I was reading this blog post on ASP.NET MVC 2's new model validation and found it to be pretty cool. I see the value in keeping things DRY by adding a couple attributes to a class property and then automagically getting client and server side validation (as long as your controller checks for valid models).
That said, imagine an ASP.NET MVC application in a greater context. Perhaps I have an ASP.NET MVC application with this validation and all, but then I want to expose things to new clients, like an iPhone or Android application (not just a mobile browser). I would have to write some web-services or something that use the same underlying repositories/services that my ASP.NET MVC app uses. Could I re-use this validation stuff?
Wouldn't I have to re-write the client-side validation? I don't see a way around this, since I'll be shipping off serialized objects to be de-serialized into non-C# classes from Objective-C or whatever.
Wouldn't I also have to re-write the server-side validation? Is there some way for me to call into the ModelState on an object outside the context of an ASP.NET MVC controller (e.g., as part of the web-services I create to talk to mobile clients).
Assuming you are talking about Data Annotations, they are not part of MVC actually. So you can use them elsewhere as you wish.
There are other work extending them - annotationscontrib is one of them and I guess you can look at how, for example ModelBinder is implemented and write your own service using that.
As for running the validations on the annotated classes one can use the Validator (thanks womp)