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Limiting visibility of abstract base classes in C#

Tag: java,c#,interface,abstract,privacy

I feel like this question may be off topic as subjective, but I'm curious.

In java I'm used to using package private and public to hide the implementation of types from other packages. I usually have an interface such as:

public interface IMyClass {
    ...
}

Any common functionality of IMyClass implementations are then defined in a private abstract class within the same package

abstract class AMyClass implements IMyClass {
    public AMyClass(...)
    ...
}

Then the derived types are public and are also defined in the IMyClass package

public class CustomMyClass extends AMyClass {
    public CustomMyClass(...){
        super(...) 
        ...
}

In c# I would like to follow the same structure, but when you extend an abstract class it has to be public. The only thing I could find to prevent other packages from extending AMyClass or using its internal functions was to make their access level internal.

public interface IMyClass{
    ...
}
...
public class AMyClass : IMyClass {
    internal AMyClass(...)
    ...
}
...
public class CustomMyClass : AMyClass {
    public CustomMyClass(...) : base(...){
        ...
}

But this style still allows other c# projects 2 ways to group the subclasses, AMyClass or IMyClass types. This seems really sloppy, especially if I want to create another abstract base class for different types of IMyClass. In that case there would now be 2 abstract classes exposed that I don't want other projects to use.

Is there a way to prevent other projects from using the abstract classes, or is it simply something to put in the projects documentation and rely on an honor system of sorts?

Best How To :

It depends on what you want. Do you want to prohibit all use of the base abstract class by client code outside the assembly? Or do you simply want to prevent inheritance of it from such code?

The latter can be achieved simply by making the constructor(s) internal. This prevents any code outside your assembly from being able to inherit it, since they don't have access to the constructor. (Note that for non-abstract classes, you can also prevent direct instantiation by making constructors protected or private…obviously there's no need to do this with abstract, since the class can't be instantiated directly anyway).

If the former, while you are correct that C# does not have Java's concept of "package", which limits what is possible with respect to access restrictions as compared to Java, you can come close.

A derived class must not be more accessible than its base class. This means that the base class (abstract or otherwise) of a public class must itself be public.

But you can do much the same sort of interface-based implementation-hiding in C# as you've done in Java by making your abstract class internal (i.e. the default access for a class in C#), then a class that inherits it also internal, while still leaving an interface those classes implement public, which provides client code using the library a way to access the public features of the class.

Since making the base class internal prevents you from making the derived class public, clients of your library won't be able to create instances of the derived class directly either. So you'll have to have a public factory method in some public class which returns the public interface instead of the derived class.

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