I have a .net-app that provides a mechanism to extend the app with plugins. Each plugin must implement a plugin-interface and must provide furthermore a constructor that receives one parameter (a resource context).
During the instantiation of the plugin-class I look via reflection, if the needed constructor exists and if yes, I instantiate the class (via Reflection). If the constructor does not exists, I throw an exception that says that the plugin not could be created, because the desired constructor is not available.
My question is, if there is a way to declare the signature of a constructor in the plugin-interface so that everyone that implements the plugin-interface must also provide a constructor with the desired signature. This would ease the creation of plugins.
I don’t think that such a possibility exists because I think such a feature falls not in the main purpose for what interfaces were designed for but perhaps someone knows a statement that does this, something like:
public interface IPlugin {
ctor(IResourceContext resourceContext);
int AnotherPluginFunction();
}
I want to add that I don't want to change the constructor to be parameterless and then set the resource-context through a property, because this will make the creation of plugins much more complicated. The persons that write plugins are not persons with deep programming experience. The plugins are used to calculate statistical data that will be visualized by the app.
Thanks for all the answers.
I’ve decided, that I let it be an interface because I don’t like to force the plugin-programmers to inherit from an abstract class so that he or she loses the possibility to inherit from an own base-class. Furthermore, deriving from an abstract class does not ensure that the plugin programmer really provides the needed constructor. It makes it only more probable (The programmer has still the possibility to add only one constructor that contains the desired parameter but that also has additional parameters, and that’s also bad. See the comments to the answer of Ken Browning).
Although I mentioned in my post that I don’t want such a property, I marked the answer of Danny Varod as accepted because I think in my situation it’s the most appropriate solution. Thanks to all who answered.
Plug-in extendability is a favorite of mine...
What I do is make sure the plug-in either implements the interface or inherits the base class of the appropriate "plugin socket".
In some places base classes are more appropriate (if the plug-in is a kind of X),
in some interfaces are more appropriate (if the plug-in does IX).
I do not pass the context to the construct, instead I use a property for that and a parameterless public constructor.
This also enables easier deserialization of plug-ins using reflection.
Interfaces cannot declare constructors. You might consider using an abstract class instead.
No, this does not exist. You are probably looking for an abstract class here.
Alternatively, you might try using a factory: make the constructor signature a method signature of another type:
public abstract class PluginFactory
{
public abstract IPlugin Create(IResourceContext context);
}
and then something like (and I always mess up this part if I want it to be short, hence the edit):
public class PluginContainer
{
public IPlugin LoadPlugin<T>(IResourceContext context) where T: PluginFactory, new()
{
var factory = new T();
return factory.Create(context);
}
}
Unfortunately, interfaces in C# can only contain methods, properties, events or indexers.
You could use and abstract class that all plugins would inherit from. You'd be able to force them to implement the constructor signature in that case.
The interface can't declare / enforce a constructor.
Define the interface and create an abstract base class that provides the most likely implementation of the constructor -- probably just saving the resource context passed in.
Encourage, but don't require, plugin authors to derive from the base class. There may be other useful methods that the base class could also provide.
Continue to use reflection to check the plugins.
As others have alluded to, using an abstract class to take care of the plumbing details is a common pattern for what you're trying to accomplish. Here is one design that avoids the need for a constructor with special parameters if the consumer inherits from the abstract base class Plugin:
public interface IPlugin
{
void Initialize(IResourceContext context);
//Other methods...
}
public abstract class Plugin : IPlugin
{
protected IResourceContext Context { get; private set; }
void IPlugin.Initialize(IResourceContext context)
{
Context = context;
}
//Abstract declaration of other methods...
}
Your code has to call Initialize behind the scenes after creating the Plugin, but this detail is hidden from typical users, as they generally don't have to implement IPlugin directly. Your typical user can just define a Plugin descendant and work with the Context property.
You might also want to look into various dependency injection frameworks (such as Ninject), though they're probably overkill for what you're doing. Still, looking at how they work may give you some ideas on different ways dependency injection can be managed.
Related
I'm developing a framework where a class inheriting from an abstract class of the framework needs to be able to specify the schema for the options it can accept when it is called to DoStuff().
I started out with an abstract GetOptionsSchema() method like this:
public abstract class Widget
{
public abstract OptionsSchema GetOptionsSchema();
public abstract void DoStuff(Options options);
}
Other developers would then extend on my framework by creating custom Widget types:
public abstract class FooWidget: Widget
{
public overide DoStuff(Options options)
{
//Do some FooWidget stuff
}
public overide OptionsSchema GetOptionsSchema()
{
//Return options for FooWidget
}
}
This works but requires the framework to create an instance of every Widget type to determine options schema they accept, even if it has no need to actually DoStuff() with any of these types.
Ultimately, I'd like to be able to determine the options schema for a specific Widget type directly from a System.Type. I would create a custom OptionsSchema attribute, but constructing these schemas is more complicated then would make sense to do in the constructor of an attribute. It needs to happen in a method.
I've seen other frameworks solve similar problems by creating a custom attribute that identifies a static method or property by name. For example the TestCaseSource attribute in NUnit.
Here's what this option might look like:
public abstract class Widget
{
public abstract void DoStuff(Options options);
}
[OptionsSchemaSource(nameof(GetOptionsSchema))]
public abstract class FooWidget: Widget
{
public overide DoStuff(Options options)
{
//Do some FooWidget stuff
}
public static OptionSchema GetOptionsSchema()
{
//Return options for FooWidget
}
}
I like how the OptionsSchemaSource attribute makes it possible to get the options schema directly from a System.Type, but this also seem much less discoverable to other developers creating custom Widget types.
With the abstract method another Widget developer knows they must override GetOptionSchema() because their code would not compile otherwise. With the OptionsSchemaSource attribute the best I could do would be to hope people read my documentation and have the framework throw an exception at run-time if it encounters a Widget with out an OptionsSchemaSource attribute.
Is there an alternative/better/recommended approach to this?
You pretty much already know everything of interest to judge what's the best approach.
As already mentioned, you cannot have static interfaces defined on your type, so there is no way you can ensure a new developer is enforced to add the attribute.
So, the two alternatives you identified are the only two I can think of.
Now, let's do a pros and cons and try to sharpen them.
Attribute
You can lessen the pain of ensuring devs put attributes on the classes with meaningful error messages. I would say that you should manage the discovery of the classes based exclusively on Attributes, not in inheritance.
If you manage everything with Attributes, you don't need to inherit from Widget.
This is a pro, because now everyone can inherit if it's desirable, and re-implement if it's preferred.
The con is that the implementation of discoverability will be more complex: you will need to use reflection at start up, get a MethodInfo, check that the method has the correct signature, give proper errors in case and invoke the method unboxing the result as needed.
Think about it: you would like a static method because you don't need to instantiate a single typed Widget instance, but actually instantiating a new Widget could very well be not a big deal.
Abstract class
Well, you enforce an inheritance chain over you developers, which could be ok, necessary or entirely optional (you judge), but you get a self documenting experience.
The apparent con is that at startup you need to instantiate a Widget for every derived type you discover, but that could very well be peanuts compared to assembly scanning and type checking and methodinfo discovery and method calls through reflection.
Ugly? Kind of. Inefficient? Not so much. And it's code that is invisible to your end user.
IMHO
I find quite a good tradeoff, when designing a framework, to put some "ugly" code inside the framework, if it means that every single implementation using the library is going to be even a little bit better.
All in all, if you're designing a library that you want to be flexible and discoverable, you should expect a developer to read at least a quick start guide. If they can read in 5 minutes a single bit of information (either "extend a base class" or "add a single or a couple attributes") and that single bit gives them an direction into discovering every aspect of widget registration, I would be ok: you can't really get much better than this.
My call: I would go the abstract class route with a smallish caveat. I really don't like having an enforced base class. So I would organize discovery at startup based on interface, IWidget, containing the GetOptionsSchema method and everything is needed to use the widget (which could be the DoStuff method, but could very well be something else). At startup you search for implementations of the interface which are not abstract, and you're good to go.
If, and only if, the only bit you really need in advance is a string or other similarly simple type, I would require an additional attribute.
[OptionsSchemaName("http://something")]
public class MyWidget : WidgetBase
{
public overide DoStuff(Options options)
{
//Do some FooWidget stuff
}
public static OptionSchema GetOptionsSchema()
{
//Return options for FooWidget
}
}
Then, your type discovery infrastructure can search for non-abstract IWidgets and throw a meaningful error right at startup like the type MyWidget is lacking an OptionsSchemaName attribute. Every implementation of IWidget must define one. See http://mydocs for information.
Bang! Nailed it!
It's not currently possible to enforce the attribute at compile time; that would've been ideal for your use case. It's also not possible to have an abstract static method, or have a static method specified in an interface; so there is no way to ensure the method is actually there at compile time, except by enforcing an instance method via abstract class or interface (which will require an instance of the type to access).
I'd go with the attribute idea - it's not unreasonable to expect developers to read documentation; even with overriding an abstract method, the developer would need to know how to construct an OptionSchema in the overridden method - back to documentation!
Assume this hypothetical situation:
I have a hierarchy of classes:
public class MyBase : System.Windows.Forms.TreeNode
{
public virtual void Init() {...}
}
Now I want to allow third parties to use MyBase to develop their derived classes like these:
public class Drv1 : MyBase { public override void Init() {...} }
public class Drv2 : MyBase { public override void Init() {...} }
I want my application be able to use Drv1 and Drv2 as plug-ins.
Now, my questions are:
Is it incorrect (or bad practice) to use classes (instead of interfaces) to set up plug-in mechanism?
Did I make a mistake I didn't use interfaces to provide THIRD-PARTIES with an interface? (because I want to persuade others to develop plug-ins for my app)
If answer of question 2 is YES, how could I use interfaces (because MyBase is derived from TreeNode) ? (this answer is critical for me)
Many thanks in advance.
Im using following rules:
If there is any code required in base then go for class.
If you need only structure or you need to "inherit" more than one class, use interfaces.
If you need both, features and multiple inheritance use both.
Its really depends what you do with that classes later on.
In your case you should be using base class as virtual method has some code in it, and you inherit from class that is 3rd party for you.
But once your business classes should use different implementation of that class then its worth of adding interfaces and use it in IoC or something.
I think going for Interfaces for only sake of it is not correct approach.
Is it incorrect (or bad practice) to use classes (instead of interfaces) to set up plug-in mechanism?
Neither C# or .NET has anything that labels this as incorrect. They describe under what circumstances your code will continue to work, and when it won't. Bad practice is a matter of opinion, but there are advantages and disadvantages to both approaches.
If answer of question 2 is YES, how could I use interfaces (because MyBase is derived from TreeNode) ? (this answer is critical for me)
If your callers need to provide a type that is derived from TreeNode, and you wish to use an interface, then you can.
public interface IMyInterface {
void Init() {...}
}
You cannot require classes implementing IMyInterface to derive from TreeNode, but you do not need to: you can ensure that the only way this gets exposed to your own application is via a generic registration method, where the generic type constraints do force the type to both derive from TreeNode and implement this interface:
public void RegisterTreeNode<T>() where T : TreeNode, IMyInterface {...}
If plugins are able to call RegisterTreeNode<Drv1>(), you're assured at compile time that it's going to match your requirements. You may of course use a different method signature, possibly one that deals with individual instances of the TreeNode class, it's the type constraints that are key here. If a caller attempts
class X : IMyInterface { public void Init() {...} }
and then
RegisterTreeNode<X>();
the compiler will simply reject this. The plugin may create instances of this X itself, but if your application never sees them, they cannot cause any harm.
Then third parties can do:
public class Drv1 : TreeNode, IMyInterface { ... }
public class Drv2 : TreeNode, IMyInterface { ... }
or even
public class Drv3 : SuperTreeNode, IMyInterface { ... }
where SuperTreeNode is derived from the standard TreeNode.
This is probably the main benefit of using an interface here: it's compatible with existing classes which provide additional functionality on top of the standard TreeNode.
This cuts both ways: the main benefit of using a common base class here, rather than an interface, would be that your own code can provide additional functionality.
P.S.: Depending on what you're after, it may also be possible to decouple this, to make your base class / interface responsible for creating TreeNode objects, rather than deriving from TreeNode. The general rule that favours this approach is called "composition over inheritance", and worth reading up on. It may or may not be a good fit for your particular use case.
In order to maintain binary backwards compatibility in .NET, you generally can't add new abstract methods to public classes and interfaces. If you do, then code built against the old version of the assembly that extends/implements your class/interface will fail at runtime because it fails to fully extend/implement the new version. For classes, however, there is a handy workaround:
public abstract class Foo {
internal Foo() { }
}
Because Foo's constructor is internal, no-one outside of my assembly can extend Foo. Thus, I can add new abstract methods to Foo without worrying about backward compatibility since I know that no class in another assembly can extend Foo.
My question is, is there a similar trick for interfaces? Can I create a public interface and somehow guarantee that no one outside of my assembly will be able to create an implementation of it?
No, you can't do that. But then, considering that the point of an interface is to define the behavior of an implementation by defining a contract, that makes sense.
What you can do, however, is create an internal interface that inherits from your public interface:
public interface IPublicInterface {
/* set-in-stone method definitions here */
}
internal interface IChildInterface : IPublicInterface {
/* add away! */
}
This should prevent any backwards compatibility issues with other assemblies while still allowing you to hide additional methods.
The downside, of course, is that you would have to remember to cast as IChildInterface when you need those, rather than simply being able to use it as an IPublicInterface
In all honesty, though, if you really wanted to define some assembly-only functionality while still requiring that the end user define their own implementations for some methods, then your best bet is probably an abstract class.
No, you can't.
But since in IL an interface is essentially just a pure abstract class (i.e. one without any implementation at all), you can use the technique you've already described and it will be practically the same.
As noted, keep in mind that this approach does restrict your type to inheriting just the fake "abstract class" interface. It can implement other interfaces, but won't be able to inherit any other type. This may or may not be a problem, depending on the scenario.
If it makes you feel better about the design, name your pure abstract class following the .NET convention for interfaces. E.g. IFoo instead of Foo.
Of course, it does imply the question: why do you want to do this? If you have no implementation at all, what harm could come from allowing other code to implement your interface?
But from a practical point of view, it's possible to enforce your rules the way you want.
I'm doing some internal domain-specific library development at the moment, and incidentally the stuff i'm trying to model mimicks "class" and "object" relations fairly well. So objects of my C# class MyClass should sort of act like a domain specific class for objects of my C# class MyObject who play the part of object or instance. Now I would like the code in MyObject to access methods of MyClass, which should not be accessible to other classes/code in the project. Any ideas how to enforce this, asside from documenting it at hoping my fellow developers will respect this.
I hope I made my question clear enough, otherwise let me know.
Best regards!
You could always split MyClass and MyObject up into another project, and define MyClass and/or MyObject as an internal class. That way it can only be accessed by other objects in that assembly.
See: http://msdn.microsoft.com/en-us/library/7c5ka91b(VS.80).aspx
The standard approach here is to declare the members internal and make sure MyClass and MyObject are part of the same assembly. That assembly should contain little else.
Additional: This is the tool that was designed for this purpose. Other languages have other means to fine-tune accessibility (C++: friend) but in .NET a simpler model was chosen.
And you don't have to take the 'nothing else' so strictly, the 2 classes could share an assembly with other related classes. you would then have to verify the no-access rule(s) manually inside that library.
I'd suggest a private nested class. That way, even if your fellow devs are writing code in the same namespace, they'll never be able to access the class.
Once the class declaration is fully enclosed within another class declaration, the class is considered nested and can only be accessed through the containing class.
Pehaps your MyObject should descend from MyClass and declare the methods in MyClas as protected.
If you don't want your consumers to invoke certain implementation specific methods you could try abstracting to interfaces or abstract base classes. That way the consumer will only 'see' the properties and methods you want them to see.
You do not have to use inheritance to provide shared functionality and you do not have to rely on member accesibility to prevent others from using methods you'd rather not expose.
For example:
public interface IDomainSpecific
{
void DoStuff();
}
public interface IDomainService
{
void HelpMeDoStuff();
}
public class DomainObject1 : IDomainSpecific
{
private readonly IDomainService _service;
public DomainObject1( IDomainService service )
{
_service = service;
}
public DoStuff()
{
// Do domain specific stuff here
// and use the service to help
_service.HelpMeDoStuff();
}
}
This uses classic constructor injection and works best when you already use dependency injection in your application, though it works perfectly well with factories as well.
The point is to keep responsibilities crystal clear. There's no chance of anybody invoking anything they shouldn't because the 'DomainObject' never knows what concrete type implements the shared service. The shared service is not exposed on the domain object either. The added bonus is testability and the possibility of swapping the service with another implementation without ever needing to touch the DomainObject.
I'm trying to come to terms with using IoC/Dependency Injection while at the same time programming to contracts rather than specific classes. The dilemma I'm having is the tension between:
Do program to interfaces for IoC: I started out with IoC relying heavily on interfaces. Judging by Spring's sample projects, interfaces are the way to go when programing to a contract with IoC.
( ... although abstract classes generally preferred: the main drawback of interfaces is that they are much less flexible than classes when it comes to allowing for evolution of APIs )
Do make class dependencies explicit via constructor
My gut feeling is that it's good programming practice to pass dependencies in to a class's constructor. Indeed, this is dependency injection.
... except you can't enforce constructor signature in interfaces/abstract clases: Neither interfaces or nor abstract classes allow for defining a constructor signature ( easily / elegantly ).
See also Framework Design Guidelines section 4.4: DO NOT define public or protected internal constructors in abstract types. ... Constructors should be public only if users will need to create instances of the type.
This question is related to a previous stackoverflow question: Interface defining a constructor signature?
But my question is:
Since you can't define a constructor in a C# interface/abstract class, as the question above asks, on a practical level:
How do you reconcile this with the sensible practice of passing dependencies in via a constructor?
Edit: Thank you for the answers. I'm hoping for some insight on what I should do in this case. Just not use contructor args? Use some sort of Init() method that does take the dependencies?
Edit2: Thanks for the great answers, very helpful.
I always think this is easier to explain with a (made up) example...
Imagine you have an ICustomerRepository interface, an IShoppingCartRepository interface and an ICheckout interface. You have concrete implementations of those interfaces - CustomerRepository, ShoppingCartRepository, and CheckoutService.
Your CheckoutService concrete class has a constructor that takes an ICustomerRepository and an IShoppingCartRepository - e.g.
public CheckoutService(ICustomerRepository customerRepository, IShoppingCartRepository shoppingCartRepository)
{
// Set fields for use in some methods later...
_customerRepository = customerRepository;
_shoppingCartRepository = shoppingCartRepository;
}
Then, when you want an ICheckoutService implementation to do some work with, you tell your IoC container which concrete class it should use for each interface type and ask it to build you an ICheckoutService. Your IoC container will go and build your classes for you, injecting the correct concrete classes into the constructor of your CheckoutService. It will also build dependencies all the way down the class heirarchy here, so if, for example your ShoppingCartRepository takes an IDatabaseSession interface in the constructor, your IoC container will inject that dependency too, as long as you have told it which concrete class to use for your IDatabaseService.
Here's some code you might use when configuring (for example) StructureMap as your IoC container (this code would typically be called during app startup):
public class AppRegistry : Registry
{
public AppRegistry()
{
ForRequestedType<ICheckoutService>().TheDefaultIsConcreteType<CheckoutService>();
ForRequestedType<ICustomerRepository>().TheDefaultIsConcreteType<CustomerRepository>();
// etc...
}
}
Then to get an instance of ICheckoutService built up and ready to go, with all the dependencies passed into the constructor for you, you would use something like:
var checkoutService = ObjectFactory.GetInstance<ICheckoutService>();
I hope that makes sense!
Your IoC container must construct an object from a concrete type, even though what you're passing around is an interface. Your constructor is not a behavior or state contract, so it does not belong in an interface or as a public member of your abstract class.
A constructor is an implementation detail, so you do not need to separate its definition from the concrete class.
You cannot define constructor signatures in interfaces. That wouldn't make sense anyway since the interface shouldn't enforce how the implementations are constructed.
Abstract classes though can indeed have constructors. They must be protected since public constructors does not make sense either. They should only be called by concrete subclasses.
The IoC principle dictates that instead of having class A know about and instantiate class B, you should instead pass in a reference to IB to the constructor of A. Then A will not need to know about class B and thus you can easily substitute class B with some other implementation of IB.
Since you're passing in an already instantiated object of class B, the IB interface doesn't need to have a constructor signature.