Tell me what is stupid about my Entity Validation (and how to improve it)

Question

I have an IEntity interface that implements an interface, IValidatable

public interface IValidatable {
    bool IsValid { get; }
    bool IsValidForPersistence { get; }
    // Rules applied at UI time (please enter your name, etc)
    IEnumerable<RuleViolation> GetRuleViolations();
    // Rules to be applied only at persistence time
    IEnumerable<RuleViolation> GetPersistenceRuleViolations();
}

public interface IEntity : IValidatable {
    int ID { get; set; }
}

and for convenience sake I've implemented my Entity class like:

 public abstract class Entity : IEntity {

        public virtual int ID { get; set; }

        public virtual bool IsValid {
            get { return RuleViolations().Count() == 0; }
        }

        public virtual bool IsValidForPersistence {
            get { return PersistenceRuleViolations().Count() == 0; }
        }

        public virtual IEnumerable<RuleViolation> GetRuleViolations() {
            return new List<RuleViolation>();
        }

        public virtual IEnumerable<RuleViolation> GetPersistenceRuleViolations() {
            return new List<RuleViolation>();
        }
    }

By default entities are valid, until GetRuleViolations() or GetPersistenceRuleViolations() are overridden.

  public partial class Company {

      public override IEnumerable<RuleViolation> GetRuleViolations() {
         if (String.IsNullOrEmpty(CompanyName))
                yield return new RuleViolation("CompanyName", "Name is required.");
     }

      public override IEnumerable<RuleViolation> GetPersistenceRuleViolations() {
         // Include standard rules too
         foreach (RuleViolation rule in RuleViolations) {
              yield return rule;
          }
        // Check some data based on a referenced entity "Bid"
        if (!Active && Bid.Active)
            yield return new RuleViolation("Active", 
               "When Active is set to false, the Bid must also be inactive.");
     }
  }

I know this is a bit naive for validation, so besides any typos, what can be improved?

Answer 1

I'd recommend looking at either the NCommon framework which has a good business rule and validation base framework, or the xVal validation framework, which also supports client side validation from the same ruleset.

Answer 2

I'm using ASP.NET MVC and doing something similar with Data Annotation Validators. I inherit however from ValidationAttribute and also override FormatErrorMessage so that I can return the errors collectively.

Answer 3

Keep in mind, if you derive an entity from another, eg. Customer from Person, you need a foeach to get the base classes violations:

  public override IEnumerable<RuleViolation> GetRuleViolations() 
  {
     // inherit base class valiations
     foreach (var violation in base.GetRuleViolations())
     {
         yield return violation;
     }

     // add own validations
     if (String.IsNullOrEmpty(CompanyName))
         yield return new RuleViolation("CompanyName", "Name is required.");
  }

Which is not so nice.

Personally, I would also look for something existing, because validation is a general problem and there are already many solutions for it.