#nullable enable

using System;

​

interface IMyInterface

{

    internal event Action MyEvent;

}

​

class MyClass : IMyInterface

{

    // IMyInterface impls

    event Action? _myevent;

    event Action? IMyInterface.MyEvent

    {

        add => _myevent += value;

        remove => _myevent -= value;

    }

    // Public API

    public void Fire()

    {

        _myevent?.Invoke();

    }

}

public class Program

{

    public static void Main()

    {

        var c = new MyClass();

        (c as IMyInterface).MyEvent += () => { Console.WriteLine("Callback"); };

        c.Fire();

    }

}