In C# 4.0, is there any way to make an otherwise private member of one class available only to a specific other class?

Question

We\'re creating an object hierarchy where each item has a collection of other items, and each item also has a Parent property pointing to its parent item. Pretty st

Answer 1

The only two things I can think of:

One:

Use sort of option number 2 you mention above (which I do constantly myself)...but make the implementation of the interface (Item) be a nested private class inside of ItemsCollection... that way only ItemsCollection knows about the setter. The interface IItem only declares a getter for Parent...and no one can cast it to Item because Item is private to ItemsCollection. So, something like:

public class ItemsCollection : ObservableCollection
{
    private class Item : IItem 
    {
        public object Parent { get; set; }
    }

    private CheckParent(IItem item)
    {
        if(item.Parent != null) throw new Exception("Item already belongs to another ItemsCollection");
        ((Item)item).Parent = this.Owner; // <-- This is where we need to access the private Parent setter
    }

    public static IItem CreateItem() { return new Item(); }
}

public interface IItem 
{
    object Parent {get; }
}

and when you want ItemsCollection to set the item Parent, case the IItem instance to Item (which does expose a setter). Only ItemsCollection can do this cast, since the Item implementation is private to ItemsCollection...so I think that accomplishes what you want.

Two:

Make it internal not private...you don't get exactly what you want, but you can use InternalsVisibleToAttribute to denote that the internal members are visible to another assembly.