I first made a new repository interface for each of my entities e.g. IClientRepository. I then made a class for each entity e.g. SqlClientRepository. I did this for a lot of my entities and then realized that they all had the same methods: Add, Update, Delete, GetAll.
So I realized that I should probably just make the one interface instead of making a separate one for each.
The problem is that now my dependency injection won't work since I can only map the interface to one repository:
Bind<IClientRepository>().To<SqlClientsRepository>().WithConstructorArgument("connectionString", WebConfigurationManager.ConnectionStrings["MyDb"].ConnectionString);
The only work-around I can see is to combine all of the repositories or go back to my first attempt. Using the first attempt would also allow me to change the return types for certain entities e.g. IClientRepository.Add() could return the new client's id, and some other entities might not need that.
Appreciate any thoughts.
You could have your ClientRepository implement both your BaseRepository and a specific IClientRepository. That way your Base can have the usual Add/Remove etc and your IClientRepository could have specialized methods (or be empty in some cases probably). Your IoC could resolve using your IClientRepository.
This is how i do it:
public class CustomerRepository : BaseRepository<Customer>, ICustomerRepository {...}
and
unityContainer.RegisterType<ICustomerRepository, CustomerRepository>();
Good luck :)
A common approach for repositories is to create a base interface like "IRepository" with the common CRUD methods or methods that all repos can use. Then for each specific repository create a new interface that derives from the base interface but includes the domain-specific methods for that repo. The same thing can be done for a base Repository implementation and the more specific implementation classes. This way you have a common place for common functionality, but you still have specific interfaces for each concrete domain-specific repository implementation.
It's hard to say for sure without knowing what you are trying to accomplish. In my latest project, there is one repository per class, IRepository<T>. That might get you out of your IoC binding trouble.
one solution is to make the Interface generic. this way, it's general enough, while still being flexible.
Related
What's the difference between adding an implementation rather than its interface, being that I have just one implementation of this interface?
// Adds a transient service by type of the implementation:
services.AddTransient(typeof(SomeConcreteService));
or
// Adds a transient service by interface of the concrete implementation type:
services.AddTransient<ISomeService, SomeConcreteService>();
services.AddTransient<ISomeService, SomeConcreteService>();
This way is preferred as it lets you use dependency injection in the correct manner.
If you use interfaces in all your controllers and then decide you want to change the concrete implementation, you will only have to edit the one line in your Startup.cs
public HomeController(ISomeService someService)
{
//..
}
When you add the implementation you will only be able to inject it as implementation.
services.AddTransient(typeof(SomeConcreteService));
Injecting this now as ISomeService will cause an error.
While this
services.AddTransient<ISomeService, SomeConcreteService>();
will allow you to inject the interface rather than the implementation.
In the end it is about loosley coupeling. It also makes your software harder to test.
If you only inject the interface you can easily test the class that uses the implementation with its given interface bc you can mock it without any troubles. If you don't and inject the real implementation, the functions of the implementation must be marked as virtual to mock them. You also need to mock the classes your implementation SomeConcreteService might be using.
Today you have one implementation of the interface. Tomorrow you may not. Others may need to extend the service in the future with a decorator, composite, or other design pattern.
Essentially, by using an interface, you are future-proofing your application for every eventuality - it can even be extended in ways that you don't foresee today, without changing a single line of code outside of your DI container registration.
If you use the concrete type, the ability to extend it is very limited. You are basically saying "this is the way it will be forever" without allowing many possibilities for extending it without changing the code. You are giving up the most useful benefit of using the DI pattern - loosely coupling your code by separating its interface from its implementation.
When implementing the repository pattern, in conjunction with entity framework, why do I see many examples using an interface for their repository class? A specific example of a reference of this is here.
What's the point of the interface? Why not just the class? Will there really need to be more than one class subscribing to that very specific interface, for just Employees, for example?
It's a frequently used pattern, quite often used specifically for unit testing, not really specific to entity framework, or repository pattern, or even any data access kind. Another great benefit is that it gives the chance to latter provide an alternate implementation without change the code using it.
Consider for example this code, that uses the dependency injection pattern:
public class EmployeeService
{
private readonly IEmployeeRepository employeeRepository;
public EmployeeService(IEmployeeRepository employeeRepository)
{
this.employeeRepository=employeeRepository;
}
public IEnumerable<Employee> GetAllEmployees()
{
IEnumerable<Employee> employeeList=this.employeeRepository.GetAll();
//Optionally do some processing here
return employeeList;
}
}
By having the interface in the repository, note that you now can work entirely using the interface, without ever mentioning the actual repository, this is the real value of it. It gives mainly two benefits:
If you want to write an automated unit test for this class, you may give it a fake implementation of the IEmployeeRepository, which would not go to the real database, but instead return a hardcoded list, so that you can test your method without worrying about the DB for now. This is called a 'Mock', and is often the main reason of putting that interface there. There are also a couple of libraries that automate that process, all relying on the fact that they generate a fake class implementing an interface. By far, that's the most common reason for putting an interface like that.
You may decide sometime in the future that you want to replace entity framework with something else, or, say, want to implement a repository to something different than a relational DB. In this case, you would write another repository, implementing the very same interface, but doing something completely different. Given that the services using it rely only on the interface that code will work entirely unmodified as long as the same contract is respected (of course, the code that actually creates the repo and gives it to the service must change, but that's another history). That way the same service works the same no matter where it reads/saves the data.
I'm working with a third party product which has provided an API. This API works by creating an implementation of a base class, and then in the app.config indicating the implementation that you want to use.
The problem with this is that it's possible to have multiple projects in this third party application. What I would like to do is create a wrapper class which implements the base class. This would look at the parameters and then look up a configuration to determine which other class to pass the processing over to, depending on which project is being used. This way we could add future projects to the system without modifying any of the existing code.
public class MyImplementation : ThirdPartyBaseClass
{
public override OnLoad(ThirdPartyType data)
{
//do stuff
}
public override Process(ThirdPartyType data)
{
//do stuff
}
}
There are about 15 methods that can be overridden. The base class methods appear to be empty because nothing happens if you don't override a method, so I would need my wrapper to be able to handle the situation where the type I need to use for this project might not implement some or all of the methods.
Anybody know of a suitable design pattern for this situation?
As said by Robert in comments, Abstract Factory seems appropriate for this one.
Check this wiki link and this dofactory link for more information on this one.
For a more concrete response, I have a few doubts.
There is an object of the base class which is got from the third party API. Now, when you say that is is possible to have multiple projects in this tool, do I take it to mean that you need to use this base class to create multiple "project" classes as defined by you?
Then, the wrapper class can have an object type of an interface IProject. This should have all the definitoins like OnLoad and Process. Each type of project will have a concrete class with the final implementation depending on the project type.
Hope this helps in giving you a direction!
I would think this would be fairly easy to implement with a good DI container (like Autofac), or even a poor-man's DI. You can choose which service to provide at run-time based on whatever criteria you choose and supply that as the concrete implementation for your implementation of the third-party API class via constructor injection.
The Decorator Pattern might help you here, together with the Abstract Factory pattern. I'd suggest a decorator (the implementation you configure in app.config) that uses a concrete factory (depending on the current configuration) to get an inner for the decorator.
If you'd like to use multiple implementations at once, you may also think of using the Composite Pattern to delegate calls to more than just a single inner.
I am brushing up on my design patterns at the moment and got a little confused when I came across this tutorial:
http://www.asp.net/mvc/tutorials/iteration-4-make-the-application-loosely-coupled-cs
If you look at listing 7 onwards, the author says it is using the decorator pattern. However, is one of the main principles of this pattern to wrap objects and ADD responsibilities and behaviour?
I think it looks more like and adapter pattern as it is adapting the MVC specific ModelStateDictionary to work with a more flexible IValidationDictionary so that different implementations can be used with the service if WPF etc were used instead. There is new responsibility or behaviour added.
Do I have this correct or not? If I'm wrong can anyone please explain why?
Thanks
I agree with you, that looks to me like the Adapter Pattern, that is, the ModelStateDictionary is abstracted behind the interface IValidationDictionary (the adapter interface) using a concrete type (the adapter) such that the implementation can be changed later.
The Decorator Pattern usually provides additional functionality via composition, exposing the same interface as the decorated type. This is usually done via sub-classing or through interface implementation.
An example of a decorator would be:
you have a repository class that fetches "objects" from the database
you have a repository decorator class that can cache objects without needing to fetch them from the database each time. This decorator class provides the cache fetching and retrieving through composition by sub-classing the original repository class and overriding the Get() method to first check the cache for the item (and Save() would be overridden to also update the cache as well as the database).
I think you're correct, and that there's an error in the post. From the article:
The Decorator pattern enables you to wrap an existing class in a new
class in order to implement an interface.
That's not exactly true - decorators do allow you to wrap one implementation inside another, but the intention usually isn't to implement another interface, but to "decorate" the instance with new functionality. The adapter pattern allows you to take two dissimilar interfaces, and modify one instance to be have like another.
so in my application I've got several different customers being "serviced". Each customer has their own implementations of various classes that are all based on interfaces.
With the latest customer being added, I've noticed there will be a lot of duplication of code from another customer but the other customer is in no other way related to them.
I've already got a default implementation for several other customers and roll new ones as i need them.
My question is how do i refactor this and still keep the code clean? If i were a dev new to this code base i would want each customer to either use the default or their own implementation of these classes... but that's a lot of duplication.
Consider using an abstract base class with abstract or virtual members. Abstract members are essentially equivalent to interface members (they have no build-in behavior, they only guarantee the method exists) whereas virtual members have a default implementation which can be overridden by derived classes.
Your question is really too vague to answer in full, but here's how you can leverage inheritance.
If you want all classes to use the same implementation of a member then that member can be implemented in the base-class.
If you want each class to have its own implementation of a member then you can either use a base-class with abstract members, or an interface.
If you want some classes to use the same implementations and others to use different implementations then implementing the default behavior in the base-class and override it as needed.
My main point is that OOP there is a spectrum of how much or little functionality is in base/abstract/concrete classes. There's no silver-bullet answer, sometimes your base classes will be skeletons and sometimes they'll be fully fleshed-out; it all depends on the specific problem at hand.
Is there some way that you could create a base class, then a specific implementation for each customer and then using some type of Dependency Injection have that load classes or functionality as needed. You want to really have a DRY system so as to avoid headaches and typos or other similar human mistakes.
You may use either inheritance (put common logic to the base class) or aggregation (spread that logic among other classes and make use them from your customers).
I'd recommend the visitor pattern:
http://en.m.wikipedia.org/wiki/Visitor_pattern
As well as the mediator pattern:
http://en.m.wikipedia.org/wiki/Mediator_pattern
Reason being that it sounds like you may benefit from decoupling, or at least more-loose-coupling, the business logic from your classes, based on what you are saying.
It's a bit difficult to know what to suggest without a better understanding of the code... but some things that have worked for me in similar situations include:
Use a Strategy, for the duplicated code. I've had most success where the strategy is encapsulated within a class implementing a known interface (one class per alternate strategy). Often in such cases I use some form of Dependency Injection framework (typically StructureMap) to pass the appropriate strategy/strategies to the class.
Use some sort of template class (or template methods) for the common item(s).
Use a Decorator to add specific functionality to some basic customer.
STW suggested that I should offer some clarification on what I mean by "Strategy" and how that differs from normal inheritance. I imagine inheritance is something you are very familiar with - something (typically a method - either abstract or virtual) in the base class is replaced by an alternate implementation in the derived class.
A strategy (at least the way I typically use it) is normally implemented by a completely different class. Often all that class will contain is the implementation for a single replaceable operation. For example if the "operation" is to perform some validation, you may have a NullValidationStrategy which does nothing and a ParanoidValidationStrategy which makes sure every McGuffin is the correct height, width and specific shade of blue. The reason I usually implement each strategy in its own class is because I try and follow the Single Responsibility Principle which can make it easier to reuse the code later.
As I mentioned above, I typically use a Dependency Injection (DI) framework to "inject" the appropriate strategy via the class constructor, but a similar results may be obtained via other mechanisms - e.g. having a SetSomeOperationStrategy(ISomeOperation StrategyToUse) method, or a property which holds the strategy reference. If you aren't using DI, and the strategy will always be the same for a given customer type, you could always set the correct choices when the class is constructed. If the strategy won't be the same for each instance of a given customer type, then you probably need some sort of customer factory (often a factory method will be sufficient).
I'd go with the answer of spinon (got my vote at least), but it's to short so let me elaborate:
Use your interfaces for the default implementation and then use dependency injection. Most tools allow you to define a scope or some criteria how to resolve something.
I assume that you do know the client at some early point of the program. So for ninject you just might want to define a "Module" for each client and load that into the kernel, depending on the client.
So I'd create a "no customization" Module and create a "ClientX" Module for every special case that uses ´Bind.To()` instead.
You end up with
a base implementation that is clean/default
a single place change for a new client (got a new one? Great. Either it works with the default or just needs a single Module that maps the interfaces to other classes)
The rest of the code shouldn't mind and get the dependencies via injection (constructor, property, whatever is easiest to go for. Constructor would probably be the nicest way) and has no special treatment at all.
You could even use a conditional binding in Ninject link text to solve the binding issue without different modules at all (although, depending on the number of clients, this might get messy and should better be separated).
I was going to suggest aggregation, as #the_joric suggests, over inheritance, but your description makes it sound like your application is already reasonably well-factored - that there aren't a lot of small classes waiting to be extracted from your existing classes. Assuming that's the case, for any given interface, if you have a perfect class for the new customer already written and ready to go, I would say go ahead and use it. If you're worried about that, for some reason, then take that perfect class, make it abstract, and create empty subclasses for your existing customer and your new customer - and if it's not quite a perfect fit, then that's the way I would go.