Keeping domain models and view models separate with ASP.NET MVC and WCF

The location, role and responsibility of objects within a software system is a common topic in the forums with plenty of disagreement about what is and isn’t correct. But first, let’s just define some commonly found objects and their roles:

Domain repository entity: Object which is an in-memory representation of a record persisted in some sort of backing data store. In most business scenarios, a repository entity will be mapped to a physical database table via an ORM tool such as NHibernate or the Microsoft Entity Framework. 

Domain model entity: An object which lives in the service layer and typically contains both logic and/or data. This is where your business rules should be defined. Often, a domain model entity and a domain repository entity will be the same thing. In other words, an object which contains business logic and is also mapped to a data store (e.g. an Order domain entity may be mapped to an Order database table, but also contains business logic to calculate the total cost of the order, including tax). This arrangement allows data to be processed by business code and persisted via the same object.

Data contract: This is a DTO (data transfer object) which is decorated with the WCFDataContract attribute. This is essentially a ‘dumb’ object containing just data and no logic, and is used to pass data across service boundaries via serialisation. Data contracts will usually by mapped to domain models to copy data from one to the other.

View model: Another type of DTO but limited to just the MVC UI layer. View model objects are used to pass data between MVC controller action methods and the views which display and capture model data.

Passing data between services and the web UI
So with these definitions in mind, it is OK (and necessary) for data contracts which are defined in your WCF service layer to be passed to and from your MVC web application. However for this to be possible, the MVC web app must have access to the data contract type declarations. The way you achieve this is to always have your WCF service implementation and WCF service & data contracts in separate assemblies. Then from your MVC web app, you can reference the project/assembly containing the service & data contracts (but do not reference the service implementation assembly).  Now your MVC web app can happily use data contracts defined in the service layer, and will still compile.

However it’s not OK for domain entities to be passed across the same service boundaries because then you blur the line between what should be a simple data transfer object and something containing logic which should never be directly exposed to your MVC views and controllers. Ignore this at your peril otherwise you will end up with business logic contaminating your UI layer which will lead to no end of problems in the future and result in a lot of refactoring and maintenance.

But how do you keep them apart?  Well from a coding point of view, one approach is that only classes that have absolutely no business logic should be decorated with DataContractand DataMember attributes. This prevents serialisation of domain entities meaning that they can’t accidentally or intentionally be used as a parameter in service contract. Another more basic check is to make sure that the web UI project never references an assembly containing domain entities (sounds obvious but I’ve seen it happen). This will keep them safely isolated from the UI. However from a physical point of view, the simple answer is that you can’t absolutely guarantee this won’t happen. It only takes one developer to unwittingly do the wrong thing and the rot starts to set in.

So if you can’t physically keep them apart, what can you do to contain the problem?  Well you have to rely on some fundamental development techniques which have been around for years: discipline, team communication and code reviews. OK I’m not going to win any awards for innovation here, but building a reliable system is more than just writing a lot of tests and then assuming everything’s OK. You have to enforce design rules and best practices which the development team sticks to, and the appropriate use of domain entities, view models, data contracts, etc. is all part of that.

Passing data within the UI
Any application with an MVC web client and WCF services will reach a point where types defined in the service layer and types defined in the web UI meet, and in most cases that will happen in your MVC controllers. But for most scenarios, that’s the only time they will share the same space. It will also help if you give data contracts and view models different names. View model names typically should reflect the view they relate to, and I usually add a ‘Model’ suffix to them for clarity (but it’s down to personal preference how you do it). However if you are using the SvcUtil tool to generate service proxies, I recommend you specify a namespace so that it’s clear which models are defined in the service layer (see this post for generating service proxies).

In a typical case where you need to get data from a view model object into a data contract object so that it can be sent to a service, all you have to do is map the properties between the two via an object initialiser, in a constructor, or using a mapping tool such as AutoMapper, although AutoMapper can be quite hungy on memory resources so be aware of this when you decide what to use. Writing your own mapping code is trivial so why use anything else?

There’s a bone of contention about whether data contracts defined in the service layer should be used as view models. Personally I don’t have a problem with it and actually prefer to have my view model types declared as data contracts in the service layer rather than in the MVC app. This is so I only have to define data annotation validation rules once rather than defining them against data contracts in the service layer, and then again for  view models in the MVC application. This makes unit/integration testing easier and reduces the chance of a property not being validated properly. As a result, data contracts declared in the service layer are passed directly into my views (so there are very few actual view models declared in the MVC app).

However you may not like this and prefer to have separate, dedicated view models because it avoids the situation where views end up being strongly-typed to classes defined in a different layer of your architecture. How you do it is entirely down to person choice.

original article by Phil Munro