Validation within Aggregate Roots - Part 2

CommandValidator.cs

  public interface IValidationHandler<in T> where T : Command {
    bool Validate(T cmd);
  }

  public interface ICommandValidator {
    bool Validate<T>(T command) where T : Command;
  }
  // Allows for all command validators to be registered on app start
  public class CommandValidator : ICommandValidator {
    private readonly IDictionary<Type, Func<object, bool>> _validationHandlers = new Dictionary<Type, Func<object, bool>>();

    public void RegisterValidator<T>(Func<T,bool> handler) where T : Command {
      _validationHandlers.Add(typeof(T), o => handler((T) o));
    }

    public bool Validate<T>(T command) where T : Command {
      Func<object, bool> handler;
      return !_validationHandlers.TryGetValue(command.GetType(), out handler) || handler(command);
    }
  }
  // Class to encapsulate all command validation rules for an Aggregate Root
  public class PatientValidationHandler : IValidationHandler<AddReferredPatient>, IValidationHandler<ChangePatientAddress> {
    public bool Validate(AddReferredPatient command) {
      if (string.IsNullOrWhitespace(command.Firstname))
        throw new CommandValidationException("firstname", "First Name is required");
    }

gistfile1.cs

// Register your command validators as you might your command handlers (N.B. this could be done with an IoC or reflection to auto-wireup)
var patientValidator = new PatientValidationHandler();
validator.RegisterValidator<AddReferredPatient>(patientValidator.Validate);
validator.RegisterValidator<ChangePatientAddress>(patientValidator.Validate);

// Then on all command sending
protected void Send<T>(T cmd) where T : Command {
  try {
    if (Validator.Validate(cmd)) {
      Bus.Send(cmd);
    }
  } 
  catch (ValidationException ex) {
    // This catches both Command Validation exception and Domain validation exceptions (which both inherit from ValidationException)
    // manage your feedback to the UI as you see fit but I've gone with the below.
    foreach(var error in ex.Errors) {
      ModelState.AddModelError(error.ParamName, error.Message);
  }
}