IT:AD:Patterns:DTO Strategy

It/all/depends/on/how/you/architect/your/application. If you have a complex business model, you may want to look into domain-driven-design. If your model is simplistic maybe a simple 2 or 3-layer (UI,BLL,DAL) will be sufficient. If your model can easily be reflected in your UI then maybe look into NakedObjects.

Chances are, however, your DTOs should almost never be mirrors of your business model. Your DTOs should be customization's of closely-related data - associated & particular to a specific context. For instance a specific user interface can be represented by a ViewModel which is composed of multiple DTOs. Your Web API services may provide endpoints for these DTOs. This doesn't mean that it's wrong to have a DTO that resembles an EF model object but DTOs can be composed of richer data than 1:1 mappings.

It also depends on the amount of loose-coupling your trying to achieve with your architecture. Is your business logic going to communicate with the database directly through EF's DbContext? In that case your BLL and DAL are indeed one layer. Not that extensible, but it may provide you with a better ROI if your time-to-market is of high importance. If you want a better seperation of concerns, you may decide to adopt the repository pattern, thus abstracting your data-access - which in the future can be easily replaced without touching the business logic.

Your DTOs should only be immutable bags of getters/setters.

Your domain model (EF POCOs) or business layer model should be more complex. This is where customizing the EF T4 POCO template becomes incredibly powerful. Make your EF POCOs partial classes so they can be extended to contain business logic, validation, cross-cutting concerns, etc etc.

I should also add you can expose your EF model via Web API and make use of OData. Unfortunately I don't have much expertise with that and maybe the community here can elaborate when to use this?

Note: I would take that as more of a philosophy. DTOs are meant only to transfer data between applications. The data shouldn't change at any point, once set.

Ref: http://forums.asp.net/p/1818473/5048008.aspx/1?Re+Map+DTO+in+a+EF+entity+using+JSON+NET