WPF ICommand vs RoutedCommand

Question

Let's have a button Command property bound to a custom command.

When should I implement ICommand and when derive from RoutedCommand? I see that RoutedCommand implements ICommand.

In which case could I need to iplement an ICommand? What about MVVM model? Which one suits better for this purpose?

Answer 1

As you have noticed the RoutedCommand class is an implementation of the ICommand interface, its main distinction if that its function is similar to that of a RoutedEvent:

The Execute and CanExecute methods on a RoutedCommand do not contain the application logic for the command as is the case with a typical ICommand, but rather, these methods raise events that traverse the element tree looking for an object with a CommandBinding. The event handlers attached to the CommandBinding contain the command logic.

The Execute method raises the PreviewExecuted and Executed events. The CanExecute method raises the PreviewCanExecute and CanExecute events.

In a case when you don't want the behavior of the RoutedCommand you'll be looking at your own implementation of ICommand. As for the MVVM pattern I can't say that one solution, it seems that everyone has their own methodology. However, here are a few approaches to this problem that I've come across:

• Using RoutedCommands with a ViewModel in WPF
• Relaying Command Logic
• Simple Command (almost identical to Relay Command but worth reading)

Answer 2

The only thing I would add to Rich McGuire's answer is that RoutedCommands (and their more prevalent descendant RoutedUICommand have to be wired up with event handlers to work correctly.

Most MVVM implementations I have come across attempt to leverage binding against the ViewModel and thus the ViewModel (and not the View) owns the CanExecute/Execute logic.

In contrast, the event handlers move that burden to the View. The handling can then be propagated to the ViewModel, but this means a slightly higher degree of coupling between ViewModel and View (casting + method call, etc.).