Question

I ‘ve recently started working with WPF using MVVM light and I have the following (simple scenario).

1. MainWindow contains a listbox of elements.

2. When one is selected and the button is clicked, I fire a command:

   ReservoirViewerCommand.Execute(null);
   

3. On the viewmodel class I instantiate the command and send a message with the selected object:

   ReservoirViewerCommand = new RelayCommand(OpenReservoir);
   private void OpenReservoir()
   {
       Messenger.Default.Send(new LaunchShowReservoirMessage(){Reservoir=SelectedReservoir});          
   }
   

   where:

   class LaunchShowReservoirMessage:MessageBase
   {
       public Reservoir Reservoir { get; set; }
   }
   

4. The mainview registers the message and opens a new child window:

   private void RegisterMessages() 
   {
       Messenger.Default.Register<LaunchShowReservoirMessage>(this,OnLaunchShowReservoir);
   }
   
   public void OnLaunchShowReservoir(LaunchShowReservoirMessage msg) {
       var showReservoir = new ReservoirViewerView();
       showReservoir.Show(); 
   }
   

What I need is that the new ViewModel (ReservoirViewerViewModel) can somehow get hold of the passed object through the message so that I can then display the details of this object on the child window.

I did some step-by-step debugging and the ViewModel constructor seems never to be reached.

Answer 1

“I did some step-by-step debugging and the ViewModel constructor seems never to be reached.”

Make sure that you are actually binding your view to your view model using one of the following:

In CodeBehind

var showReservoir = new ReservoirViewerView();
showReservoir.DataContext = ViewModelLocator.ReservoirViewerViewModel; //static property
//OR showReservoir.DataContext = new ReservoirViewerViewModel();
showReservoir.Show();

In Xaml View

<Window x:Class="Garmin.Cartography.AdminBucketTools.ChildWindowView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        
    DataContext="{Binding Path=ReservoirViewerViewModel, Source={StaticResource Locator}}">
    <!--Use the non-static property in your ViewModelLocator-->

    <Grid />
</Window>

In Xaml Resources

<DataTemplate DataType="{x:Type viewmodels:ReservoirViewerViewModel}">
    <views:ReservoirViewerView/>
</DataTemplate>

“What I need is that the new ViewModel (ReservoirViewerViewModel) can somehow get hold of the passed object through the message so that I can then display the details of this object on the child window.”

Simply register for the same message in your ReservoirViewerViewModel class:

Messenger.Default.Register<LaunchShowReservoirMessage>(this, (msg) =>
{
    var reservoir = msg.Reservoir;
});

FYI, if you derive your message class from GenericMessage<[content type]> instead of MessageBase, then you can use the already-defined Content property of the GenericMessage class. For example:

public class LaunchShowReservoirMessage: GenericMessage<Reservoir>
{
    public LaunchShowReservoirMessage(Reservoir content) : base(content) { }
}

And then:

Messenger.Default.Register<LaunchShowReservoirMessage>(this, (msg) =>
{
    var reservoir = msg.Content;
});