Tightly Coupled Controllers in Visual Basic .NET

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Tightly Coupled Controllers
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As you saw earlier, a controller class typically invokes a class on the application s service layer. The application s service layer is responsible for implementing the application logic and is ultimately in charge of any orchestration required that involves other components in other layers, such as BLL and DAL. Note that the controller belongs to the presentation layer. Hence, in observance of the principle of layering, it can talk only to its closest layer the business layer. The service layer is just part of it. Here s a typical method on a controller:
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public ActionResult Find(int customerId) { // Some simple forms of validation might take place here. Classic server-side // validation against business rules will occur in the service layer class. ... // Invoke the business logic var service = new CustomerServices(); var customer = service.FindById(id);
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4 Inside Controllers
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// Deal with possibly wrong return values caused by server exceptions ... // Fill the view model container this.ViewData["Customer"] = customer; // Trigger the view return this.View(); }
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The method uses a service layer class to get information about the specified customer. In turn, the service layer class CustomerServices uses a specific repository object to wrap any data access code:
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public class CustomertServices { public IList<Customer> FindAll() { // Get any necessary data from the DAL var rep = new CustomerRepository(); var customer = rep.GetAll(); // Return data return customers; } public Customer FindById(int id) { // Get any necessary data from the DAL var rep = new CustomerRepository(); var customer = rep.GetByID(id); // Return data return customer; } }
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This code works just fine. But both the CustomerController and CustomerServices classes hold references to explicit objects and use the new operator to get instances. This code is not ideal from a testing perspective because dependencies are hard-coded. You need to find a way to inject in the controller class any dependencies it might have on the service layer. Furthermore, you also need to refactor the service layer to be injected with dependencies on repositories.
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ServiceLayerContext Class
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The simplest way to inject a reference to the service layer in a controller class is to add an ad hoc property. Instead of repeating this property for each new controller class, you might want to add it to a tailor-made superclass.
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Part II The Core of ASP.NET MVC
As discussed earlier, the ApplicationContext class does the trick. This class is a container of all dependencies a controller might have. In particular, it exposes a reference to a ServiceLayerContext object, as shown here:
public class ApplicationContext { public ApplicationContext(ServiceLayerContext context) { Initialize(context, null); } ... // Properties public ServiceLayerContext ServiceLayerContext { get; private set; } ... }
The service layer is not made of a single class. Generally, you ll likely have a service layer class for each controller that needs to invoke the middle tier. The ServiceLayerContext object is, then, another global container for all service layer classes you happen to have:
public class ServiceLayerContext { public ICustomerServices CustomerService { get; private set; } ... }
As you might have noticed, the ServiceLayerContext class uses an abstraction of the content service class the ICustomerServices interface. That s precisely the trick. Here s how the aforementioned Find method on the CustomerController class looks now:
// ApplicationContext is inherited from custom controller superclass public ActionResult Find(int customerId) { // Some simple forms of validation might take place here. Classic server-side // validation against business rules will occur in the service layer class. ... // Invoke the business logic var service = ApplicationContext.ServiceLayerContext.CustomerService; var customer = service.FindById(id); this.ViewData["Customer"] = customer; // Fill the view model container and trigger the view ... }
4 Inside Controllers
If you design the controller to work against an abstraction and manage to provide a concrete object that implements that abstraction, you end up with an extensible and maintainable design and gain a lot in terms of testability.
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