MVC Validation using Data Annotations - Model classes or View Model classes?

As far as best practices is concerned I would say: in neither of them. Validation should be separate. Frameworks such as FluentValidation allow you to completely separate your validation logic from your models.

But to answer your question I would put validation into View Models as those are the classes you are binding your controller actions to. You could also have multiple View Models that are tied to the same model but with different validation rules.

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