I have a component that needs to call a specific service depending on the input it receives. So my component has to look at the input and based on a configuration that says "for this input call this service with this data" needs to call the proper service. The services have a common signature method and a specific one (each).
I thought about an abstract class that includes the signatures for all three methods. The implementation for the two services will override all three methods (throwing NotImplementedException for the methods that are not supported by current service). A component that could be initialized with a map (that for each input type will have the type of the service to be called) will also be defined.
Do you have a better approach to cope this scenario ?
Factory pattern has this definition:
Define an interface for creating an
object, but let subclasses decide
which class to instantiate. Factory
Method lets a class defer
instantiation to subclasses
Sounds like what you want, right?
Microsoft calls this The Provider Model Design Pattern. Although since your methods do not implement all methods perhaps it isn't a great fit.
At its most basic level the pattern is:
An abstract base class defining a
contract. The abstract base class has
all the abstract methods and
properties required to implement the
public API it supports.
Configuration information. Once an
implementation of the feature provider
class is created, it must be described
in the configuration section. The description of the provider within configuration provides all the information so that the provider can be instantiated in the running application.
The abstract base class usually should support factory methods to create new objects.
The Strategy design pattern is well suited for your problem.
A strategy encapsulates an algorithm and that needs to be executed depending on the type of data you have as input.
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.
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Interface vs Base class
(38 answers)
Closed 9 years ago.
what is the main utility of Interface. we know that we can implement dynamic behaviour using interface but i guess it is not only the utility. so i like to know when we have to write interface and when we need to go for abstract class.
show me 5 or 10 most important uses of interface in real life scenario.
another main use is coming to my mind that project manager or team lead will implement basic skeleton through interface and other developer follow it.
so please guys show me with sample code few most important use of interface which we can do with abstract class or concrete class.
one guy told me like this way which is not very clear to me
interfaces are defined contracts between classes or structs, consumers can exchange the implementation by a different one as long as the same contract is met that is the method names and signature that compose a specification that classes and structs can work against rather than working against a concrete implementation.
The important part about interfaces is to know when to use them and as a matter of fact it's quite simple, when you want two or more unrelated objects to have the same common functionality but not necessarily the same implementation you will want to use interfaces; otherwise, when you have related objects that have a shared functionality and implementation then you may consider to use an abstract class instead of an interface.
this thing is not clear specially
when you want two or more unrelated objects to have the same common functionality but not necessarily the same implementation you will want to use interfaces; otherwise, when you have related objects that have a shared functionality and implementation then you may consider to use an abstract class instead of an interface.
it would be nice if anyone explains with sample code when to go for interface & when abstract class.
show me few best important area which is always handle with interface with sample code or best interface uses with sample code.thanks
Some of microsoft recommendation from this link
If you anticipate creating multiple versions of your component,
create an abstract class. Abstract classes provide a simple and easy
way to version your components. By updating the base class, all
inheriting classes are automatically updated with the change.
Interfaces, on the other hand, cannot be changed once created. If a
new version of an interface is required, you must create a whole new
interface.
If the functionality you are creating will be useful across a wide
range of disparate objects, use an interface. Abstract classes
should be used primarily for objects that are closely related,
whereas interfaces are best suited for providing common
functionality to unrelated classes.
If you are designing small, concise bits of functionality, use
interfaces. If you are designing large functional units, use an
abstract class.
If you want to provide common, implemented functionality among all
implementations of your component, use an abstract class. Abstract
classes allow you to partially implement your class, whereas
interfaces contain no implementation for any members.
I won't answer all you questions. I just want to give you some hints.
The main difference between an interface and an abstract class is, that a c# class can implement multiple interfaces even if they declare the same members. And it can even implement those equally named members differently by implementing the interface explicitly.
If you derive from an abstract class, you also "inherit" al its dependencies. For example if a method in an abstract class uses another class from a different assembly, you have to reference that assembly. --> Compile order --> No parallel build
Mocking in unittest can be trickier when using abstract classes with base functionality
Let's take for instance some Data Access Objects which can retrieve data from a DB, a SAOP Service, a REST Service or even an XML file.
You would use Interfaces to ensure what kind of operations they offer to the rest of the application. You can also say that those interfaces describe the Domain and how they interact with it.
public interface IUserDao
{
User GetUserById(int id);
void AddUser(User u);
....
}
This IUserDao can be implemented by using WCF, Entity Framework, XmlDocuments, and many other techniques, the controller or other parts of the application don't care about the details as long as they have those abstracted methods to retrieve and add a user.
On the other hand the same Data Access Objects can have a base class which can for instance initialize some connections or open the XmlDocument, ...
public abstract BaseDao
{
public Connection GetNewConnection()
{
....
}
// or similar functions which are used by DAOs accessing the same data source (DB, XML, ...)
}
So as it was described, you can use interfaces to hide implementation details and bring the implementation to a more absract level, this way, less skilled developers or developers more interested in the domain specific aspects (some specific calculation, ...) can contribute without the need to understand how exactly they need to retrieve and store the data from / to the database.
Also it is easier to exchange functionality, for instance you can start with a simple xml file but soon you'll realize that you'll need a whole DB - you can keep the interfaces and implement the classes with DB access.
On the other hand abstract classes share basic functionality (technical functionality), which is so basic that it is used by many classes but shouldn't be instantiated alone. You could exchange Abstract Classes for some utility classes with static methods, but than you would loose the advantages of OOP.
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.
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Interface vs Base class
When should I choose inheritance over an interface when designing C# class libraries?
So I'm writing my first real program in C#. The program will scrape data from four different websites. My plan is to have one parent class that will look like this:
class Scraper
{
string scrapeDate(url);
string scrapeTime(url);
//&c.
}
Then I will have four classes that inherit from it.
The other option is to make Scraper an interface and have four classes implementing it.
What are the differences between these approaches?
Class inheritance represents an "is-a" relationship, e.g., a Tank is a Vehicle. If your situation doesn't at least meet this, choose interface over inheritance.
If the proposed base class doesn't provide default implementations for your methods, this would be another reason to choose interface over inheritance.
In C#, you can only inherit from one class but multiple interfaces. This would be yet another reason to choose interface over inheritance.
Get the idea?
Inheritance is, in my opinion, the better approach when it is possible. Implementing common features in a base class helps ensure that the underlying implementation is consistent, while implementing an interface guarantees only that the, well, interface is consistent. Inheritance wherever possible is one leg of the OOP tripod, and limits code duplication.
I use interfaces when I have objects that can't or shouldn't have a common base class, but need to be provide similar functionality. Classes that implement a common interface may (and probably do) expose additional functionality, may implement multiple interfaces, etc.
For instance: in one application, my data access layer is built around "provider" classes, which insulate business objects and database proxy objects from the database. In one instance, I have a provider which interacts with a SQL Server database and another for communicating with Oracle CRM On Demand (aka, Why God Why). The both implement an agnostic interface so that the client code doesn't care which data store it's dealing with, or the quirks of working with each.
The Oracle provider communicates with a hosted server via a collection of web services, manages connection sessions, batches requests, etc. The SQL provider uses a set of stored procedures. Although both interfaces accept the same data classes, the Oracle provider transforms the data to match their esoteric (to put it lightly) schema. With this implementation, I could easily add a provider to use an XML data store, a different RDBMS, or a stub/mock unit test.
In this situation, it doesn't make much sense for these things to have a common base class and, in a few instances, it's impossible.
Honestly, do both.
public interface IScraper
{
string ScrapeDate(string url);
}
public abstract class Scraper : IScraper
{
public string ScrapeDate(string url)
{
// default implementation
}
}
There's advantages either way, but those are difficult to quantify without knowing more about your requirements. However, there's no reason you can't do both. Having an interface for your class makes it mockable for testing purposes as well.
Something else to consider though; if the functionality for each of your derived classes are similar enough, it may be easier to simply have a single class that takes parameters to the constructor.
An interface contains only the signatures of methods, delegates or events. The implementation of the methods is done in the class that implements the interface.
A class can implement multiple interfaces.
A class can have only one direct base class.
Refering to Abstract Class versus Interface.
There are some similarities and
differences between an interface and
an abstract class:
A class may implement several
interfaces.
A class may inherit only one abstract
class.
An interface cannot provide any code,
just the signature.
An abstract class can provide
complete, default code and/or just the
details that have to be overridden.
An interface cannot have access
modifiers for the subs, functions,
properties etc everything is assumed
as public
An abstract class can contain access
modifiers for the subs, functions,
properties
Interfaces are used to define the
peripheral abilities of a class. In
other words both Human and Vehicle can
inherit from a IMovable interface.
An abstract class defines the core
identity of a class and there it is
used for objects of the same type.
If various implementations only share
method signatures then it is better to
use Interfaces.
If various
implementations are of the same kind
and use common behaviour or status
then abstract class is better to use.
If we add a new method to an Interface
then we have to track down all the
implementations of the interface and
define implementation for the new
method.
If we add a new method to an
abstract class then we have the option
of providing default implementation
and therefore all the existing code
might work properly.
No fields can be defined in interfaces
An abstract class can have fields and
constrants defined
Interfaces allows to define the structure of common behaviors.
Inheritance is useful if you can extract a common implementation of one or more specific behaviors.
Basically if several classes scrape a date the same way, it make sense to put scrapeDate in a base class; otherwise use only an interface and define the specific scrapeDate in every class that implement your interface.
If you have common functionality, you should use inheritance - the functionality will then be available in all child classes, and each child class can extend or override the parent class code.
If you have something consuming your classes, you would use interfaces to ensure that all of the classes implement the same methods and properties, but not necessarily the same functionality.
Main differences:
an interface has no implementation at all, whereas an abstract base class can implement common functionality
a class can only inherit from one base class, but it can implement multiple interfaces
In your case, it's likely that all your scraper classes will need some common features, so it makes sense to make them all inherit from a common base class
What you appear to have here is really best served as an Interface. If you were to have some common logic you wished to include or some common data members you wished to include then you would use a Base Class and inherit from it. What you are doing is requiring each child to implement a minimum set of logic.
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.