The technique below is one technique, others exist, but the design goals of this approach are:

1. The ViewModel cannot know or reference the View
2. The ViewModel should should not care how the View displays or interacts with the user (it could be a dialog box, a flyout, etc.)
3. The ViewModel must be Portable Class Library compatible and View agnostic, e.g. not need to know about dialog buttons, etc.
4. Choices the user makes in the View dialog should result in Commands being executed on the ViewModel
5. The ViewModel must not specify the content of the message text, button labels, etc. – the View should be responsible for the text/content of the message

One way to think about these design goals is that the ViewModel is asking for some semantic message/dialog to be displayed in the View.

Defining a Message to be used with the MVVM Light Messenger

The way the ViewModel “tells” the View to create a dialog is by sending a message using the MVVM Light Messenger.

First we define a custom message:

using System.Windows.Input;
using GalaSoft.MvvmLight.Messaging;

namespace Project.Portable.ViewModel.Messages
{
    public class ShowDialogMessage : MessageBase
    {
        public ICommand Yes { get; set; }
        public ICommand No { get; set; }
        public ICommand Cancel { get; set; }
    }
}

This message allows us to define the commands that the view will call, based on the choice that the user makes in the dialog.

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This article is part of the 50 Apps by Christmas series.

I developed a Silverlight app a few years ago and I wanted to see how easy it would be to port to a Windows Store app. You can download free trial of SilverKeys for Windows 8.

The Original Silverlight App

Here’s a screenshot of the original Silverlight app:

The app helps amateur composers try out different scales and also helps them understand what common chord progressions are often used.

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