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    <p>I personally don't think it violates DRY since the model and view-model (I prefer the term presenter) don't point to the same information. For instance your VM and M both have a Title property, but your VM's Title property could also include validation, whereas your model's Title property could assume validity.</p> <p>While it's true that The VM may contain all of the properties of the model, there is also the possibility of having validations (e.g. Title must be non-blank), data-dependencies, bindable UI-specific properties (icons, colors, brushes, etc.) which aren't part of the view.</p> <p>Essentially all UI patterns have similar "duplication" in the way you state it: namely cascading modifications. Try changing a model in MVC without changing the controller.</p> <p>That being said, MVVM (or any UI pattern designed to separate UI, logic, and state) can be overly tedious for simple cases such as your example. When logic becomes little more than state-pass through, the value separating the controller/presenter/view-model decreases.</p> <p>In your specific case, if there really isn't any logic, validation, or UI specific properties that your VM isn't surfacing, and your model doesn't have to be persisted, serialized, or made backwards compatible with an existing structure (or adding the logic to do so in your VM is trivial), I would strongly consider combining the M and VM to avoid creating properties whose sole purpose is to get/set the underlying model's properties.</p>
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    1. COI would never, ever assume validity at the model level. A major benefit of abstracting out the model is that you can replace the UI layer with something else (a different UI, a web service) and gain code reuse. If you delegate model validation to the original UI, the model becomes impossible to correctly re-use.
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      1. USEric J.
    2. CO@ Eric J: Late reply, but you make an excellent point. In most of the MVVM examples and suggested by the WPF/SL binding infrastructure itself, the models are behaviorally light (e.g. Fowler's anemic domain model) deferring validation to databind-time, e.g. the VM layer. I'm curious as to your thoughts and approaches regarding databinding and limiting its intrusion into domain models while at the same time relying on the domain models for validation.
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    3. CO@EricJ., on contrary, the domain model (aggregate) invariants must be enforced to provide model **consistency**. What is the purpose of a model if it does not to enforce its own consistency?
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      1. USSergey Brunov
 

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