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I personally think there are no hard and fast rules... well, there is one - be pragmatic. Pragmatically speaking, a view model is a model, and so is a data model. Both are classes, independent of the UI, and surface state as properties, operations as methods, and notifications as events. The thing that I think in my mind differentiates them is how general they are. I see a model as generally usable across multiple views, vs. a view model optimized for a particular view. I would personally never abstract for the sake of abstracting. I would never surface top-level properties for every model property and implement it by delegating to an underlying model. This increases work. It increases the amount of code to be tested. It requires propagation of metadata, change notifications etc. If there is some actual logic to add, then yes, there would be properties on the view model I'd expose and delegate to the underlying model as appropriate. Even there I'd ask if it is reasonable/appropriate to expose those as computed properties on the model (view model and data model are both models). As far as surfacing DAL types directly vs. not, that in my mind is somewhat orthogonal. It depends on other factors - how much do you want to abstract the DAL, how useful a DAL type is - for example, if a DAL type has a lot of foreign keys, a presentation model equivalent or projection might be more useful where there is some denormalization done. Sometimes security might be the reason to write a presentation model/projection - eg. I don't want to send email addresses to the client, and instead want an alternate representation of email addresses. In my sample, I used a DAL type directly, to simplify and not have an overload of concepts in a single sample. I do want to blog about presentation models and projections in a dedicated manner... and so didn't want to mix posts of viewmodel and .net ria services with presentation model concepts at the same time.
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1. POMVVM Experts need your opinion on MVVM and Dataforms
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1. PTAnswer
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1. USNikhil Kothari
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