Please see the code below:

public interface ISport     {         int FitnessLevel { get; }         int Players { get; }         int Duration { get; }     }      public class Football : ISport     {         public int FitnessLevel { get; } = 5;         public int Players { get; } = 11;         public int Duration { get; } = 90;     }      public class Person     {         private readonly ISport _sport;         private Guid _id;         private readonly int _fitnessLevel;          public Person(ISport sport, Guid id, int fitnessLevel)         {             _id = id;             _sport = sport;             _fitnessLevel = fitnessLevel;             if (sport == null)             {                 throw new ArgumentException("Sport cannot be null.", "Sport");             }             if (_sport.FitnessLevel > fitnessLevel)             {                 throw new ArgumentException("Fitness Level is not high enough.", "FitnessLevel");             }             if (id == Guid.Empty)             {                 throw new ArgumentException("ID cannot be empty.", "ID");             }         }     } 

and the test code below:

ISport sport = new Football(); Person person = new Person(sport, Guid.NewGuid(), 3); 

The Person constructor throws an exception in this case because the persons fitness level is not enough to play the sport in question.

Please focus on the code below:

if (_sport.FitnessLevel > fitnessLevel) {     throw new ArgumentException("Fitness Level is not high enough.", "FitnessLevel"); } 

Notice that the Football class is used to validate the Person. Is this a normal to approach validation like this:

1) A Value Objects member is used to validate an entity

2) A Value Objects member is used to validate another value object

I am asking this because I have never seen validation approached like this before and I am trying to follow the principle of least astonishment these days.