Interface declare event and the concrete class implement the interface

 1 class Program
 2     {
 3         static void Main(string[] args)
 4         {
 5             EventDemo();
 6             Console.ReadLine();
 7         }
 8 
 9         static void EventDemo()
10         {
11             MathClass mc = new MathClass();
12             mc.Age = 10;
13             mc.ChangedEvent += Mc_ChangedEvent;
14             mc.Age = 20;
15         }
16 
17         private static void Mc_ChangedEvent(object sender, ChangedEventArgs e)
18         {
19             Console.WriteLine($"The old value is {e.OldValue},new value is {e.NewValue}");
20         }
21     }
22 
23     interface IMath
24     {
25         void Add(int x, int y);
26          
27         int Age { get; set; }
28 
29         event EventHandler<ChangedEventArgs> ChangedEvent;         
30     }
31 
32     public class MathClass : IMath
33     {
34         private int age;
35         public int Age 
36         { 
37             get
38             {
39                 return age;
40             }
41             set
42             {
43                 if(value!=age)
44                 {
45                     int oldValue = age;
46                     age = value;
47                     int newValue = value;
48                     OnChanged(oldValue, newValue);
49                 }
50             }
51         }
52 
53         public event EventHandler<ChangedEventArgs> ChangedEvent;
54 
55         protected virtual void OnChanged(int oldValue,int newValue)
56         {
57             var handler = ChangedEvent;
58             handler?.Invoke(this, new ChangedEventArgs(oldValue, newValue));
59         }
60 
61         public void Add(int x, int y)
62         {
63             Console.WriteLine($"x+y={x + y}");
64         }
65     }
66 
67     public class ChangedEventArgs:EventArgs
68     {
69         public int OldValue { get; set; }
70         public int NewValue { get; set; }
71 
72         public ChangedEventArgs(int oldValue,int newValue)
73         {
74             OldValue = oldValue;
75             NewValue = newValue;
76         }
77     }