private string Execute(PersonStub person) { var handler = new ValidationHandler <PersonStub>( x => !string.IsNullOrWhiteSpace(x.FirstName), x => $"{nameof(PersonStub.FirstName)} [{x.FirstName}] should not be null or whitespace."); var message = string.Empty; handler.InvalidResult += (object sender, Contract.EventArgs.ValidationHandlerArgs e) => message = e.ValidationResult.ErrorMessage; handler.Execute(person); return(message); }