Question

I’m building a library where I’ve got a pattern like the following:

public class Foo : Foo_Impl
{
}

public class Foo_Impl
{
}

I don’t want other developers to accidentally use the Foo_Impl class. What options are available to me to hide this? I’d also like to hide it from other classes in the same assembly it’s defined in. Ideally I’d have loved to do:

public class Foo : Foo_Impl
{
  private class Foo_Impl
  {
  }
}

But that doesn’t work for a variety of reasons.

Answer 1

Make Foo_Impl an abstract class. This won’t stop other developers from deriving from it, but it will make it impossible to create an instance of Foo_Impl directly – it will need to be instantiated by creating a derived object, such as Foo.

public abstract class Foo_Impl
{
    public Foo_Impl()
    {
    }
}

public class Foo : Foo_Impl
{
    public Foo() // implicitly calls the base class's constructor
    {
    }
}

–

var works = new Foo(); // Works
var error = new Foo_Impl(); // Compiler error

Per Thomas Levesque’s suggestion, you also have the option of making the abstract constructor internal:

public abstract class Foo_Impl
{
    internal Foo_Impl()
    {
    }
}

public class Foo : Foo_Impl
{
    public Foo() // implicitly calls the base class's constructor
    {
    }
}

This will prevent developers from inheriting from Foo_Impl from outside the assembly.