Private method call using delegate

Question

I am reading CLR via C# 4th edition. In the chapter covering delegates (chapter 17), it is written that:

The FeedbackToConsole method is defined as private inside the Program type, but the Counter method is able to call Program’s private method. In this case, you might not expect a problem because both Counter and FeedbackToConsole are defined in the same type. However, this code would work just fine even if the Counter method was defined in another type. In short, it is not a security or accessibility violation for one type to have code that calls another type’s private member via a delegate as long as the delegate object is created by code that has ample security/ accessibility.

I am experimenting with this idea, but I'm not able to call a private method from another type. Here is my code:

namespace DelegatesEvents
{
    class Program
    {
        public static void Main(string[] args)
        {
            Counter(1, 2, new FeedBack(DelegateEventsDemo.FeedBackToConsole));
            Console.ReadLine();
        }

        private static void Counter(int p1, int p2, FeedBack feedBack)
        {
            for (int i = p1; i <= p2; i++)
            {
                if (feedBack != null)
                {
                    feedBack(i);
                }
            }
        }
    }
}

namespace DelegatesEvents
{
    internal delegate void FeedBack(Int32 val);

    public class DelegateEventsDemo
    {
        private static void FeedBackToConsole(Int32 val)
        {
            Console.WriteLine("Good morning delegates" + val);
        }
    }
}

When I declare FeedBackToConsole as private, it can not be called in the delegate, as expected if I was trying to call it otherwise.

Where am I going wrong?

Answer 1

In short, it is not a security or accessibility violation for one type to have code that calls another type’s private member via a delegate as long as the delegate object is created by code that has ample security/accessibility.

DelegateEventsDemo has to create the delegate object. So we add public GetFeedBackDelegate method (you can add property as well) which returns the delegate object referencing private method.

namespace DelegatesEvents
{
    internal delegate void FeedBack(Int32 val);

    public class DelegateEventsDemo
    {
        private static void FeedBackToConsole(Int32 val)
        {
            Console.WriteLine("Good morning delegates" + val);
        }

        internal static FeedBack GetFeedBackDelegate()
        {
            return FeedBackToConsole;
        }
    }
}

And used by the code:

namespace DelegatesEvents
{
    class Program
    {
        public  static void Main(string[] args)
        {
            Counter(1, 2, DelegateEventsDemo.GetFeedBackDelegate());
            Console.ReadLine();
        }

        private static void Counter(int p1, int p2, FeedBack feedBack)
        {
            for (int i = p1; i <= p2; i++)
            {
                if (feedBack != null)
                {
                    feedBack(i);
                }
            }
        }
    }
}