I feel like this should be very possible.
I have an interface, let's call it IJerry. Now, I have a class in variable x. That class implements IJerry perfectly. The thing is, that class does not ever reference IJerry. It just happens to have a perfect, compliant signature with IJerry.
Make sense? Let's say you create a class called MyClass that implements INotifyPropertyChanged. Then you delete the MyClass : INotifyPropertyChanged declaration from the class but you LEAVE the implementation inside the class.
Is there a way to determine if the class "implements" an interface even if it does not make an explicit reference to it?
Not easily.
You would have to read the fields, method, and properties of the interface using reflection, and then check if the class has them (again using reflection)
Alternately, if you are using C#4, you could just forget about IJerry, and put MyClass in a dynamic variable, and then you C# figure out at run-time for it has the methods being called.
There's a lot more to implementing an interface than meets the eye. For one, implementation methods are virtual, even though you don't use that keyword. In fact, you're not allowed to use that keyword. For another, the compiler rearranges the methods to match the method table layout of the interface. Removing the inherited interface from the declaration is guaranteed to make the result incompatible. The methods won't be virtual anymore.
What you are pursuing is called 'dynamic dispatch'. Implemented in the DLR and integrated into .NET 4.0 and the C# 4.0 language. Re-inventing the System.Reflection code and making it efficient is a major undertaking.
You would have to use reflection to see if x had methods that matched the ones on IJerry. The real question is, what are you going to do with the answer? Prior to version 4, C# doesn't support "duck typing", so in order to use your class where an IJerry is required you have to write adapter code.
Related
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 created a class called MostRecentStack<T> which is a stack that only keeps a certain number of items, dropping the ones at the bottom to make room for new ones. I'd like to have a variable that can store a reference to either a regular ("infinite") stack, or one of my custom type, depending on the circumstances, but C# defines no generic "stack" interface. Normally this wouldn't be a problem, but I'd like System.Collections.Generic.Stack<T> to implement the interface as well.
As long as a class provides the required members, is there any way to, in the interface definition, tell the compiler to consider a class as implementing the interface? I'd like to be able to do this without having to use as or other methods of typecasting.
The exact thing you're asking for isn't possible. However, something like should be very similar to what you want:
public class CompatibleStack<T> : System.Collections.Generic.Stack<T>, IYourStackInterface<T>
{
}
The CompatibleStack is functionally equivalent to System.Collections.Generic.Stack, except it now implements IYourStackInterface.
As long as System.Collections.Generic.Stack has all the right members to implement IYourStackInterface, this should compile fine. And you can pass a CompatibleStack around as an IYourStackInterface without any problems.
No, it is not possible to add new interface to existing class that you don't own. Options:
if you get instance of the class via some dependency injection controller you may be able to wrap class with proxy that will implement interface by calling matching methods.
you can simply derive from existing class and add interface (if it is not sealed) and start using your class.
in your particular case as Baldrick pointed out you can do reverse - derive from existing class and implement interface.
you can try to use dynamic to get some duck typing (as both classes will have matching methods) for some performance, readability and strong type cost.
Side note: in general C# does not support duck typing, but there is one case (foreach) where implementing interface is not strictly required - just having correct methods on collection is enough to support foreach.
I was trying NBuilder in my unit test. An excellent library. However, I could not explain the following structure of classes and interfaces.
In FizzWare.NBuilder namespace:
ISingleObjectBuilder
SingleObjectBuilderExtensions
In FizzWare.NBuilder.Implementation
IObjectBuilder`
ObjectBuilder
SingleObjectBuilderExtensions is simply a wrapper on IObjectBuilder.
The client code should usually use a class named Builder which has a static method that gives you ISingleObjectBuilder. You never need to instantiate any of the classes in client code.
Now, I dont get the point of the SingleObjectBuilderExtensions. Does it give any kind of design benefit? Why not the methods are directly in ISingleObjectBuilder specially when the two interfaces are in same namespace.
ISingleObjectBuilder is an interface; interfaces cannot provide implementation. That would mean that every implementation of ISingleObjectBuilder would need to provide the implementation.
However, in many cases, a method has a pre-defined behaviour, and just needs access to other members of the interface (i.e. just members of ISingleObjectBuilder), so there is no benefit in making each implementation provide this.
Additionally, it is not a good idea to add more members to an existing interface, since that would be a breaking change for all existing implementations.
Extension methods solve both of these issues:
the extension method will work for all implementations of ISingleObjectBuilder
it doesn't change the existing API, so all existing implementations will continue to be valid
Having it in the same namespace simply makes it convenient. It is a likely bet that any code using ISingleObjectBuilder already has a using directive importing that namespace; therefore, most code will already see the extension method in the IDE simply by pressing . in the IDE.
To add a concrete example, LINQ-to-Objects works on IEnumerable<T>. There are lots of IEnumerable<T> implementations. If each of them had to write their own First(...), FirstOrDefault(...), Any(...), Select(...) etc methods, that would be a huge burden - bot would provide no benefit as the implementations would be pretty much identical in most cases. Additionally, fitting this onto the interface retrospectively would have been disastrous.
As a side note: per-type versions of a method always take precedence over extension methods, so if (in the case of LINQ-to-Objects) you have a type that implements IEnumerable<T> (for some T), and that type has a .First() instance method, then:
YourType foo = ...
var first = foo.First();
will use your version, not the extension methods.
Extension methods are only syntactic sugar. They are normal static methods that work with public data the class they "extend" exposes.
There doesn't seem to be need for those extension methods to work with private fields and it also gives you ability to have interface instead of abstract class. And everyone knows interfaces are better choice than abstract classes.
And the reason for both to be in same namespace is to avoid declaring new namespace just to use the extension methods. How many times did it happen to you, that you tried to use LINQ only to notice there are no methods in intellisense or the code didn't compile. Reason being there was not System.Linq namespace included.
Is there a way to not be forced to include members with the MustOverride property when you inherit? I'm working on a custom MembershipProvider, and I only need access to a few members. It's purely an aesthetic thing - I just hate having to stub out 100 lines of unused member declarations.
You can right click the inherited class and select "Implement this ..." and it will create the method definitions for you throwing NotImplementedException() until you rewrite the method's code.
MustOverride (abstract in C#) specifies that it must be overridden. There is no way around this.
You can have Visual Studio stub out the features for you, but they need to exist in order for the class to be instantiated.
One other option: If you'll be doing multiple versions, you can make a class that implements the members (throwing NotImplementedException, or doing nothing), and then derive from THAT class. Your concrete version only would need to have the specific methods overriden that you wish.
No, there is not.
Btw, MustOverride is usually called abstract in C#.
Yes there is... Declare your class as abstract, too! :-P
Now, seriously, just think about it. If you would not implement an abstract member in a non-abstract class, imagine what would happen at runtime, when calling a method without implementation? In C++, this was possible, and you would get a "Pure virtual function call" error.
There's something I want to customize in the System.Web.Script.Services.ScriptHandlerFactory and other .NET stuff inside an internal class. Unfortunately, it's an internal class. What options do I have when trying to customize a method in this class?
You might find this recent article enlightening. Basically, it says that you can't override anything marked internal, and the source is about as authoritative as it gets. Best you can hope for is an extension method.
The internal keyword signifies that a unit of code (class, method, etc.) is "public" to the assembly it is in, but private to any other assembly.
Because you are not in the same assembly, you cannot do anything. If it wasn't internal you could use the new keyword on the method you're overriding (to hide the original implementation) when extending the class.
In short: you are to be SOL.
The only thing i can think of you could do is write a proxy class, where one of your private fields is the class you'd want to extend and you implement all it's methods and proxy their calls. that way you can still customize output, but you'd have to get your class used, and considering it's marked internal, i'm not sure that's possible without some serious hacking.
using System;
...
using System.Web.Script.Services
namespace MyGreatCompany.ScriptServices
{
public class MyScriptHandlerFactory /* implement all the interfaces */
{
private ScriptHandlerFactory internalFactory;
public MyScriptHandlerFactory()
{
internalFactory = new ScriptHandlerFactory();
}
...
}
}
This could make what you want to accomplish possible, but it won't be pretty.
I believe you can use Reflection to get around the access modifiers on a class, so perhaps you can use Reflection.Emit to generate a type that inherits from an internal type (but NOT the sealed modifier), though I can't find an example of this online.
This certainly works for accessing private members of classes, and probably for inheritance of non-sealed classes. But it doesn't help much if the target methods are not already marked virtual.
It depends on the assembly. This could possibly violate some licensing (although its similar to some sort of static linking), and maybe even make deployment a nightmare, but you could consider:
Decompile and copy the code over to your own project; modify as needed
Recompile/patch the assembly and add an "InternalsVisibleToAttribute"