Inheritance trees and protected constructors in C#

Question

Given the following inheritance tree, what would be the best way of implementing it in a way that works?

abstract class Foo<T> : IEnumerable<T>
{
    public abstract Bar CreateBar();
}

class Bar<T> : Foo<T>
{
    // Bar's provide a proxy interface to Foo's and limit access nicely.
    // The general public shouldn't be making these though, they have access
    // via CreateBar()
    protected Bar(Foo base)
    {
        // snip...
    }
}

class Baz<T> : Foo<T>
{
    public Bar CreateBar()
    {
        return new Bar(this);
    }
}

This fails with: 'Bar.Bar()' is inaccessible due to its protection level.

I don't want the constructor being public, only classes that inherit from Foo should be able to create Bars. Bar is a specialised Foo, and any type of Foo should be able to create one. Public internal is an 'option' here, as the majority of the predefined extensions to Foo will be internal to the DLL, but I consider this a sloppy answer, since anyone who comes along later who wants to create their own type of Foo or Baz (which is likely to happen) will be stuck with a default CreateBar() implementation, which may or may not meet their needs.

Perhaps there is a way of refactoring this to make it work nicely? I'm banging my head on the wall trying to design this so it'll work though.

Edit (More info):

Slightly more concrete: Foo is implementing IEnumerable and long story short, Bar is providing the same interface, but to a limited subset of that enumerable object. All Foo's should be able to create subsets of themselves (ie. Bar) and return it. But I don't want to have everyone who ever wants to implement a Foo to have to worry about this, because Bar will do the proxying and worry about limiting the range, etc.

Answer 1

Would it be possible for you to make Baz a nested type within Bar? That's the only way you'll give it more access to Bar than it would otherwise have. Just having the same parent class only gives it access to protected members of Foo, and Foo doesn't have special access to Bar. I suspect there are other tortuous ways of doing this with nested types, but really it's going to be quite unpleasant for maintenance engineers.

It's quite an odd design though, to force one derived class to create an instance of a different class derived from the same base class. Is that really what you need? Perhaps if you put this in more concrete terms it would be easier to come up with alternative designs.

Answer 2

C# doesn't provide a direct equivalent of the C++ friend keyword. Seems like your design is requiring this sort of construct.

In C++ you could designate that a specific class has access to the private/protected members of another class by using "friend". Note: this is not the same as C# internal, modifier which gives access to all classes within the same assembly.

Looking at your design, it seems you are trying to do something along the lines that would require the C++ style friend. Jon Skeet is right, the usual design in C# to make up for this is to use a nested class.

This forum post explains further and shows some examples of how to do this.

Answer 3

You can access Bar's constructor through a nested type within Foo:

abstract class Foo<T> : IEnumerable<T>
{
  public abstract Bar<T> CreateBar();

  protected Bar<T> CreateBar(Foo<T> f) { return new FooBar(f); }

  private class FooBar : Bar<T> 
   { public FooBar(Foo<T> f) : base(f) {}   
   }
}

class Bar<T> : Foo<T>
{ protected Bar(Foo<T> @base) {}
}

class Baz<T> : Foo<T>
{
    public override Bar<T> CreateBar() 
    {
        return CreateBar(this);
    }
}

Answer 4

Okay, new answer:

1. Split Bar into an interface and a concrete class.
2. Express the public abstract method in terms of IBar.
3. Make Bar a private nested class in Foo, implementing IBar. Give it an internal constructor which you can call from Foo.
4. Write a protected method in Foo which creates an instance of Bar from itself. Classes deriving from Foo can use this to implement the abstract method if just proxying is good enough, and classes with more complicated needs can just implement IBar directly. You could even change the abstract method to a virtual one, and create a new Bar from "this" by default.

EDIT: One variant on this would be to make Bar a protected nested class within Foo, with a public constructor. That way any derived class would be able to instantiate it for themselves, but no unrelated class would be able to "see" it at all. You'd still need to separate the interface from the implementation (so that the interface can be public) but I think that's a good thing anyway.

Answer 5

Forget, for a moment, that Bar derives from Foo. If you do this, it sounds like the problem is "How do I make it so that every subclass of Foo can create a Bar, even if the subclass doesn't have access to Bar's constructor?"

That's a pretty easy problem to solve:

public class Foo
{
   protected static Bar CreateBarInstance()
   {
      return new Bar();
   }
   public virtual Bar CreateBar()
   {
      return CreateBarInstance();
   }
}

public class Bar
{
    internal Bar()
    {
    }
}

public class Baz : Foo
{
    public override Bar CreateBar()
    {
        Bar b = base.CreateBar();
        // manipulate the Bar in some fashion
        return b;
    }
}

If you want to guarantee that the subclasses of Foo don't have access to Bar's constructor, put them in a different assembly.

Now, to derive Bar from Foo, it's a simple change:

public class Bar : Foo
{
    internal Bar()
    {
    }
    public override Bar CreateBar()
    {
        throw new InvalidOperationException("I'm sorry, Dave, I can't do that.");
    }

}

"I don't want the constructor being public." Check.

"Only classes that inherit from Foo should be able to create Bars." Check.

"Any type of Foo should be able to create one." Check,

"Anyone who comes along later who wants to create their own type of Foo or Baz (which is likely to happen) will be stuck with a default CreateBar() implementation, which may or may not meet their needs." This is pretty highly dependent on what has to happen in the Foo.CreateBar() method, I think.