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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
Is there a concept in C# of class definition and implementation similar to what you find in C++?
I prefer to keep my class definitions simple by removing most, if no every, implementations details (it depends on several factors as you may know, but generally I move towards leaving most member implementation details outside the class definition). This has the benefit of giving me a bird's eye view of the class and its functionality.
However in C# it seems I'm forced to define my member functions at the point of declaration. Can this be avoided, or circumvent some way?
During my apprenticeship of C#, this is one aspect that is bothering me. Classes, especially complex ones, become increasingly harder to read.
This is really a case of needing to step back and see the bigger picture. Visual studio has many, many tools to help you write and manipulate your code, from outlining, #regions, class view, class diagrams, the Code Definition Window and many more.
C# isn't C++, if you try to make it so then you'll trip over yourself and no-one else will be able to read your code.
A day spent learning to use the Visual Studio tools will repay the investment many times over in terms of productivity and you'll soon wonder how you ever lived with that C++ way of doing things.
Update in response to comments
I have long since stopped regarding my code as simple text files. I regard code as an organic thing and I find that allowing myself to rely on a feature-rich IDE lets me move up and down levels of abstraction more easily and enhances my productivity no end. I suppose that could be a personal trait and perhaps it is not for everyone; I have a very 'visual' mind and I work best when I can see things in pictures.
That said, a clever IDE is not an excuse for poor style. There are best practices for writing "clean code" that don't require an smart IDE. One of the principles of clean code is to keep the definition of something near its use and I think that could be extended to cover declaration and definition. Personally, I think that separating the declaration and definition makes the code less clear. If you are finding that you get monster classes that are hard to understand, then that might be a sign that you're violating the Single Responsibility Principle.
The reason for separate definition and declaration in c/C++ is because C++ uses a single pass compiler, where forward references cannot be resolved later, unlike C# and its two-pass compiler which can happily find references regardless of the order of declaration. This difference stems from the different design philosphies of the compilers: C/C++ considers each source file to be a unit of compilation, whereas in C# the entire project is considered to be the unit of compilation. I suppose when you are used to working in the C/C++ way then separating the declaration and definition can appear to be a desirable element of style, but I personally believe that keeping declaration and use (or in this case declaration and definition) enhances, rather then reduces, readability. I used to be a C programmer myself until I started using C# in 2001. I always loved C and thought it's way of doing things was the 'bees knees'. These days when I read C/C++ code I think it looks absolutely horrendous and I can't believe we used to put up with working that way. It's all a matter of what you are used to, I suppose.
If you're using Visual Studio, you can take advantage of the Class View. You can also use the expand/collapse features of the source code editor.
In the improbable case that your tools don't help, you can always write a quick utility that will summarize the class for you.
If the class has been compiled, you can use Reflector to view the class, too.
No, there is no concept of implementation and header files in C# like you find in C/C++. The closest you can come to this is to use an interface, but the interface can only define the public members of your class. You would then end up with a 1-to-1 mapping of classes and interfaces, which really isn't the intent for how interfaces are to be used.
You could get a similar result by defining an interface for each of your classes which they then implement.
It sounds like you're referring to interfaces. In c#, you can define all of your member functions in an interface, and then implement them in another class.
In C# you could fake it with partial classes and partial members to a point, however, forward declarations and prototypes go the way of the dodo bird with your newer languages. Class View, Class Diagrams, Intellisense, et al, all help to remove the potential need for those "features".
Define an interface.
Then it's nice to be able to automatically implement the interface using a nice code assist tool.
If you find that a class is hard to read or difficult to understand, that's often a sign that the class is trying to do too much. Instead of trying to duplicate C++'s separation of declarations and definitions, consider refactoring the troublesome class into several classes so that each class has less responsibility.
Whenever it's possible or desirable, I'll go with the previous responses and define an interface. but it's not always appropriate.
alternatively, you can work around this "problem" by using some static code inspection tools. Resharper's "File Structure" window will give you exactly what you want. you can also use the built in "Class View" from visual studio. but I prefer the former.
The prototyping that I guess you are referring to does not really exist in C#. Defining interfaces as others have suggested will give you a point where you have declarations of your methods collected, but it's not the same thing as prototypes, and I am not so sure that it will help you in making your implementation classes easier to read.
C# is not C++, and should probably not be treated as C++.
Not sure what you mean by your classes continue to grow and become hard to read. Do you mean you want a header file like view of a class's members? If so, like John suggested, can't you just collapse the implementation so you don't have to see it?
If you don't want every class to implement a certain thing, then interfaces are probably the way to go (like others are saying).
But as a side thought, if your classes themselves get more and more complex as a your write the program, perhaps it's more of a design issue than a language problem? I think a class should have one responsibility and not take on more and more responsibilities as the program grows, rather the number of classes and how old classes are used should grow and get more complex as you continue to develop your software?
There are two remedies for this to make it more C++-ish:
Create an interface file that declares all method signatures and properties
Implement that interface in a class across multiple files by using the partial modifier on the class definitions
Edits:
// File: ICppLikeInterface.cs
public interface ICppLikeInterface
{
...
}
// File: CppLikeImplementation1.cs
public partial class CppLikeImplementation : ICppLikeInterface
{
...
}
// File: CppLikeImplementation2.cs
public partial class CppLikeImplementation : ICppLikeInterface
{
...
}
The C++ way of separating interface into a header file is mostly (I think) due to an early design decision when C was created to allow fast, incremental compilations during the "old days", as the compiler throws away any meta data, contrary to Smalltalk. This is not a matter with C# (nor Java) where tens of thousands of lines compiles within seconds on recent hardware (C++ still doesn't)
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.
I have been reading that creating dependencies by using static classes/singletons in code, is bad form, and creates problems ie. tight coupling, and unit testing.
I have a situation where I have a group of url parsing methods that have no state associated with them, and perform operations using only the input arguments of the method. I am sure you are familiar with this kind of method.
In the past I would have proceeded to create a class and add these methods and call them directly from my code eg.
UrlParser.ParseUrl(url);
But wait a minute, that is introducing a dependency to another class. I am unsure whether these 'utility' classes are bad, as they are stateless and this minimises some of the problems with said static classes, and singletons. Could someone clarify this?
Should I be moving the methods to the calling class, that is if only the calling class will be using the method. THis may violate the 'Single Responsibilty Principle'.
From a theoretical design standpoint, I feel that Utility classes are something to be avoided when possible. They basically are no different than static classes (although slightly nicer, since they have no state).
From a practical standpoint, however, I do create these, and encourage their use when appropriate. Trying to avoid utility classes is often cumbersome, and leads to less maintainable code. However, I do try to encourage my developers to avoid these in public APIs when possible.
For example, in your case, I feel that UrlParser.ParseUrl(...) is probably better handled as a class. Look at System.Uri in the BCL - this handles a clean, easy to use interface for Uniform Resource Indentifiers, that works well, and maintains the actual state. I prefer this approach to a utility method that works on strings, and forcing the user to pass around a string, remember to validate it, etc.
Utility classes are ok..... as long as they don't violate design principles. Use them as happily as you'd use the core framework classes.
The classes should be well named and logical. Really they aren't so much "utility" but part of an emerging framwework that the native classes don't provide.
Using things like Extension methods can be useful as well to align functionality onto the "right" class. BUT, they can be a cause of some confusion as the extensions aren't packaged with the class they extend usually, which is not ideal, but, still, can be very useful and produce cleaner code.
You could always create an interface and use that with dependency injection with instances of classes that implement that interface instead of static classes.
The question becomes, is it really worth the effort? In some systems, the answer in yes, but in others, especially smaller ones, the answer is probably no.
This really depends on the context, and on how we use it.
Utility classes, itself, is not bad. However, It will become bad if we use it the bad way. Every design pattern (especially Singleton pattern) can easily be turned into anti-pattern, same goes for Utility classes.
In software design, we need a balancing between flexibility & simplicity. If we're going to create a StringUtils which is only responsible for string-manipulation:
Does it violate SRP (Single Responsibility Principle)? -> Nope, it's the developers that put too much responsibilities into utility classes that violate SRP.
"It can not be injected using DI frameworks" -> Are StringUtils implementation gonna varies? Are we gonna switch its implementations at runtime? Are we gonna mock it? Of course not.
=> Utility classes, themselve, are not bad. It's the developers' fault that make it bad.
It all really depends on the context. If you're just gonna create a utility class that only contains single responsibility, and is only used privately inside a module or a layer. Then you're still good with it.
I agree with some of the other responses here that it is the classic singleton which maintains a single instance of a stateful object which is to be avoided and not necessarily utility classes with no state that are evil. I also agree with Reed, that if at all possible, put these utility methods in a class where it makes sense to do so and where one would logically suspect such methods would reside. I would add, that often these static utility methods might be good candidates for extension methods.
I really, really try to avoid them, but who are we kidding... they creep into every system. Nevertheless, in the example given I would use a URL object which would then expose various attributes of the URL (protocol, domain, path and query-string parameters). Nearly every time I want to create a utility class of statics, I can get more value by creating an object that does this kind of work.
In a similar way I have created a lot of custom controls that have built in validation for things like percentages, currency, phone numbers and the like. Prior to doing this I had a Parser utility class that had all of these rules, but it makes it so much cleaner to just drop a control on the page that already knows the basic rules (and thus requires only business logic validation to be added).
I still keep the parser utility class and these controls hide that static class, but use it extensively (keeping all the parsing in one easy to find place). In that regard I consider it acceptable to have the utility class because it allows me to apply "Don't Repeat Yourself", while I get the benefit of instanced classes with the controls or other objects that use the utilities.
Utility classes used in this way are basically namespaces for what would otherwise be (pure) top-level functions.
From an architectural perspective there is no difference if you use pure top-level "global" functions or basic (*) pure static methods. Any pros or cons of one would equally apply to the other.
Static methods vs global functions
The main argument for using utility classes over global ("floating") functions is code organization, file and directory structure, and naming:
You might already have a convention for structuring class files in directories by namespace, but you might not have a good convention for top-level functions.
For version control (e.g. git) it might be preferable to have a separate file per function, but for other reasons it might be preferable to have them in the same file.
Your language might have an autoload mechanism for classes, but not for functions. (I think this would mostly apply to PHP)
You might prefer to write import Acme:::Url; Url::parse(url) over import function Acme:::parse_url; parse_url();. Or you might prefer the latter.
You should check if your language allows passing static methods and/or top-level functions as values. Perhaps some languages only allow one but not the other.
So it largely depends on the language you use, and conventions in your project, framework or software ecosystem.
(*) You could have private or protected methods in the utility class, or even use inheritance - something you cannot do with top-level functions. But most of the time this is not what you want.
Static methods/functions vs object methods
The main benefit of object methods is that you can inject the object, and later replace it with a different implementation with different behavior. Calling a static method directly works well if you don't ever need to replace it. Typically this is the case if:
the function is pure (no side effects, not influenced by internal or external state)
any alternative behavior would be considered as wrong, or highly strange. E.g. 1 + 1 should always be 2. There is no reason for an alternative implementation where 1 + 1 = 3.
You may also decide that the static call is "good enough for now".
And even if you start with static methods, you can make them injectable/pluggable later. Either by using function/callable values, or by having small wrapper classes with object methods that internally call the static method.
They're fine as long as you design them well ( That is, you don't have to change their signature from time to time).
These utility methods do not change that often, because they do one thing only. The problem comes when you want to tight a more complex object to another. If one of them needs to change or be replaced, it will be harder to to if you have them highly coupled.
Since these utility methods won't change that often I would say that is not much problem.
I think it would be worst if you copy/paste the same utility method over and over again.
This video How to design a good API and why it matters by Joshua Bloch, explains several concepts to bear in mind when designing an API ( that would be your utility library ). Although he's a recognized Java architect the content applies to all the programming languages.
Use them sparingly, you want to put as much logic as you can into your classes so they dont become just data containers.
But, at the same time you can't really avoid utilites, they are required sometimes.
In this case i think it's ok.
FYI there is the system.web.httputility class which contains alot of common http utilities which you may find useful.