Allow only outer class to instantiate inner class but inner class should be public

Question

I was wondering if it was possible in c# to do:

public class Outer 
{
    public class Inner {}   

    public Inner CreateInner() 
    {   
        return new Inner(); // should only be allowed inside this method
    }
}

where you can only create a new instance of the Inner class inside an Outer class method. I want to do this because I need better control over what Inner class is created and it helps me mock the return value of CreateInner and verify that it was called. Is this possible?

@EhsanSajjad If you make the class private, he wont be able to expose it. — Blue, Aug 29 '19 at 08:06
I was googling for ages and for some reason I could not find that duplicate post. Thanks for that. — Mayron, Aug 29 '19 at 08:10
I think we all face the same problem at one time or another: the answer exists but we don't have the right keywords. — ProgrammingLlama, Aug 29 '19 at 08:12

score 4 · Answer 1 · answered Aug 29 '19 at 08:07

Making the default constructor private does not work, as you have probably already discovered.

However, you could declare a public nested interface and return that instead, and make the concrete class entirely private.

Something like this:

public class Outer
{
    public interface IInner
    {
        string SomeMethod();
    }

    class Inner: IInner
    {
        public string SomeMethod()
        {
            return "Hello, World!";
        }
    }

    public IInner CreateInner()
    {
        return new Inner(); // should only be allowed inside this method
    }
}

Oh, of course! Thanks a lot :) – Mayron Aug 29 '19 at 08:09 — Mayron, Aug 29 '19 at 08:09

Allow only outer class to instantiate inner class but inner class should be public

1 Answers1