Whilst working on my network engine I came across a problem that I initially solved with an interface, but it turned out that the class that implemented this interface exposed public methods that I really wanted to be private, an example will explain the problem.

public interface IMessageReceiver
{
    void Receive(string message);
}

public interface IMessageProvider
{
    IMessageReceiver MessageReceiver;
    void Update();
}

public class ConcreteReceiver: IMessageReceiver
{
    private IMessageProvider _MessageProvider;

    public ConcreteReciever(IMessageProvider messageProvider)
    {
        _MessageProvider = messageProvider;
        _MessageProvider.MessageReceiver = this;
    }
    public void Receive(string message)
    {
        Console.WriteLine(message);
    }

    public void Update()
    {
        MessageProvider.Update();
    }
}

When calling ConcreteReceiver.Update(), this will call MessageProvider.Update(), then, the concrete implementation of IMessageProvider should call IMessageReceiver.Receive, effectively like a callback.

Now the issue in my case was that I did not want the Receive method of ConcreteReceiver to be public, but of course any concrete type that implements an interface must implement those methods as public, after pondering a while I realised the design was actually flawed anyway, what I really wanted was a callback, so I changed my design as follows.

public interface IMessageProvider
{
    Action<string> ReceiveAction;
    void Update();
}

public class ConcreteReceiver
{
    private IMessageProvider _MessageProvider;

    public ConcreteReciever(IMessageProvider messageProvider)
    {
        _MessageProvider = messageProvider;
        _MessageProvider.ReceiveAction = Receive;
    }
    private void Receive(string message)
    {
        Console.WriteLine(message);
    }

    public void Update()
    {
        MessageProvider.Update();
    }
}

So now my Receive method on my ConcreteReceiver can be private, since I am now using a generic Action<T> delegate, this prevents my public API exposing methods that are really meant to be only used internally.