Implementing Business Rules Domain Policies

Business Rules - Domain Policies is created to ensure that the logical rules in an object are received from the outside. The rules of business are in a chain of successive chains, which may also reveal a more complex business rule.

I want to continue with a code example to understand the logic. In repository Domain Policies implemented by interface definition ;

 public interface IIssueAssignmentPolicy
    {
        void CheckAssignment(Issue issue, User user);
    }

These definition and below implementation located in Core layer.

public class IssueAssignmentPolicy : IIssueAssignmentPolicy
    {
        private readonly IIssueRepository _issueRepository;
        private readonly IIssueAssignmentConfiguration _configuration;

        public IssueAssignmentPolicy(IIssueRepository issueRepository, IIssueAssignmentConfiguration configuration)
        {
            _issueRepository = issueRepository ?? throw new ArgumentNullException(nameof(issueRepository));
            _configuration = configuration ?? throw new ArgumentNullException(nameof(configuration));
        }

        public void CheckAssignment(Issue issue, User user)
        {
            if(_issueRepository.GetOpenIssueCountOfUser(user.Id) >= _configuration.MaxConcurrentOpenIssueCountForAUser)
            {
                throw new IssueAssignmentException($"Can not assign more than {_configuration.MaxConcurrentOpenIssueCountForAUser} open issues to a user!");
            }
        }
    }

After definitions of rules, we should use this policies into domain functions as below way;

  public void AssignTo([Required] User user, [Required] IIssueAssignmentPolicy policy)
        {
            Check.NotNull(user, nameof(user));
            Check.NotNull(policy, nameof(policy));

            policy.CheckAssignment(this, user);

            AssignedUserId = user.Id;
        }

So by this way, we can able to manage domain related business rules into the Core Entity objects. That means we protect to our entities unmanaged modifications.