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Where should validation reside when using ViewModels with MVC (MVVM), on the Model or the ViewModel? (Or both or neither)? And why? I bring this up especially in light of V2 of ASP.NET MVC coming out soon.
What about complex/custom validation?
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A model is usually more closely related to how your data is stored (database, services, etc.) and the model will closely resemble those. The ViewModel on the other hand is closely related to how your data is presented to the user. It is usually a flatten version of your model, denormalized, etc.
Model validation refers to the process of confirming that the model actually achieves its intended purpose. In most situations, this will involve confirmation that the model is predictive under the conditions of its intended use.
The right place for validation is the Model. This makes most sense because you are doing validation on the data, which is what the model represents. In terms of the CRUD updates, the model should always be used somehow. If you are changing data from the view, you should have validations being checked.
Any business specific validation should be in the Model. Any web site specific validation could be done in both the Model and/or ViewModel. This way your business logic (Models) can move behind a different interface (Windows, Web Forms, etc.) and your validation will stay intact.
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