happening that explains why the property is being set to null. I added the same code used in the selection changed event to a LostFocus event on the listboxt item and still received the same result.

I'd appreciate any help with understanding why the public property SelectedItemID is being set to null when the expander button which is part of the listbox control is being set to null. And of course I would REALLY appreciate any help in learning how prevent the property from being set to null and retaining the bound ID.

Update I have attempted to remove the datacontext reference from the Expander as this was suggested to be the issue. From what I have since this is a data template item it "steps" out of the visual tree and looses reference to the datacontext of the control which is inherited from the parent object. If I attempt to set the datacontext in code for the control all bindings to properties are lost.

My next attempt was to set the datacontext for the expander control within the constructor as

private SearchViewModel _ViewModel;
    public srchCFS()
    {
        InitializeComponent();
        this.cfsExpander.DataContext = this._ViewModel;
    }

This approach does not seem to work as InvokeCommandAction is never fired. This command only seems to trigger if data context is set on the expander.

thanks in advance

With this line you create a new SearchViewModelDataSource using its default constructor.

DataContext="{Binding Source={StaticResource SearchViewModelDataSource}}"

I guess this is why you find null because this is the default value for reference type. You can resolve the issue by setting DataContext to the same instance used to the main controll (you can do it by code after all components are initialized).

Hope this help!

Edit

I don't think that binding may be lost after setting datacontext from code. I do it every time I need to share something between two or more model.
In relation to the code you've written :

private SearchViewModel _ViewModel;
public srchCFS()
{
    InitializeComponent();
    this.cfsExpander.DataContext = this._ViewModel;
}

Instead of using this.cfsExpander you can try to use the FindName method. Maybe this will return you the correct instance.

object item = this.FindName("expander_name");
if ((item!=null)&&(item is Expander))
{
    Expander exp = item as Expander;
    exp.DataContext = this._ViewModel;
 }

Try if its work for you.
Of course, this._ViewModel has to expose a property of type ICommand named GetCfsResultCommand but I think this has been already done.

While this was a hacky approach I found an intermediate solution to get the listbox item value to the view model. I ended up using the selection changed event and passing the value directly to a public property wihtin my view model. Not the best approach but it resolved the issue short term

private void ResultListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (((ListBox)sender).SelectedItem != null)
            _ViewModel.SelectedItemID = (((ListBox)sender).SelectedItem as QueryResult).ID.ToString();
            MySelectedValue = (((ListBox)sender).SelectedItem as QueryResult).ID.ToString();
        this.NotifyPropertyChanged(_ViewModel.SelectedItemID);
    }

For this to fire I did have to also setup a property changed handler within the view to push the change to the VM. You can disregard the MySelectedValue line as it is secondary code I have in place for testing.

For those intereted the generic property changed handler

        public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(string info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }