Implementing MVVM in WPF without using System.Windows.Input.ICommand

Question

I'm trying to implement a WPF application using MVVM (Model-View-ViewModel) pattern and I'd like to have the View part in a separate assembly (an EXE) from the Model and ViewModel parts (a DLL).

The twist here is to keep the Model/ViewModel assembly clear of any WPF dependency. The reason for this is I'd like to reuse it from executables with different (non-WPF) UI techs, for example WinForms or GTK# under Mono.

By default, this can't be done, because ViewModel exposes one or more ICommands. But the ICommand type is defined in the System.Windows.Input namespace, which belongs to the WPF!

So, is there a way to satisfy the WPF binding mechanism without using ICommand?

Thanks!

Answer 1

You should be able to define a single WPF custom routed command in your wpf layer and a single command handler class. All your WPF classes can bind to this one command with appropriate parameters.

The handler class can then translate the command to your own custom command interface that you define yourself in your ViewModel layer and is independent of WPF.

The simplest example would be a wrapper to a void delegate with an Execute method.

All you different GUI layers simply need to translate from their native command types to your custom command types in one location.

Answer 2

WinForms doesn't have the rich data binding and commands infrastructure needed to use a MVVM style view model.

Just like you can't reuse a web application MVC controllers in a client application (at least not without creating mountains of wrappers and adapters that in the end just make it harder to write and debug code without providing any value to the customer) you can't reuse a WPF MVVM in a WinForms application.

I haven't used GTK# on a real project so I have no idea what it can or can't do but I suspect MVVM isn't the optimal approach for GTK# anyway.

Try to move as much of the behavior of the application into the model, have a view model that only exposes data from the model and calls into the model based on commands with no logic in the view model.

Then for WinForms just remove the view model and call the model from the UI directly, or create another intermediate layer that is based on WinForms more limited data binding support.

Repeat for GTK# or write MVC controllers and views to give the model a web front-end.

Don't try to force one technology into a usage pattern that is optimized for another, don't write your own commands infrastructure from scratch (I've done it before, not my most productive choice), use the best tools for each technology.

Answer 3

Instead of the VM exposing commands, just expose methods. Then use attached behaviors to bind events to the methods, or if you need a command, use an ICommand that can delegate to these methods and create the command through attached behaviors.

Answer 4

Off course this is possible. You can create just another level of abstraction. Add you own IMyCommand interface similar or same as ICommand and use that.

Take a look at my current MVVM solution that solves most of the issues you mentioned yet its completely abstracted from platform specific things and can be reused. Also i used no code-behind only binding with DelegateCommands that implement ICommand. Dialog is basically a View - a separate control that has its own ViewModel and it is shown from the ViewModel of the main screen but triggered from the UI via DelagateCommand binding.

See full Silverlight 4 solution here Modal dialogs with MVVM and Silverlight 4