public Cat(Animal friend)
     : this()
 {
     Friend = friend;
 }
 public Duck(Animal friend, int age)
     : this()
 {
     Interlocked.Increment(ref CtorFriendAge);
     Age = age;
     Friend = friend;
 }
 public Duck(Animal friend)
     : this()
 {
     Friend = friend;
 }
 public Duck(int age, Animal friend)
     : this()
 {
     Interlocked.Increment(ref CtorAgeFriend);
     Age = age;
     Friend = friend;
 }
 public Dog(Animal friend)
     : this()
 {
     Friend = friend;
 }