Wpf – ICommand vs RoutedCommand

icommandmvvmrouted-commandswpf

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 implement an ICommand?
What about MVVM model? Which one suits better for this purpose?

Best Answer

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)

Related Solutions

C# – How should the ViewModel close the form

I was inspired by Thejuan's answer to write a simpler attached property. No styles, no triggers; instead, you can just do this:

<Window ...
        xmlns:xc="clr-namespace:ExCastle.Wpf"
        xc:DialogCloser.DialogResult="{Binding DialogResult}">

This is almost as clean as if the WPF team had gotten it right and made DialogResult a dependency property in the first place. Just put a bool? DialogResult property on your ViewModel and implement INotifyPropertyChanged, and voilà, your ViewModel can close the Window (and set its DialogResult) just by setting a property. MVVM as it should be.

Here's the code for DialogCloser:

using System.Windows;

namespace ExCastle.Wpf
{
    public static class DialogCloser
    {
        public static readonly DependencyProperty DialogResultProperty =
            DependencyProperty.RegisterAttached(
                "DialogResult",
                typeof(bool?),
                typeof(DialogCloser),
                new PropertyMetadata(DialogResultChanged));

        private static void DialogResultChanged(
            DependencyObject d,
            DependencyPropertyChangedEventArgs e)
        {
            var window = d as Window;
            if (window != null)
                window.DialogResult = e.NewValue as bool?;
        }
        public static void SetDialogResult(Window target, bool? value)
        {
            target.SetValue(DialogResultProperty, value);
        }
    }
}

I've also posted this on my blog.

R – How to move the RoutedCommand handler from View-codebehind to ViewModel

You'll expose a property off your ViewModel that references the command.

class MyViewModel
{
    public RoutedCommand SaveCmd{ get{ return Commands.SaveCustomer; } }
}

Then in the XAML

<Button Command="{Binding SaveCmd}" />

However you might find it easier to use the RelayCommand so that you can define the actual command logic in your model as well.

Best Answer

Related Solutions

C# – How should the ViewModel close the form

R – How to move the RoutedCommand handler from View-codebehind to ViewModel

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