MVVMLight Messenging

Basically there are two possible ways to send a message, first using an overload of the RaisePropertyChanged and secondly by using the Messenger.Default.Send command. For the address detail screen I used the messaging overload of RaisePropertyChanged. 

public AddressViewModel SelectedAddress
{
  set
  {
    var oldValue = selectedAddress;
    selectedAddress = value;
    this.RaisePropertyChanged(() => SelectedAddress, oldValue, selectedAddress, true);
    NavigationService.Navigate("/Views/AddressDetailView.xaml");
  }
  get
  {
    return selectedAddress;
  }
}

The RaisePropertyChanged method includes an overload that accepts a boolean argument, called broadcast. This last argument of the RaisePropertyChanged event indicates whether or not a message should be broadcasted regarding this change. MVVM Light, under the covers construct and sends a message of typePropertyChanged<AddressViewModel>. So any subscribers listening for this specific message will get it delivered via the messaging infrastructure.

public AddressDetailViewModel(IAddressRepository addressRepository, 
  IRideRepository rideRepository)
{
  this.addressRepository = addressRepository;
  this.rideRepository = rideRepository;

  SaveAddressCommand = new RelayCommand(SaveAddress);
  CancelCommand = new RelayCommand(Cancel);
  DeleteCommand = new RelayCommand(Delete);

  Messenger.Default.Register<PropertyChangedMessage<AddressViewModel>>(
    this,
    message =>
      {
        SelectedAddress = null;
        SelectedAddress = message.NewValue;
        if (message.NewValue != null)
        {
          whichAddress = message.NewValue.WhichAddress;
        }
      });
}

The entire AddressViewModel is sent as payload with the message and is directly set to the SelectedAddressproperty of the AddressDetailViewModel. This property is bound to the different fields on the view which in its turn gets refreshed so that the data is directly shown on the screen.

By communicating using messages instead of direct references, you decouple both view models.This will increase the flexibility and testability of your view models.