[Question] MVVM Validation and Messaging Together? ObservableValidator and ObservableRecipient

[khdc]

I'm using the awesome MVVM library and have some classes that need BOTH validation and messaging.

I see ObservableValidator and ObservableRecipient, but nothing that does both.

Am I missing something?

How can I have a view-model that does both?

Thank you for making a great MVVM library!

Hello khdc, thank you for your interest in Windows Community Toolkit!

I have automatically added a "needs triage" label to help get things started. Our team will analyze and investigate the issue, and escalate it to the relevant team if possible.. Other community members may also answer the question and provide feedback 🙌

[Kyaa-dost]

Looping in @Sergio0694 ⬆️

[Sergio0694]

Hi @khdc, glaad to hear you're enjoying the MVVM Toolkit so far! 😄

The issue you mentioned is a known one, which is really just a consequence of the fact that C# doesn't support multiple inheritance. The ObservableValidator class contains a number of fields that are necessary to support its functionality, and we didn't want to force all consumers of just ObservableMessenger to have to always incur in that overhead per-instance even when the validation logic wasn't actually being used (and vice versa). Unfortunately there isn't really a good way to combine different base classes from two different hierarchy branches in C#, so I think in this case you might want to just define a custom base class that will extend ObservableValidator and inject some messenger support.

The simplest version could be just like this:

public abstract class ObservableRecipientWithValidation : ObservableValidator
{
    protected ObservableRecipientWithValidation()
        : this(WeakReferenceMessenger.Default)
    {
    }

    protected ObservableRecipientWithValidation(IMessenger messenger)
    {
        Messenger = messenger;
    }

    protected IMessenger Messenger { get; }
}

And if you only register your messages once, then you'd be able to use all the various extensions to add message handlers (either using the IRecipient<T> interface, or with the explicit handlers). If your viewmodel also require the same ability as ObservableRecipient to be enabled/disabled, then you'd need to add the IsActive/OnActivated/OnDeactivated logic as well.

Alternatively, and depending on your preferences, you could also go with a "compositional" approach instead, eg. by having an ObservableRecipient viewmodel that has a child viewmodel of type ObservableValidator, and binding the various UI bits to the right components depending on what your application needs. This would require a slightly different architecture but wouldn't require the introduction of a new base class. Of course, it depends on how often you need to do this in your scenario 🙂

Hope this helps!

[khdc]

This really helps a lot! Thank you for taking the time to help me understand the issue and giving me some sample code! Above and beyond! 💯

[Kyaa-dost]

Closing the issue as the response was provided above.