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copied!<p>Always validate at both levels.</p> <p>You need to validate the view models because you want to feed back to the user as quickly and easily as possible if they've done something wrong. You also don't want to be bothering the rest of your domain logic if the model is invalid.</p> <p>But, you will also want to verify that everything's happy in the domain, once the view model has been validated. For simple models, these checks may be the same, and so it does look like duplicating logic, however as soon as your application grows so you may have multiple user interfaces, or many different applications using the same domain models, it becomes so important to check within the domain.</p> <p>For example, if your application grows so you end up providing an API to customers to interact directly with the application programmatically, it becomes a necessity to validate the domain models, since you cannot guarantee that the user interface used has validated the data to the standard that you need (or even validated it at all). There is an argument to say that the data received by APIs should be validated in much the same way as the view models are validated, and that's probably a good idea since that is achieving the same goal as the view model validation is. But regardless of the route in (either from a UI or the API), you will want to always guarantee that the data is valid, so defining that in a central place is ideal.</p> <p>The aims of the two levels of validation is different, too. I would expect a view model validation to inform me of <strong>all</strong> problems (such as missing first name, last name is too long, DoB is not a date). However, I think it would be ok for the domain logic to fail on the first error, and just report that one. Again, for simple models, it may be possible to collect all errors and report them all back, however the more complex an application gets, the harder it gets to anticipate all errors, especially if the logic will change depending on the data. But, as long as only good data gets past, that should be fine!</p>

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