Interface declare event and the concrete class implement the interface

class Program
    {
        static void Main(string[] args)
        {
            EventDemo();
            Console.ReadLine();
        }

        static void EventDemo()
        {
            MathClass mc = new MathClass();
            mc.Age = 10;
            mc.ChangedEvent += Mc_ChangedEvent;
            mc.Age = 20;
        }

        private static void Mc_ChangedEvent(object sender, ChangedEventArgs e)
        {
            Console.WriteLine($"The old value is {e.OldValue},new value is {e.NewValue}");
        }
    }

    interface IMath
    {
        void Add(int x, int y);
         
        int Age { get; set; }

        event EventHandler<ChangedEventArgs> ChangedEvent;         
    }

    public class MathClass : IMath
    {
        private int age;
        public int Age 
        { 
            get
            {
                return age;
            }
            set
            {
                if(value!=age)
                {
                    int oldValue = age;
                    age = value;
                    int newValue = value;
                    OnChanged(oldValue, newValue);
                }
            }
        }

        public event EventHandler<ChangedEventArgs> ChangedEvent;

        protected virtual void OnChanged(int oldValue,int newValue)
        {
            var handler = ChangedEvent;
            handler?.Invoke(this, new ChangedEventArgs(oldValue, newValue));
        }

        public void Add(int x, int y)
        {
            Console.WriteLine($"x+y={x + y}");
        }
    }

    public class ChangedEventArgs:EventArgs
    {
        public int OldValue { get; set; }
        public int NewValue { get; set; }

        public ChangedEventArgs(int oldValue,int newValue)
        {
            OldValue = oldValue;
            NewValue = newValue;
        }
    }