I've noticed that a lot of developers define an interface for EVERY class that is going to be injected using DI framework. What are the advantages of defining Interfaces for every class?
Letting your application components (the classes that contain the application logic) implement an interface is important, since this promotes the concept of:
Program to an interface, not an implementation.
This is effectively the Dependency Inversion Principle. Doing so allows you to replace, intercept or decorate dependencies without the need to change consumers of such dependency.
In many cases developers will be violating the SOLID principles when having an almost only one-to-one mappings between classes and an interfaces in their applications. One of the principles that is almost certainly violated in that case is the Open/closed principle, because when every class has its own interface, it is not possible to extend (decorate) a set of classes with cross-cutting concerns (without dynamic proxy generation trickery that is).
In the systems I write, I define two generic interfaces that cover the bulk of the code of the business layer. They are called ICommandHandler<TCommand> and an IQueryHandler<TQuery, TResult>:
public interface ICommandHandler<TCommand>
{
void Handle(TCommand command);
}
public interface IQueryHandler<TQuery, TResult> where TQuery : IQuery<TResult>
{
TResult Handle(TQuery query);
}
Besides the nice side effect of not having to define many interfaces, this allows great flexibility and ease of testing. You can read more about it here and here.
Depending on the system I write, I might also use interfaces such as:
IValidator<T> for validating messages
ISecurityValidator<T> for applying security restrictions on messages
IRepository<T>, the repository pattern
IAuthorizationFilter<T> for applying authorization/security filtering on IQueryable<T> queries.
Depending on the system I write, somewhere between 80% and 98% of all components implement one of these generic interfaces I define. This makes applying cross-cutting concerns to those so called joinpoints trivial.
If you don't design to interfaces, you are going to be hamstrung when it comes time to refactor your code and/or add enhancements. Using a DI framework is not really at issue when it comes to designing to an interface. What DI gives you is late-binding and much better ability to write unit tests.
Related
When I write Java code, I found annotation based libraries are very popular, e.g. hibernate, Jackson, Gson, Spring-MVC. But in Scala, most of the popular libraries are not providing annotations, or provided but recommend non-annotation approaches, e.g. squerly, slick, argonaut, unfiltered, etc.
Sometimes, I found the annotations are easier to read and maintain, but why people are not so interested in them?
One reason is that annotations often have to be used at declaration-site. Hence, you have to "pollute" your domain models with code not relevant to your business logic. Solutions based on macros or type classes on the other hand are usually applied on use-site. This allows higher reusability of your domain models.
E.g., what if you need different serialization logic for different tasks? With annotations you have usually no other choice than implementing an additional representation of your model with modified annotations. With type classes (probably automatically derived through macros), you have to just implement another instance and inject it accordingly to your needs.
Macros and implicits can often be used as a substitute for annotations and have the benefit of being statically checked.
Complexity of the interface is another factor affecting coupling. The
more complex each interface is, the higher will be the degree of
coupling
The above quoted sentence is from A concise introduction to Software Engineering ,Chapter 6: Design
I don't know how the interface can be used in communication between modules of component diagram ?
Interfaces are implemented (exposed) by the components.
Other components reference this interfaces and use them to create objects, invoke methods, etc. This relationship between a component and the interface it uses is called dependency.
The more interfaces you have in your system, the complexer your system is.
Actually, the factor that influences the complexity much more than pure interface count is the nature and the structure of just mentioned dependencies. A solid system architecture has low number of dependencies, organized in a clear, clean and simple manner.
Especially dangerous are so called circular dependencies. They should be fully avoided.
A simple example:
As we know in OOP that interface provides a set of operations without implementation but
class is the opposite.
in Object oriented design ,we use uml the interface has a set of operations without implementation
and the class also has a set of operations without implementation(i know class has attributes in addition to its operations)?
so, what is the difference in UML?
As we know in OOP that interface provides a set of operations without implementation but class is the opposite.
Not quite true - abstract classes are classes that have one or more methods declared but not defined (in C++ and Java these are abstract methods). You can have a class defined with all its methods abstract - in which case there is close similarity with an interface.
One key idea in UML, though, is that an interface is a set of methods exposed to other classes or components. The purpose is to define a set of operations.
However, moving to programming, a method may be made abstract to aid development (e.g. by ensuring all subclasses have an implementation). This method might be purely internal to the class.
One last observation: the term interface and class in UML are not quite synonymous to interface and class in a language, say Java. For example, Java does not allow multiple class inheritance. Instead Java has the interface which allows a class to implement multiple types (not classes - a subtle difference)
EDIT
Quick note technical words:
Declare: Stating to the system that a variable or operation exists and its type or signature
Define: Same as declaring, but additionally providing a complete implementation of a variable or operation
Interface: A set of declarations of operations
Type: An object's interface(s) and other operations
Class: An object's class defines (not declares) how the object is implemented, including its internal state and the implementation of its operations
Define is to Declare as Class is to Type.
(see What is the difference between Type and Class?)
The purpose of interface is to define a set of operations but we are do the same for class also define a set of operations?
So the purpose of the interface is to declare (not define) a set of public operations that other objects want to use. A class (in UML) is the complete set of operations (public and private). A class (in Java, C++, etc.) additionally defines all non-abstract operations.
So the key is the intent: When other components of the system want to use a set of operations, use interface. When you're using UML to describe an implementation (of a component, algorithm, etc.) use class.
when I go to class that assumed to implement those operations I can't see any implementation for those operations as a diagram describe those operations or anything give a sign for implementation?
UML tool is for modelling and so deliberately avoids providing a place where you enter operation definitions - that is left for later. The idea is that you:
Define the model in UML
Use the UML tool to generate code in the target language
(And some allow you to import your code back into the tool to modify the model with any changes made during implementation. This is called "round-trip" modelling, something which the old TogetherJ product excelled at)
This deliberate gap (you might say deficiency) means that 'define' vs. 'declare' in UML is meaningless. Sorry.
Perhaps you've just seen models created for describing an overview, rather than modelling the system fully, but you can model the behaviour of a class's operations in most UML tools, and some tools also model the behaviour sufficiently that it can be executed .
The behaviour associated with an operation can be modelled using UML state machines, using UML action semantics or in several other ways. Quite often this is left out of the model - it is not always useful to go to that level of detail, so the implementations may just be hinted at in the documentation associated with the operation. But concrete classes in UML definitely have concrete behaviours associated with their operations, so the difference between UML and programming is that UML focuses on behaviour rather than implementation.
According to Wikipedia -
Unified Modeling Language (UML) is a standardized general-purpose
modeling language in the field of object-oriented software
engineering. The Unified Modeling Language includes a set of graphic
notation techniques to create visual models of object-oriented
software-intensive systems.
So, most important thing is UML is general-purpose and graphical. It is not only about classes and interfaces.
UML offers a standard way to visualize a system's architectural blueprints.
Software Construction Needs a Plan. Structure diagrams, Behavior diagrams, Interaction diagrams helps to Visualise In Multiple Dimensions and Levels of Detail which is
Appropriate For Both New and Legacy Systems.
Unified and Universal, Accommodates Parallel Development of Large Systems.
When I think of UML, one term which comes to mind is software quality. One thing that has plagued the software industry in recent year is poor software design. While the software industry has done fairly well for the last decade, the impact of globalization is changing the ways in which software is designed.
I have business logic that could either sit in a business logic/service layer or be added to new members of an extended domain class (EF T4 generated POCO) that exploits the partial class feature.
So I could have:
a) bool OrderBusiness.OrderCanBeCancelledOnline(Order order) .. or (IOrder order)
or
b) bool order.CanBeCancelledOnline() .. i.e. it is the order itself knows whether or not it can be cancelled.
For me option b) is more OO. However option a) allows more complex logic to be applied e.g. using other domain objects or services.
At the moment I have a mix of both and this doesn't seem elegant.
Any guidance on this would be much appreciated!
The key thing about OO for me is that you tell objects to do things for you. You don't pull attributes out and make the decisions yourself (in a helper class or other).
So I agree with your assertion about option b). Since you require additional logic, there's no harm in performing an operation on the object whilst passing references to additional helper objects such that they collaborate. Whether you do this at the time of the operation itself, or pre-populate your order object with those collaborating entities is very much dependent upon your current situation.
You can also use extension methods to the POCO's to wrap your bll methods.
So you can keep using your current bll's.
in c# something like:
public static class OrderBusiness <- everything must be static, class and method
{
public static bool CanBeCancelledOnline(this Order order) <- notice the 'this'
{
logic ...
And now you can do order.CanBeCancelledOnline()
This is likely to depend on the complexity of your application and does require some judgement that comes with experience. The short answer is that if your project is anything more than a pretty simple one then you are best off putting your logic in the domain classes.
The longer answer:
If you place your logic within a service layer you are affectively following the transaction script pattern, and ending up with an anaemic domain model. This can be a valid route, but it generally works best with simple and small projects. The problem is that the transaction script layer (your service layer) becomes more complicated to maintain as it grows.
So the alternative is to create a rich domain model that contains the logic within it. Keeping logic together with the class it applies to is a key part of good OO design, and in a complex project pretty essential. It usually requires a bit more thought and effort initially, which is why for very simple projects people sometimes use the transaction script pattern.
If you are unsure about which to go with it is not normally a too difficult job to refactor your logic to move it from your service layer to the domain, but you need to make the call early enough that the job is not too large.
Contrary to one of the answers, using POCO classes does not mean you can't have business logic in your domain classes. POCO is about not applying framework specific structures to your domain classes, such as methods and interfaces specific to a particular ORM. A class with some functions to apply business logic is clearly still a Plain-Old-CLR-Object.
A common question, and one that is partially subjective.
IMO, you should go with Option A.
POCO's should be exactly that, "plain-old-CLR" objects. If you start applying business logic to them, they cease to be POCO's. :)
You can certainly put your business logic in the same assembly as your POCO's, just don't add methods directly to them, create helper classes to facilitate business rules. The only thing your POCO's should have is properties mapping to your domain model.
Really depends on how complex your business rules are. In our application, the busines rules are very straightforward, so we use Option A.
But if your business rules start to get messy, consider using the Specification Pattern.
Back in December, there was this post that was answered with "it is ok to use concret types [for simple object]".
But I keep seeing more and more simple entities with interfaces in sample projects, and even the very large Enterprise application I just took control over (counting 89 interfaces and going).
Is it that people are not picking the best approach, and just shotgunning the project with the "my project is loosely-coupled!" approach?
Or, am I missing something. I can unit test with concret types for my IService, IFactory and IRepository implmentations I have (and works quite well). I am also building my first "Anticorruption Layer" for abstracting a lot of these 3rd party tools out away from the main domain. This anticorruption layer has a number of Facades, Translators, and Adapters - all of which are loosely coupled (or planned to be).
So, is there something I missing about Entities having Interfaces?
public interface IContent
{
Int32 ContentID {get; set;}
}
IList<IContent> list = new List();
Edit: I should also mention that that the enterprise app I have that has all of these interfaces, has zero unit tests. lol
It is more important that entities that have responsibility conform to an interface than it is for simple data objects. If you can define the entity in terms of methods, then, yes, you'll benefit from an interface. I can't see that an object that will simply be used as a DTO within the application gains any great advantage by having an interface.
That said, there is certainly benefit from abstracting away "entities" created by a third party tool, or a framework like L2S, in my opinion.