Overview of the MkayAttribute.

MkayAttribute.ctor.cs

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class MkayAttribute : ValidationAttribute, IClientValidatable
{
  private readonly string _ruleSource;
  private readonly string _defaultErrorMessage;
  private readonly Lazy<ConsCell> _cell;

  public MkayAttribute(string ruleSource)
  {
    _ruleSource = ruleSource;
    _defaultErrorMessage = "Respect '{0}', mkay?".With(ruleSource);
    _cell = new Lazy<ConsCell>(() => new ExpParser(ruleSource).Parse());
  }

  protected ConsCell Tree
  {
    get { return _cell.Value; }
  }

  public IEnumerable<ModelClientValidationRule> GetClientValidationRules(
    ModelMetadata metadata, ControllerContext context) ...

  protected override ValidationResult IsValid(
    object value, ValidationContext validationContext) ...
}