This issue comes up for me so often in my coding that I'm astonished I can find so little reference to it, and would value other people's thoughts and ideas.
I define lots of APIs, for the frameworks I work on, and within large domain models that I want to break up. Those APIs consist almost entirely of interfaces (meaning, in my case, C# interfaces). I find, over and over again, that I want to distinguish between two kinds of interface. In the absence of finding any more widely used terms, I define these two as follows:
'Role' interfaces are intended to be implemented by objects outside of the API, in order that those objects can be used as arguments for methods defined on the API.
'Result' interfaces are implemented by objects inside the API and made available to other parts of the system via the API. The intent of defining a result interface rather than exposing the object that implements it is to restrict the view of the object to the outside world.
To pick one example, a Payments sub-system might define IPayableItem as a Role interface, implemented by many types in other parts of the application in order that Payments may be generated for them. Those generated Payment objects may be retrieved via the API but defined by the Result interface IPayment.
The only way I can currently distinguish these is by naming convention and/or commenting. Ideally, I would like the distinction enforced by the language, and have it enforce the rule: you can't implement a Result interface outside the API, only use it. But C# doesn't provide any such mechanism. (Can anyone advise me of a language that does?). I could define an attribute, but this still wouldn't enforce anything.
Another important significance of the distinction lies in Semantic Versioning of the API. If I add a new member to a Role interface then this should be seen as a breaking change (and hence a first-level version) - because any existing external implementations will need to add that member. But if I add a member to what I deem to be a 'Result' interface then it should only be my own code that is impacted - it is just a new feature (second-level version) for everyone else. But with no enforced distinction between the two types there's some risk that people are implementing the Result interfaces and hence their code would be broken.
Has anyone else encountered this dilemma? If so, how have you dealt with it? I look forward to your answers.
But please don't both to respond with either of the following arguments (which I have heard all too often):
My Result interfaces should be abstract classes rather than interfaces. This does not solve the problem, and potentially makes it worse, since external code can sub-class them.
I should be returning the concrete type and ensuring that anything I don't want accessible outside the API is marked 'internal'. There are lots of cases where I need things inside the API to be public, e.g. to be accessible to other frameworks (not going through the API).
I think what you're asking is it possible to expose an interface, but determine that a given instance is one you created?
If so, you could also create an internal private interface, and mark all your implementations as also implementing the private interface. Then upon being given an object from the outside world, verify it has the internal interface implementation as well.
public interface IPublic
{
...
}
internal interface ISecret { }
public class PublicImplementation : IPublic, ISecret
{
...
}
Only you can implement the ISecret, so even if someone implements the IPublic and passes it to you, it will fail the ISecret test.
Related
I am developing a set of classes that implement a common interface. A consumer of my library shall expect each of these classes to implement a certain set of static functions. Is there anyway that I can decorate these class so that the compiler will catch the case where one of the functions is not implemented.
I know it will eventually be caught when building the consuming code. And I also know how to get around this problem using a kind of factory class.
Just curious to know if there is any syntax/attributes out there for requiring static functions on a class.
Ed Removed the word 'interface' to avoid confusion.
No, there is no language support for this in C#. There are two workarounds that I can think of immediately:
use reflection at runtime; crossed fingers and hope...
use a singleton / default-instance / similar to implement an interface that declares the methods
(update)
Actually, as long as you have unit-testing, the first option isn't actually as bad as you might think if (like me) you come from a strict "static typing" background. The fact is; it works fine in dynamic languages. And indeed, this is exactly how my generic operators code works - it hopes you have the static operators. At runtime, if you don't, it will laugh at you in a suitably mocking tone... but it can't check at compile-time.
No. Basically it sounds like you're after a sort of "static polymorphism". That doesn't exist in C#, although I've suggested a sort of "static interface" notion which could be useful in terms of generics.
One thing you could do is write a simple unit test to verify that all of the types in a particular assembly obey your rules. If other developers will also be implementing the interface, you could put that test code into some common place so that everyone implementing the interface can easily test their own assemblies.
This is a great question and one that I've encountered in my projects.
Some people hold that interfaces and abstract classes exist for polymorphism only, not for forcing types to implement certain methods. Personally, I consider polymorphism a primary use case, and forced implementation a secondary. I do use the forced implementation technique fairly often. Typically, it appears in framework code implementing a template pattern. The base/template class encapsulates some complex idea, and subclasses provide numerous variations by implementing the abstract methods. One pragmatic benefit is that the abstract methods provide guidance to other developers implementing the subclasses. Visual Studio even has the ability to stub the methods out for you. This is especially helpful when a maintenance developer needs to add a new subclass months or years later.
The downside is that there is no specific support for some of these template scenarios in C#. Static methods are one. Another one is constructors; ideally, ISerializable should force the developer to implement the protected serialization constructor.
The easiest approach probably is (as suggested earlier) to use an automated test to check that the static method is implemented on the desired types. Another viable idea already mentioned is to implement a static analysis rule.
A third option is to use an Aspect-Oriented Programming framework such as PostSharp. PostSharp supports compile-time validation of aspects. You can write .NET code that reflects over the assembly at compile time, generating arbitrary warnings and errors. Usually, you do this to validate that an aspect usage is appropriate, but I don't see why you couldn't use it for validating template rules as well.
Unfortunately, no, there's nothing like this built into the language.
While there is no language support for this, you could use a static analysis tool to enforce it. For example, you could write a custom rule for FxCop that detects an attribute or interface implementation on a class and then checks for the existence of certain static methods.
The singleton pattern does not help in all cases. My example is from an actual project of mine. It is not contrived.
I have a class (let's call it "Widget") that inherits from a class in a third-party ORM. If I instantiate a Widget object (therefore creating a row in the db) just to make sure my static methods are declared, I'm making a bigger mess than the one I'm trying to clean up.
If I create this extra object in the data store, I've got to hide it from users, calculations, etc.
I use interfaces in C# to make sure that I implement common features in a set of classes.
Some of the methods that implement these features require instance data to run. I code these methods as instance methods, and use a C# interface to make sure they exist in the class.
Some of these methods do not require instance data, so they are static methods. If I could declare interfaces with static methods, the compiler could check whether or not these methods exist in the class that says it implements the interface.
No, there would be no point in this feature. Interfaces are basically a scaled down form of multiple inheritance. They tell the compiler how to set up the virtual function table so that non-static virtual methods can be called properly in descendant classes. Static methods can't be virtual, hence, there's no point in using interfaces for them.
The approach that gets you closer to what you need is a singleton, as Marc Gravell suggested.
Interfaces, among other things, let you provide some level of abstraction to your classes so you can use a given API regardless of the type that implements it. However, since you DO need to know the type of a static class in order to use it, why would you want to enforce that class to implement a set of functions?
Maybe you could use a custom attribute like [ImplementsXXXInterface] and provide some run time checking to ensure that classes with this attribute actually implement the interface you need?
If you're just after getting those compiler errors, consider this setup:
Define the methods in an interface.
Declare the methods with abstract.
Implement the public static methods, and have the abstract method overrides simply call the static methods.
It's a little bit of extra code, but you'll know when someone isn't implementing a required method.
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Interface vs Abstract Class (general OO)
I can see their advantage in coordination of a developing team, or code that might be further developed by others.
But if not, is there a reason to use them at all? What would happen if I omit them?
Abstract – I'll be able to instantiate it. No problem. If it doesn't make sense – I won't.
Interface – I have that functionality declared in all classes deriving from it anyway.
Note: I'm not asking what they are. I'm asking whether they're helpful for anything but coordination.
Both are what I call contracts and can be used in the following fashion by an individual developer:
Abstract
Allows for polymophism of differing derived implementations.
Allows one to create base functionality which can be dictated or not that the derived class be required to implement.
Allows for a default operation to be runtime consumed if the derived does not implement or required to implement.
Provides a consistency across derived objects which a base class pointer can utilize without having to have the actual derived; hence allows generic operations on a derived object from a base class reference similar to an Interface in runtime operation.
Interface
Allows a generic pattern of usage as a defacto contract of operation(s).
This usage is can be targetted to the process in hand and allows for the
surgically precise operations for that contract.
Used to help with
factory patterns (its the object returned), mocking of data during
unit tests and the ability to replace an existing class (say from a
factory returning the interface) with a different object and it
doesn't cause any consumer of the factory any pain of refactoring due to the adherence of the interface contract.
Provides a pattern of usage which can be easily understood away from the static of the rest of the class's implementation.
Long story short are they required to get a job done? No.
But if you are into designing systems which will have a lifespan of more than one cycle, the upfront work by said architect will pay off in the long run whether on a team or by an individual.
++Update
I do practice what I preach and when handing off a project to other developers it was nice to say
Look at the interface IProcess which all the primary business classes adhere to. That process defines a system of goals which can help you understand the purpose and the execution of the business logic in a defined way.
While maintaining and adding new functionality to the project the interfaces actually helped me remember the flow and easily add new business logic into the project.
I think if you're not coordinating with others, it does two things
helps keep your from doing weird things to your own code. Imagine
your write a class, and use it in multiple projects. You may evolve
it in one project so that it is unrecognizable from it's cousin in
another project. Having an abstract class or interface makes you
think twice about changing the function signatures.
it gives you flexibility going forward - plenty of classic examples here. Use
the generic form of the thing you're trying to accomplish, and if
you decide you need a different kind later (streamreaders are a
great example, right?) you can more easily implement it later.
Abstract - you can instantiate a child of it, but what is more important, it can has its own non abstract methods and fields.
Interface - more "rough" one in regard of abstract, but in .NET you can have multiple inheritance. So by defining interface you can lead consumer of your interface(s) to subscribe to different contracts(interfaces), so present different "shapes" of specified type.
There are many reasons to use either construct even if you are not coordinating with anyone. The main use is that both actually help express the developper intent, which may help you later figure out why you choose the design you actually chose. They also may allow for further extensibility.
Abstract class allow you to define one common implementation that will be shared across many derived classes while delegating some of the behavior to the child classes. It allows the DRY (don't repeat yourself) principle to avoid having the same code repeated everywhere.
Interfaces expresses that your class implements one specific contract. This has a very useful uses within the framework, among which:
Use of library functionality that necessitate the implementation of some Interface. Examples are IDisposable, IEquatable, IEnumerable...
Use of constraints in generics.
Allow mocking of interfaces (if you do unit testing) whithout having to instanciate a real object.
Use of COM objects
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I have a function that returns same kind of objects (query results) but with no properties or methods in common. In order to have a common type I resorted using an empty interface as a return type and "implemented" that on both.
That doesn't sound right of course. I can only console myself by clinging to hope that someday those classes will have something in common and I will move that common logic to my empty interface. Yet I'm not satisfied and thinking about whether I should have two different methods and conditionally call next. Would that be a better approach?
I've been also told that .NET Framework uses empty interfaces for tagging purposes.
My question is: is an empty interface a strong sign of a design problem or is it widely used?
EDIT: For those interested, I later found out that discriminated unions in functional languages are the perfect solution for what I was trying to achieve. C# doesn't seem friendly to that concept yet.
EDIT: I wrote a longer piece about this issue, explaining the issue and the solution in detail.
Although it seems there exists a design pattern (a lot have mentioned "marker interface" now) for that use case, i believe that the usage of such a practice is an indication of a code smell (most of the time at least).
As #V4Vendetta posted, there is a static analysis rule that targets this:
http://msdn.microsoft.com/en-us/library/ms182128(v=VS.100).aspx
If your design includes empty interfaces that types are expected to implement, you are probably using an interface as a marker or a way to identify a group of types. If this identification will occur at run time, the correct way to accomplish this is to use a custom attribute. Use the presence or absence of the attribute, or the properties of the attribute, to identify the target types. If the identification must occur at compile time, then it is acceptable to use an empty interface.
This is the quoted MSDN recommendation:
Remove the interface or add members to it. If the empty interface is being used to label a set of types, replace the interface with a custom attribute.
This also reflects the Critique section of the already posted wikipedia link.
A major problem with marker interfaces is that an interface defines a contract for implementing classes, and that contract is inherited by all subclasses. This means that you cannot "unimplement" a marker. In the example given, if you create a subclass that you do not want to serialize (perhaps because it depends on transient state), you must resort to explicitly throwing NotSerializableException (per ObjectOutputStream docs).
You state that your function "returns entirely different objects based on certain cases" - but just how different are they? Could one be a stream writer, another a UI class, another a data object? No ... I doubt it!
Your objects might not have any common methods or properties, however, they are probably alike in their role or usage. In that case, a marker interface seems entirely appropriate.
If not used as a marker interface, I would say that yes, this is a code smell.
An interface defines a contract that the implementer adheres to - if you have empty interfaces that you don't use reflection over (as one does with marker interfaces), then you might as well use Object as the (already existing) base type.
You answered your own question... "I have a function that returns entirely different objects based on certain cases."... Why would you want to have the same function that returns completely different objects? I can't see a reason for this to be useful, maybe you have a good one, in which case, please share.
EDIT: Considering your clarification, you should indeed use a marker interface. "completely different" is quite different than "are the same kind". If they were completely different (not just that they don't have shared members), that would be a code smell.
As many have probably already said, an empty interface does have valid use as a "marker interface".
Probably the best use I can think of is to denote an object as belonging to a particular subset of the domain, handled by a corresponding Repository. Say you have different databases from which you retrieve data, and you have a Repository implementation for each. A particular Repository can only handle one subset, and should not be given an instance of an object from any other subset. Your domain model might look like this:
//Every object in the domain has an identity-sourced Id field
public interface IDomainObject
{
long Id{get;}
}
//No additional useful information other than this is an object from the user security DB
public interface ISecurityDomainObject:IDomainObject {}
//No additional useful information other than this is an object from the Northwind DB
public interface INorthwindDomainObject:IDomainObject {}
//No additional useful information other than this is an object from the Southwind DB
public interface ISouthwindDomainObject:IDomainObject {}
Your repositories can then be made generic to ISecurityDomainObject, INorthwindDomainObject, and ISouthwindDomainObject, and you then have a compile-time check that your code isn't trying to pass a Security object to the Northwind DB (or any other permutation). In situations like this, the interface provides valuable information regarding the nature of the class even if it does not provide any implementation contract.
What is need of interfaces in c# ? as we are writing abstract method in interfaces. instead of that we can directly implement those methods in class.
Interfaces don't support implementation, so you cannot supply any default implementations as you can with abstract classes. Additionally, interfaces are not restricted to hierarchies, so they are more flexible than abstract classes.
You do not need to use interfaces in C#. They are useful and appropriate in some circumstances, but not in all circumstances. A handy rule of thumb that I use is that if, in your project, you only have one class that implements an interface, you do not need that interface.
Note: one possible counter to this rule of thumb is that you may need to write a second implementing class in the future, which may justify the use of the interface. I do not necessarily agree, as I think a considerable amount of time in programming is wasted anticipating future scenarios which never materialize.
You may want to read up on polymorphism.
For myself, I find a lot of use out of interfaces when I have similar objects but completely different implementations of the same methods.
Additionally, you can implement multiple interfaces but inherit only one abstract class. I find this very useful because my business objects have a better representation.
When writing any N-Tiered application which separates out business logic from the presentation I think you will start to find many uses for interfaces.
Interface is needed exactly as described in books: to define a contract between components. They are one of the best ways to expose certain functionality to other modules while preserving encapsulation.
For example:
1) try, without an interface, to expose some piece of functionality implemented in assembly 'A', to assembly 'B', with no actual implementation visible to assembly 'A'.
2) Even worse - if we consider .NET remoting scenarios, where the server must expose certain
functionality to the client, while the functionality is implemented and hosted on the server side. In this case an assembly is published to a client, where interfaces for the server-hosted classes are defined.
Think of Interfaces as contracts.
You create a contract that a class must follow. For example, if our Dog object must have a Walk method, it's defining class must implement this method.
In order to force every dog class (inherited or not) to implement this method, you must make them adhere to a contract i.e assign an interface which specifies that method.
An interface is a construct that enforces particular classes to follow strict rules of implementation.
The reason for this is that you end up with Dog objects (inherited or not) that now, by default, have a Walk method. This means you can pass these objects as parameters, safe in the knowledge that you can call the Walk method on any Dog class (inherited or not) and it will deffinately be implemented.
If you want to write testable code, you will usually need to employ interfaces. When unit testing, you may have ClassA which depends upon ClassB which Depends upon ClassC etc, but you only want to test ClassA. You certainly don't want to create a ClassC to pass to a ClassB just to instantiate ClassA.
In that case, you make ClassA depend upon IClassB (or some more generic name, most likely that does not imply anything about the ClassB implementation) and mock out IClassB in your tests.
It is all about dependency management for me.
You need interfaces when you want to think of a disparate set of classes as all being the same type of object. Say, for example, you have a set of classes that all read their configuration from a file. One way to handle this is to have all the classes implement the appropriate methods to read a configuration from a file. The trouble with this is that then any code that uses those classes and wants to configure them needs to know about all the different classes so that it can use the methods on them.
Another way is to have them all derive from a single base class. That way any code using the classes need only know about the base class -- it can treat any of the derived classes as the base class and use those methods defined in the base class to do the configuration. This isn't so bad, but it has the major drawback -- since C# doesn't support multiple inheritance -- of limiting your inheritance chain. If you want to be able to have the same sort of ability for some of the classes, but not all of them for a different set of behavior, you're stuck implementing it for all of them anyway. Not good.
The last way is to use interfaces to define the behavior. Any class wanting to implement the behavior need only implement the interface. Any class wanting to use the behavior need only know about the interface and can then use any class that implements it. Classes can implement any interface or even multiple interfaces so you have granular "allocation" of behavior among classes. You can still use base classes and inheritance hierarchies to provide the main behavior of a class in a shared way, but the class is also free to implement other interfaces to give itself more behavior and still retain the convenience of classes that use it to know only about the interface.
Interfaces allow implementers to use their own base class. With abstract classes, implementers are forced to use the given base class to implement the interface even if it actually makes more sense for the implementer to use a project-specific base class.
Interfaces are used to define the behaviour/properties of classes without specifying the implementation. If you begin thinking about classes as fulfilling roles rather than just being a bundle of methods and properties then you can begin assigning multiple roles to classes using interfaces - which is not possible using straight inheritance in C# as you can only inherit from a single class. By making clients of the classes depend on the interfaces (roles) rather than the class itself your code will be more loosely coupled and probably better designed.
Another benefit of this is if you are writing unit tests they are more likely to be focussed on behaviour rather than state. Coding against interfaces makes it very easy to make mock/stub implementations for this type of testing.
Not everything will need interfaces - but most things with behaviour probably (i.e. more than just a bundle of data) should have them IMHO as you'll always have two implementations: a real one in your application and one or more fake ones in your tests.
Interface is a contract that defines the signature of the functionality. So if a class is implementing
a interface it says to the outer world, that it provides specific behavior.
Example if a class is
implementing ‘Idisposable’ interface that means it has a functionality to release unmanaged
resources. Now external objects using this class know that it has contract by which it can dispose
unused unmanaged objects.
As presented by GoF 1st principle: Program to an interface not an implementation.
This helps in many ways. It's easier to change the implementation, it's easier to make the code testable and so on...
Hopes this helps.
I am developing a set of classes that implement a common interface. A consumer of my library shall expect each of these classes to implement a certain set of static functions. Is there anyway that I can decorate these class so that the compiler will catch the case where one of the functions is not implemented.
I know it will eventually be caught when building the consuming code. And I also know how to get around this problem using a kind of factory class.
Just curious to know if there is any syntax/attributes out there for requiring static functions on a class.
Ed Removed the word 'interface' to avoid confusion.
No, there is no language support for this in C#. There are two workarounds that I can think of immediately:
use reflection at runtime; crossed fingers and hope...
use a singleton / default-instance / similar to implement an interface that declares the methods
(update)
Actually, as long as you have unit-testing, the first option isn't actually as bad as you might think if (like me) you come from a strict "static typing" background. The fact is; it works fine in dynamic languages. And indeed, this is exactly how my generic operators code works - it hopes you have the static operators. At runtime, if you don't, it will laugh at you in a suitably mocking tone... but it can't check at compile-time.
No. Basically it sounds like you're after a sort of "static polymorphism". That doesn't exist in C#, although I've suggested a sort of "static interface" notion which could be useful in terms of generics.
One thing you could do is write a simple unit test to verify that all of the types in a particular assembly obey your rules. If other developers will also be implementing the interface, you could put that test code into some common place so that everyone implementing the interface can easily test their own assemblies.
This is a great question and one that I've encountered in my projects.
Some people hold that interfaces and abstract classes exist for polymorphism only, not for forcing types to implement certain methods. Personally, I consider polymorphism a primary use case, and forced implementation a secondary. I do use the forced implementation technique fairly often. Typically, it appears in framework code implementing a template pattern. The base/template class encapsulates some complex idea, and subclasses provide numerous variations by implementing the abstract methods. One pragmatic benefit is that the abstract methods provide guidance to other developers implementing the subclasses. Visual Studio even has the ability to stub the methods out for you. This is especially helpful when a maintenance developer needs to add a new subclass months or years later.
The downside is that there is no specific support for some of these template scenarios in C#. Static methods are one. Another one is constructors; ideally, ISerializable should force the developer to implement the protected serialization constructor.
The easiest approach probably is (as suggested earlier) to use an automated test to check that the static method is implemented on the desired types. Another viable idea already mentioned is to implement a static analysis rule.
A third option is to use an Aspect-Oriented Programming framework such as PostSharp. PostSharp supports compile-time validation of aspects. You can write .NET code that reflects over the assembly at compile time, generating arbitrary warnings and errors. Usually, you do this to validate that an aspect usage is appropriate, but I don't see why you couldn't use it for validating template rules as well.
Unfortunately, no, there's nothing like this built into the language.
While there is no language support for this, you could use a static analysis tool to enforce it. For example, you could write a custom rule for FxCop that detects an attribute or interface implementation on a class and then checks for the existence of certain static methods.
The singleton pattern does not help in all cases. My example is from an actual project of mine. It is not contrived.
I have a class (let's call it "Widget") that inherits from a class in a third-party ORM. If I instantiate a Widget object (therefore creating a row in the db) just to make sure my static methods are declared, I'm making a bigger mess than the one I'm trying to clean up.
If I create this extra object in the data store, I've got to hide it from users, calculations, etc.
I use interfaces in C# to make sure that I implement common features in a set of classes.
Some of the methods that implement these features require instance data to run. I code these methods as instance methods, and use a C# interface to make sure they exist in the class.
Some of these methods do not require instance data, so they are static methods. If I could declare interfaces with static methods, the compiler could check whether or not these methods exist in the class that says it implements the interface.
No, there would be no point in this feature. Interfaces are basically a scaled down form of multiple inheritance. They tell the compiler how to set up the virtual function table so that non-static virtual methods can be called properly in descendant classes. Static methods can't be virtual, hence, there's no point in using interfaces for them.
The approach that gets you closer to what you need is a singleton, as Marc Gravell suggested.
Interfaces, among other things, let you provide some level of abstraction to your classes so you can use a given API regardless of the type that implements it. However, since you DO need to know the type of a static class in order to use it, why would you want to enforce that class to implement a set of functions?
Maybe you could use a custom attribute like [ImplementsXXXInterface] and provide some run time checking to ensure that classes with this attribute actually implement the interface you need?
If you're just after getting those compiler errors, consider this setup:
Define the methods in an interface.
Declare the methods with abstract.
Implement the public static methods, and have the abstract method overrides simply call the static methods.
It's a little bit of extra code, but you'll know when someone isn't implementing a required method.