View values are not inserted but retrieved from ViewModel - mvvm

Question

I am trying to create an application on the basis of the WAF framework following the MVVM pattern. Currently, my solution consists of two projects (each equipped with MEF and MAF references):
*.Application (holding controllers and viewmodels)
*.Presentation (holding the actual view files)
I am creating the binding between view and viewmodel via the ViewModel interface - see code fragments below. Further, all classes are made available via the MEF framework inside the App.xaml.cs file. Here, the controller is also initialized. In the easiest case, I want to show a string value in a label of the main window.
Here is the problem: If I start the application, the value of the second label only shows the fallback value, but the get method of the property is being called properly (checked via debugging mode). The binding between View and ViewModel seems to be correct - if I change the binding path in the xaml to a non existent property, I get an output that the property can not be found in the ViewModel. My impression is that there could be a problem with the events for view updating? Any suggestions on this strange behaviour?
Here is the expert of the ViewModel:
[Export]
public class MainWindowViewModel : ViewModel<IMainWindowView>
{
private string _labelContent;
public string LabelContent
{
get { return _labelContent; }
set { SetProperty(ref _labelContent, value); }
}
[ImportingConstructor]
public MainWindowViewModel(IMainWindowView view) : base(view)
{
}
}
Here is the exerpt of the controller:
[Export(typeof(IMainWindowController))]
public class MainWindowController : IMainWindowController
{
private MainWindowViewModel _mainWindowViewModel;
public MainWindowViewModel MainWindowViewModel
{
get { return _mainWindowViewModel; }
set { _mainWindowViewModel = value; }
}
[ImportingConstructor]
public MainWindowController(MainWindowViewModel mainWindowViewModel)
{
_mainWindowViewModel = mainWindowViewModel;
}
public void Initialize()
{
_mainWindowViewModel.LabelContent = "stfu";
}
}
The view interface:
public interface IMainWindowView : IView
{
}
And the view itself:
[Export(typeof(IMainWindowView))]
public partial class MainWindow : Window, IMainWindowView
{
public MainWindow()
{
InitializeComponent();
}
}
<Window x:Class="MyCompany.Product.Redesign.Presentation.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<StackPanel>
<Label Content="Test" />
<Label Name="MyLabel" Content="{Binding Path=LabelContent, FallbackValue=Fallback}" />
</StackPanel>
</Window>

Answer 1

Are you sure, that the view that is displayed really is the instance with the ViewModel-Instance you are setting the property on?
First, make sure that you don't have a view set as the Application's StartupUri-Property in the App.xaml. Then make sure, that you call View.Show() through your ViewModel. You are then certain that you really set the property on the instance that is being displayed:
App.xaml
<Application <!-- note: no StartupUri Property -->
x:Name="App" x:Class="YourProject.Presentation.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
ShutdownMode="OnMainWindowClose">
</Application>
MainViewController.cs (with method declaration in IMainViewController.cs)
public void Run()
{
_mainWindowViewModel.Show();
}
App.xaml.cs
_controller = mainExportProvider.GetExportedValue<IMainViewController>();
_controller.Initialize();
_controller.Run();
MainViewModel.cs (with method declaration in IMainViewModel.cs)
public void Show()
{
ViewCore.Show();
}
This should do the trick. Otherwise, you might be seeing a view instance that you don't have a reference to. Thus you are setting a property on a ViewModel whos view isn't being displayed.