Advantage of passing a Lambda expression instead of an Object

Question

I've been reviewing FluentValidation which is a nice validation library by the way but I noticed that they pass LambdaExpression in instead of an object property and I like to learn the advantage of this usage:

using FluentValidation;

public class CustomerValidator: AbstractValidator<Customer> {
  public CustomerValidator() {
    RuleFor(customer => customer.Surname).NotEmpty();
    RuleFor(customer => customer.Forename).NotEmpty().WithMessage("Please specify a first name");
    RuleFor(customer => customer.Company).NotNull();
    RuleFor(customer => customer.Discount).NotEqual(0).When(customer => customer.HasDiscount);
    RuleFor(customer => customer.Address).Length(20, 250);
    RuleFor(customer => customer.Postcode).Must(BeAValidPostcode).WithMessage("Please specify a valid postcode");
  }

  private bool BeAValidPostcode(string postcode) {
    // custom postcode validating logic goes here
  }
}

Customer customer = new Customer();
CustomerValidator validator = new CustomerValidator();
ValidationResult results = validator.Validate(customer);

bool validationSucceeded = results.IsValid;
IList<ValidationFailure> failures = results.Errors;

As you can see RuleFor(customer => customer.Surname).NotEmpty(); instead, RuleFor(customer.Surname).NotEmpty(); wouldn't be enough and clean?

Jon Skeet · Accepted Answer · 2012-08-23 17:51:51Z

6

instead, RuleFor(customer.Surname).NotEmpty(); wouldn't be enough and clean?

No. Because at the time when you're calling RuleFor, you don't have a customer - therefore you can't access their surname.

As is usually the case with delegates, you're passing around code to be executed at a later time. The expression customer.Surname is an expression which would be evaluated immediately in the context of an existing customer variable.

Now if we had the mythical infoof operator, we could do it without creating a delegate. We could potentially write something like:

RuleFor(infoof(Customer.Surname)).NotEmpty()

which would be lovely. The RuleFor method would take that property reference and evaluate it later against a given customer. Wonderful.

Unfortunately, we don't have that operator - so the easiest way of expressing the idea of "when you've got a customer, get hold of the Surname property" is to use a delegate. It's also very flexible, as it can work with (almost) any code you want to execute.

answered Aug 23, 2012 at 17:51

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Tarik Over a year ago

I see it now. Thanks for the prompt answer! +1

Tarik Over a year ago

I have one thing in my mind if you don't mind. What if there was a instance variable defined in the AbstractValidation<T> class such as T obj and when calling validate(customer), T obj could be set the customer instance and RuleFor(obj.Name) could be used.

Jon Skeet Over a year ago

Not really, because the whole point is to set up the validator once in the constructor, and have that apply to multiple instances over time. We want to call RuleFor once, not once per instance we validate.

Tarik Over a year ago

But isn't that validating one instance at a time, I mean when saying Validate it gets the reference in order to validate its properties. Wouldn't be the same passing in "instance variable"? Since Validate will switch the reference address of the instance field.

Jon Skeet Over a year ago

@DorianGray: You call Validate, you pass in the value, sure - but that's not when the RuleFor code runs. The delegate that you pass into the RuleFor call runs during the Validate. That's the point. You're setting up code to run later on.

|

Jon Senchyna · Accepted Answer · 2012-08-23 17:53:52Z

1

If you were to pass customer.SurName, you would be pointing to the value of the SurName property of a specific customer instance. Using a lambda function allows the validation logic to take a customer object when the validation logic is invoked, and then retrieve the SurName property of that customer.

answered Aug 23, 2012 at 17:53

Jon Senchyna

8,0872 gold badges28 silver badges46 bronze badges

Collectives™ on Stack Overflow

Advantage of passing a Lambda expression instead of an Object

2 Answers 2

6 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related