I’m using the S#arp Architecture. I can’t remember where I read it, but I understand that view models should be stored at the service layer, and your views should submit the view model to the service for processing.
My question then is this: Which layer should construct the view model? Should it be at the service layer, with the controller requesting it? Or should the controller construct the view model itself? There is also a question about updating the view model since, if it contains collections, and the model state is invalid, you will also need to repopuplate any lists.
Any suggestions?
I create view models inside controllers. Controllers take domain entities (retrieved from database by model binders), possibly inside other view models, contact repositories for additional data, create new view model, and pass it to appropriate view (or redirect). So controllers responsibility is to prepare view/viewmodel according to input domain data (and handle errors of course).
You can look here for alternative to creating view models in controller. This technique moves view model creation outside actions, so that not only controller actions accept pure domain objects, but they also return pure domain objects. I wouldn’t say it’s appropriate in all cases, but it’s very interesting to learn.
The above technique, related to AutoMapper, also raised questions similar to “should I pass viewmodels to service layer”. No you don’t. If you need to pass complex object to service or domain layer, you define this object in the appropriate service/domain layer and use it to pass data to those layers. This object then can be easily mapped to/from view models (for example, using AutoMapper). But your lower layers (service/domain) should not be coupled to upper layers (view/controllers). Not in this case, not in others. Never low level layers should depend on something defined above them.