Accessing parent member from child of child class

Question

Today I faced a very odd interview question. The interviewer asked me:-

There is a class Parent and it has a method GetData. Class Child1 inherits Parent and Child2 inherits Child1. What you can do in Parent class so that the method "GetData" will be accessible from Child1 but not from Child2?

I tried a few "obvious" ways but they either didn't compile or didn't restrict access. It seems like there shouldn't be a way to do this, but I look forward to seeing the answer! — BradleyDotNET, May 23 '14 at 18:19

score 2 · Answer 1 · answered May 23 '14 at 18:42

2

Kind of a weird setup, but here's another option that works because nested classes can access private members of the outer class:

public class Parent
{
    public Parent()
    {
        GetData();
    }

    private void GetData()
    {
    }        

    public class Child1 : Parent
    {
        public Child1()
        {
            GetData();
        }
    }

}

class Child2 : Parent.Child1
{
    public Child2()
    {
        GetData(); //compiler error, inaccessible due to protection level
    }
}

answered May 23 '14 at 18:42

Jason P

26,984
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1

damn you, you beat me to it – Kik May 23 '14 at 18:43
Would this be an acceptable answer, however, since it means you now have to refer to Parent.Child1 and Child2 in completely different syntax? – David May 23 '14 at 18:49
You can instantiate them the same way (`var c1 = new Child1(); var c2 = new Child2();`). Is that what the interviewer wanted... who knows? – Jason P May 23 '14 at 18:53

score 2 · Answer 2 · answered May 23 '14 at 18:43

Parent class can mark it's method as private, then declare the first Child inside of it's own declaration.

    public class Job
    {

        private void Test()
        {

        }

        public class JobChild : Job
        {
            public JobChild()
            {
                //works
                this.Test();
            }

        }
    }


    public class JobChildTwo : Job.JobChild
    {
        public JobChildTwo()
        {
            //doesn't work
            this.Test();
        }
    }

score 1 · Answer 3 · answered May 23 '14 at 18:29

1

If we assume that Parent and Child1 exist in Assembly A while Child2 exists in Assembly B, and Assembly A does not expose its internals to Assembly B whilst Assembly B references Assembly A, then you can mark GetData as internal, at which point it will be accessible to Child1 but not Child2.

The side effect here is that it would be visible to the entire assembly.

Note that protected internal would have the opposite effect - it would make GetData visible to child2, since protected internal is explicitly "protected OR internal" as per MSDN

answered May 23 '14 at 18:29

David

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I would not of thought of different assemblies. Cool solution! – BradleyDotNET May 23 '14 at 18:30
It's honestly the only solution I could think of - very strange question indeed! – David May 23 '14 at 18:30
is it possible in languages other than c#. – gmtek May 23 '14 at 18:38
@Gulshan to my knowledge, it should be possible with the same setup in any other .NET language (VB, F#) and possibly with C++ or Java. Not familiar enough with other languages beyond that, or at least no familiar enough with assembly-level access restrictions. – David May 23 '14 at 18:41
1

@Gulshan - You can take advantage of private inheritance in C++ like described [here](http://stackoverflow.com/a/1372858/632337) but this in not supported in C#. – Rohit Vats May 23 '14 at 18:45

Accessing parent member from child of child class

3 Answers3