How to restrict access to nested class member to enclosing class?

Question

Is it possible to specify that members of a nested class can be accessed by the enclosing class, but not other classes ?

Here's an illustration of the problem (of course my actual code is a bit more complex...) :

public class Journal
{
    public class JournalEntry
    {
        public JournalEntry(object value)
        {
            this.Timestamp = DateTime.Now;
            this.Value = value;
        }

        public DateTime Timestamp { get; private set; }
        public object Value { get; private set; }
    }

    // ...
}

I would like to prevent client code from creating instances of JournalEntry, but Journal must be able to create them. If I make the constructor public, anyone can create instances... but if I make it private, Journal won't be able to !

Note that the JournalEntry class must be public, because I want to be able to expose existing entries to client code.

Any suggestion would be appreciated !

---

UPDATE: Thanks everyone for your input, I eventually went for the public IJournalEntry interface, implemented by a private JournalEntry class (despite the last requirement in my question...)

Answer 1

Actually there is a complete and simple solution to this problem that doesn't involve modifying the client code or creating an interface.

This solution is actually faster than the interface-based solution for most cases, and easier to code.

public class Journal
{
  private static Func<object, JournalEntry> _newJournalEntry;

  public class JournalEntry
  {
    static JournalEntry()
    {
      _newJournalEntry = value => new JournalEntry(value);
    }
    private JournalEntry(object value)
    {
      ...

Answer 2

If your class is not too complex, you could either use an interface which is publicly visible and make the actual implementing class private, or you could make a protected constructor for the JornalEntry class and have a private class JornalEntryInstance derived from JornalEntry with a public constructor which is actually instantiated by your Journal.

public class Journal
{
    public class JournalEntry
    {
        protected JournalEntry(object value)
        {
            this.Timestamp = DateTime.Now;
            this.Value = value;
        }

        public DateTime Timestamp { get; private set; }
        public object Value { get; private set; }
    }

    private class JournalEntryInstance: JournalEntry
    { 
        public JournalEntryInstance(object value): base(value)
        { }
    }
    JournalEntry CreateEntry(object value)
    {
        return new JournalEntryInstance(value);
    }
}

If your actual class is too complex to do either of that and you can get away with the constructor being not completely invisible, you can make the constructor internal so it is only visible in the assembly.

If that too is infeasible, you can always make the constructor private and use reflection to call it from your journal class:

typeof(object).GetConstructor(new Type[] { }).Invoke(new Object[] { value });

Now that I think about it, another possibility would use a private delegate in the containing class which is set from the inner class

public class Journal
{
    private static Func<object, JournalEntry> EntryFactory;
    public class JournalEntry
    {
        internal static void Initialize()
        {
            Journal.EntryFactory = CreateEntry;
        }
        private static JournalEntry CreateEntry(object value)
        {
            return new JournalEntry(value);
        }
        private JournalEntry(object value)
        {
            this.Timestamp = DateTime.Now;
            this.Value = value;
        }

        public DateTime Timestamp { get; private set; }
        public object Value { get; private set; }
    }

    static Journal()
    {
        JournalEntry.Initialize();
    }
        
    static JournalEntry CreateEntry(object value)
    {
        return EntryFactory(value);
    }
}

This should give you your desired visibility levels without needing to resort on slow reflection or introducing additional classes / interfaces

Answer 3

Make JournalEntry a private nested type. Any public members will be visible only to the enclosing type.

public class Journal
{
    private class JournalEntry
    {
    }
}

If you need to make JournalEntry objects available to other classes, expose them via a public interface:

public interface IJournalEntry
{
}

public class Journal
{
    public IEnumerable<IJournalEntry> Entries
    {
        get { ... }
    }

    private class JournalEntry : IJournalEntry
    {
    }
}

Answer 4

A simpler approach is to just use an internal constructor, but make the caller prove who they are by supplying a reference that only the legitimate caller could know (we don't need to be concerned about non-public reflection, because if the caller has access to non-public reflection then we've already lost the fight - they can access a private constructor directly); for example:

class Outer {
    // don't pass this reference outside of Outer
    private static readonly object token = new object();

    public sealed class Inner {
        // .ctor demands proof of who the caller is
        internal Inner(object token) {
            if (token != Outer.token) {
                throw new InvalidOperationException(
                    "Seriously, don't do that! Or I'll tell!");
            }
            // ...
        } 
    }

    // the outer-class is allowed to create instances...
    private static Inner Create() {
        return new Inner(token);
    }
}

Answer 5

In this case you could either:

1. Make the constructor internal - this stops those outside this assembly creating new instances or...
2. Refactor the JournalEntry class to use a public interface and make the actual JournalEntry class private or internal. The interface can then be exposed for collections while the actual implementation is hidden.

I mentioned internal as a valid modifier above however depending on your requirements, private may be the better suited alternative.

Edit: Sorry I mentioned private constructor but you've already dealt with this point in your question. My apologies for not reading it correctly!

Answer 6

For generic nested class =)

I know this is an old question and it has already an accepted answer, nevertheless for those google swimmers who may have a similar scenario to mine this answer may provide some help.

I came across this question for I needed to implement the same feature as the OP. For my first scenario this and this answers worked just fine. Nevertheless I needed also to expose a nested generic class. The problem is that you can not expose a delegate type field (the factory field) with opened generic parameters without making your own class generic, but obviously this is not what we want, so, here is my solution for such scenario:

public class Foo
{
    private static readonly Dictionary<Type, dynamic> _factories = new Dictionary<Type, dynamic>();

    private static void AddFactory<T>(Func<Boo<T>> factory)
        => _factories[typeof(T)] = factory;

    public void TestMeDude<T>()
    {
        if (!_factories.TryGetValue(typeof(T), out var factory))
        {
            Console.WriteLine("Creating factory");
            RuntimeHelpers.RunClassConstructor(typeof(Boo<T>).TypeHandle);
            factory = _factories[typeof(T)];
        }
        else
        {
            Console.WriteLine("Factory previously created");
        }

        var boo = (Boo<T>)factory();
        boo.ToBeSure();
    }

    public class Boo<T>
    {
        static Boo() => AddFactory(() => new Boo<T>());

        private Boo() { }

        public void ToBeSure() => Console.WriteLine(typeof(T).Name);
    }
}

We have Boo as our internal nested class with a private constructor and we mantain on our parent class a dictionary with these generic factories taking advantage of dynamic. So, each time TestMeDude is called, Foo searches for whether the factory for T has already been created, if not it creates it calling nested class' static constructor.

Testing:

private static void Main()
{
    var foo = new Foo();

    foo.TestMeDude<string>();
    foo.TestMeDude<int>();
    foo.TestMeDude<Foo>();

    foo.TestMeDude<string>();

    Console.ReadLine();
}

The output is:

Answer 7

The solution Grizzly suggested does make it a bit hard to create the nested class somewhere else but not impossible,like Tim Pohlmann wrote someone can still inherit it and use the inheriting class ctor.

I'm taking advantage of the fact that nested class can access the container private properties, so the container asks nicely and the nested class gives access to the ctor.

 public class AllowedToEmailFunc
{
    private static Func<long, EmailPermit> CreatePermit;

    public class EmailPermit
    {
        public static void AllowIssuingPermits()
        {
            IssuegPermit = (long userId) =>
            {
                return new EmailPermit(userId);
            };
        }

        public readonly long UserId;

        private EmailPermit(long userId) 
        {
            UserId = userId;
        }
    }

    static AllowedToEmailFunc()
    {
        EmailPermit.AllowIssuingPermits();
    }

    public static bool AllowedToEmail(UserAndConf user)
    {
        var canEmail = true; /// code checking if we can email the user
        if (canEmail)
        {
            return IssuegPermit(user.UserId);
        }
        else
        {
            return null
        }

    }
}

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This solution is not something I would do on a regular day on the job, not because it will lead to problems in other places but because it's unconventional (I've never seen it before) so it might cause other developers pain .