When we are using EF (f.e.) via MVC, we can use ModelState.IsValid to detect a model can pass DataAnnotations metadata or not. But how can I use DataAnnotations metadata in a desktop (win-forms / wpf) application?
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In fact, I want to create an object same as ModelState (a dictionary that can save properties and messages associated with each). Then, wrap the DAL by a validation-layer, in VL use metadata to validate models, that the VL can be used in any project. I know I should use reflector, but I haven't any experience. Thanks to any help and suggestion.
If you are using EF 4.1/4.1 dbcontext, it has a built in validation API that can check Data Annotation rules as well as IValidatableObject.Validate. I'm not quite sure that I understand your goal, but if it is to have the validation in the data layer then you can just use what's built in. (Here's an overview http://msdn.microsoft.com/en-us/data/gg193959).
If you want your own validator that is separate from the data layer, then look at the System.ComponentModel.DataAnnotations namespace for methods and other logic you can leverage to do your own validation.