RegsiterCommandValidator.cs

    public class RegsiterCommandValidator : MessageValidator<RegisterCommand>
    {
        private readonly IDbContext _context;

        public RegsiterCommandValidator(IDbContext context)
        {
            _context = context;
        }

        public override ValidationResult Validate(RegisterCommand command)
        {
            ValidateAnnotations(command);

            if (_context.Users.Any(x => x.UserName == command.UserName))
                _result.AddError(new ValidationFailure("UserName", "The username is invalid."));

            if (IsFiveOclock())
                _result.AddError(new ValidationFailure("", "Go to happy hour!"));

            return _result;
        }

        private bool IsFiveOclock()
        {
            //it is somewhere!
            return true;
        }
    }