Allow access to but prevent instantiation of a nested class by external classes

Question

I\'m looking to define a nested class that is accessible to the container class and external classes, but I want to control instantiation of the nested class, such that only

Answer 1

How about abstraction via an interface?

public class Container
{
    public interface INested
    {
        /* members here */
    }
    private class Nested : INested
    {
        public Nested() { }
    }

    public INested CreateNested()
    {
        return new Nested();  // Allow
    }
}

class External
{
    static void Main(string[] args)
    {
        Container containerObj = new Container();
        Container.INested nestedObj;

        nestedObj = new Container.Nested();       // Prevent
        nestedObj = containerObj.CreateNested();  // Allow

    }
}

You can also do the same thing with an abstract base-class:

public class Container
{
    public abstract class Nested { }
    private class NestedImpl : Nested { }
    public Nested CreateNested()
    {
        return new NestedImpl();  // Allow
    }
}

class External
{
    static void Main(string[] args)
    {
        Container containerObj = new Container();
        Container.Nested nestedObj;

        nestedObj = new Container.Nested();       // Prevent
        nestedObj = containerObj.CreateNested();  // Allow

    }
}

Answer 2

It is impossible to declare class in such way. I think that the best way for you would be to declare class as private and expose it through public interface:

class Program
{
    static void Main(string[] args)
    {
       // new S.N(); does not work
        var n = new S().Create();
    }
}

class S
{
    public interface IN
    {
        int MyProperty { get; set; }
    }
    class N : IN
    {
        public int MyProperty { get; set; }
        public N()
        {

        }
    }

    public IN Create()
    {
        return new N();
    }
}