I'm in the middle of refactoring an analytics api which needs to allow clients to send events as HTTP GET requests, with their "Hit" data encoded in the URL's QueryString Parameters.
My API's job is to recieve these events, and then place the "valid" events onto a queue in another part of the system for processing.
Some Hits have the same shape. The only thing that makes them different is the value of the type parameter, which all events must have at a minimum.
The problem I've encountered is that based on the Hit type, I'd like to be able to assume the type of each field given to me, which requires model binding. Of course. Currently, I can only find out what model to validate against after checking the value of type - which risks making the API excessively "stringly typed"
An example route would be:
GET https://my.anonymousanalytics.net/capture?type=startAction&amount=300&so_rep=true
Therefore, my Hit would be:
{
type: "startAction",
amount: 300,
so_rep: true
}
Which, hypothetically, could be bound to the Model StackOverflowStartHitModel
class StackOverflowStartHitModel {
public string type { get; } // Q: Could I force the value of this to be "startAction"?
? public int amount { get; }
public boolean so_rep { get; }
}
Why am I asking this here? Well I'm normally a JavaScript developer, but everyone who I'd normally turn to for C# wisdom is off work with the flu.
I have experimented with the [FromQuery] attribute decorator, but my concern is that for Hits that are the exact same shape, I might not be able to tell the difference between whether it is a startAction or an endAction, for example.
you're going to need to have a validation engine of some sort, but do not confuse this with your UI model validation. It sounds like you really have one model with a number of valid states which really is business logic.
Your model looks like this:
public class StackOverflowModel
{
public string type { get; set;}
public int amount { get; set; }
public bool so_rep { get; set;}
}
it doesn't matter what value your type field has and you don't need to hard-code it either, it will be captured as is and then it can be checked against valid states.
There are a number of ways to do this, that I can think of.
One option would be to create a list of valid rules ( states ) and then simply check if your input model matches any of them. One way to implement something like this could be with a library like FluentValidation. You can see an example here: Validation Rules and Business Rules in MVC
Another option would be to use some sort of Pattern Matching techniques like described here: https://learn.microsoft.com/en-us/dotnet/csharp/pattern-matching
Whichever option you go with, make sure you put this validation stuff in a separate class, maybe even a separate project. You can then add tests for each rule that you have to make sure everything works. This will also keep your controller light.
You haven't given examples of valid and invalid states, but I am guessing you're really talking about variations of those 3 parameters such as, when type is "something" then amount can only be < 200 and so_rep can only be "whatever". This can be done quite nicely with the FluentValidation library.
In CQS (Command Query Separation) it is common to have commands with a "void" return value and Queries with a return type. (or so I have learned...)
Now I wonder if this COMMAND is valid then, because basically, we are doing the same thing as in a query, just with the "out" keyword instead of using the return type:
public class LogTrace{
public Guid CorrelationId { get; }
public DateTime Timestamp { get; }
}
public class Logger{
public void Log(string message, out LogTrace trace){
trace = new LogTrace(){//Fill properties};
//Log the message (+ trace)
}
}
CQS is not about "not having return types" but about not having queries change data or commands return state - it doesn't prescribe anything about how or when you should use language constructs because it's a language-agnostic concept.
Whether you return data from your command via an explicit return value or implicitly via the use of out parameters is really immaterial; you're still violating the same principle.
If anything, using out is worse: not only do you violate the principle but you also add unnecessary complexity to your code by trying to undermine yourself with a technicality. If you really need to return a value, you should use a proper return value for it.
I'd also question why you are trying to conform to architectural patterns that apparently do not fit with what you're trying to do (or further analyse what makes you think you need to return a value and address that underlying issue, depending on your perspective).
Usually when you do CQS, you generally have ICommandHander<> and IQueryHandler<> so you can add decorators and get some really powerful cross-cutting concerns implemented. I went a step further and created a ICommandQueryHandler<>. There are gray areas, like Stacks/Queues where to get the data, you have to change it first. Thus, it is better to follow the standard as best you can, but just have something in place that you only use as a last resort when all the other options aren't going to work - and then use that each time to be consistent.
Q: `Can Model Classes contain Methods, and Constructor ?" [BEST PRACTICE]
I have an employee Model class which consists of the Employee's name, age, salary etc...
public class Employee
{
public string Name { get; set; }
public int Age { get; set; }
public int Salary { get; set; }
}
so can the above Model Class contain some self updating methods [like updating the employee's age every 1 hour with a timer or something] and\or constructors or it is better to handle all that things from the executer ?
Why couldn't they? The real answer is: it depends on your use case. Patterns, such as something being a model class, are recommendations, common ways of doing things, not unbreakable dogmas.
Note: The example “updating the employee's age every 1 hour with a timer or something” is fundamentally wrong. Storing age is fundamentally wrong, unless you are storing a snapshop to some moment in time. A much better solution would be to store date of birth, and compute the age. For example, by defining an Age property with a getter only.
A model should contain all logic related to the model (this is DSL) so, yes it can update itself each hour. and when u define auto properties, it's same as defining setters and getter, so of course you can add methods, and constructors as well.
Let me put my 2 cents here.
I will start with this: It can be dangerous
There is nothing wrong with adding simple CRUD-like methods in your models, but... it thins the layer of separation between your model and your business logic.
I've many times witnessed these methods getting 'enhanced' over time and at some point ending up with logic which does not fit the purpose of a model. The result: hard dependency between your 'model' and your 'controller'.
It's not that it is wrong, but it opens 'portals' which someone can abuse.
If you want to have a more strict architecture, you can consider adding your operations in a 'Manager-like' class:
Car {
string type;
int price;
}
CarManager {
public CarManager(Car car) {...}
public ChangePrice(...) {...}
public ChangeType(...) {...}
}
... the manager class can now apply validations, exception handling, logging etc, without the concern that it violates your architecture.
Conclusion:
Yes, you can add simple CRUD operations in your model.
Concern: mind their usage.
Edit:
Missed that there is an ask to "updating the employee's age every 1 hour with a timer or something".
In my book this is a no no :). The model's purpose should be limited to data storage only. Periodically updating information is a pure business logic operation.
It all depends on how the class is being used.
There is nothing in the language that does not allow it. But depending on what frameworks you are using and how you are using the classes in your code as to where to put different pieces of functionality.
There is no best answer for every situation.
Danger ... Danger Dr. Smith... Philosophical post ahead
The purpose of this post is to determine if placing the validation logic outside of my domain entities (aggregate root actually) is actually granting me more flexibility or it's kamikaze code
Basically I want to know if there is a better way to validate my domain entities. This is how I am planning to do it but I would like your opinion
The first approach I considered was:
class Customer : EntityBase<Customer>
{
public void ChangeEmail(string email)
{
if(string.IsNullOrWhitespace(email)) throw new DomainException(“...”);
if(!email.IsEmail()) throw new DomainException();
if(email.Contains(“#mailinator.com”)) throw new DomainException();
}
}
I actually do not like this validation because even when I am encapsulating the validation logic in the correct entity, this is violating the Open/Close principle (Open for extension but Close for modification) and I have found that violating this principle, code maintenance becomes a real pain when the application grows up in complexity. Why? Because domain rules change more often than we would like to admit, and if the rules are hidden and embedded in an entity like this, they are hard to test, hard to read, hard to maintain but the real reason why I do not like this approach is: if the validation rules change, I have to come and edit my domain entity. This has been a really simple example but in RL the validation could be more complex
So following the philosophy of Udi Dahan, making roles explicit, and the recommendation from Eric Evans in the blue book, the next try was to implement the specification pattern, something like this
class EmailDomainIsAllowedSpecification : IDomainSpecification<Customer>
{
private INotAllowedEmailDomainsResolver invalidEmailDomainsResolver;
public bool IsSatisfiedBy(Customer customer)
{
return !this.invalidEmailDomainsResolver.GetInvalidEmailDomains().Contains(customer.Email);
}
}
But then I realize that in order to follow this approach I had to mutate my entities first in order to pass the value being valdiated, in this case the email, but mutating them would cause my domain events being fired which I wouldn’t like to happen until the new email is valid
So after considering these approaches, I came out with this one, since I am going to implement a CQRS architecture:
class EmailDomainIsAllowedValidator : IDomainInvariantValidator<Customer, ChangeEmailCommand>
{
public void IsValid(Customer entity, ChangeEmailCommand command)
{
if(!command.Email.HasValidDomain()) throw new DomainException(“...”);
}
}
Well that’s the main idea, the entity is passed to the validator in case we need some value from the entity to perform the validation, the command contains the data coming from the user and since the validators are considered injectable objects they could have external dependencies injected if the validation requires it.
Now the dilemma, I am happy with a design like this because my validation is encapsulated in individual objects which brings many advantages: easy unit test, easy to maintain, domain invariants are explicitly expressed using the Ubiquitous Language, easy to extend, validation logic is centralized and validators can be used together to enforce complex domain rules. And even when I know I am placing the validation of my entities outside of them (You could argue a code smell - Anemic Domain) but I think the trade-off is acceptable
But there is one thing that I have not figured out how to implement it in a clean way. How should I use this components...
Since they will be injected, they won’t fit naturally inside my domain entities, so basically I see two options:
Pass the validators to each method of my entity
Validate my objects externally (from the command handler)
I am not happy with the option 1 so I would explain how I would do it with the option 2
class ChangeEmailCommandHandler : ICommandHandler<ChangeEmailCommand>
{
// here I would get the validators required for this command injected
private IEnumerable<IDomainInvariantValidator> validators;
public void Execute(ChangeEmailCommand command)
{
using (var t = this.unitOfWork.BeginTransaction())
{
var customer = this.unitOfWork.Get<Customer>(command.CustomerId);
// here I would validate them, something like this
this.validators.ForEach(x =. x.IsValid(customer, command));
// here I know the command is valid
// the call to ChangeEmail will fire domain events as needed
customer.ChangeEmail(command.Email);
t.Commit();
}
}
}
Well this is it. Can you give me your thoughts about this or share your experiences with Domain entities validation
EDIT
I think it is not clear from my question, but the real problem is: Hiding the domain rules has serious implications in the future maintainability of the application, and also domain rules change often during the life-cycle of the app. Hence implementing them with this in mind would let us extend them easily. Now imagine in the future a rules engine is implemented, if the rules are encapsulated outside of the domain entities, this change would be easier to implement
I am aware that placing the validation outside of my entities breaks the encapsulation as #jgauffin mentioned in his answer, but I think that the benefits of placing the validation in individual objects is much more substantial than just keeping the encapsulation of an entity. Now I think the encapsulation makes more sense in a traditional n-tier architecture because the entities were used in several places of the domain layer, but in a CQRS architecture, when a command arrives, there will be a command handler accessing an aggregate root and performing operations against the aggregate root only creating a perfect window to place the validation.
I'd like to make a small comparison between the advantages to place validation inside an entity vs placing it in individual objects
Validation in Individual objects
Pro. Easy to write
Pro. Easy to test
Pro. It's explicitly expressed
Pro. It becomes part of the Domain design, expressed with the current Ubiquitous Language
Pro. Since it's now part of the design, it can be modeled using UML diagrams
Pro. Extremely easy to maintain
Pro. Makes my entities and the validation logic loosely coupled
Pro. Easy to extend
Pro. Following the SRP
Pro. Following the Open/Close principle
Pro. Not breaking the law of Demeter (mmm)?
Pro. I'is centralized
Pro. It could be reusable
Pro. If required, external dependencies can be easily injected
Pro. If using a plug-in model, new validators can be added just by dropping the new assemblies without the need to re-compile the whole application
Pro. Implementing a rules engine would be easier
Con. Breaking encapsulation
Con. If encapsulation is mandatory, we would have to pass the individual validators to the entity (aggregate) method
Validation encapsulated inside the entity
Pro. Encapsulated?
Pro. Reusable?
I would love to read your thoughts about this
I agree with a number of the concepts presented in other responses, but I put them together in my code.
First, I agree that using Value Objects for values that include behavior is a great way to encapsulate common business rules and an e-mail address is a perfect candidate. However, I tend to limit this to rules that are constant and will not change frequently. I'm sure you are looking for a more general approach and e-mail is just an example, so I won't focus on that one use-case.
The key to my approach is recognizing that validation serves different purposes at different locations in an application. Put simply, validate only what is required to ensure that the current operation can execute without unexpected/unintended results. That leads to the question what validation should occur where?
In your example, I would ask myself if the domain entity really cares that the e-mail address conforms to some pattern and other rules or do we simply care that 'email' cannot be null or blank when ChangeEmail is called? If the latter, than a simple check to ensure a value is present is all that is needed in the ChangeEmail method.
In CQRS, all changes that modify the state of the application occur as commands with the implementation in command handlers (as you've shown). I will typically place any 'hooks' into business rules, etc. that validate that the operation MAY be performed in the command handler. I actually follow your approach of injecting validators into the command handler which allows me to extend/replace the rule set without making changes to the handler. These 'dynamic' rules allow me to define the business rules, such as what constitutes a valid e-mail address, before I change the state of the entity - further ensuring it does not go into an invalid state. But 'invalidity' in this case is defined by the business logic and, as you pointed out, is highly volitile.
Having come up through the CSLA ranks, I found this change difficult to adopt because it does seem to break encapsulation. But, I agrue that encapsulation is not broken if you take a step back and ask what role validation truly serves in the model.
I've found these nuances to be very important in keeping my head clear on this subject. There is validation to prevent bad data (eg missing arguments, null values, empty strings, etc) that belongs in the method itself and there is validation to ensure the business rules are enforced. In the case of the former, if the Customer must have an e-mail address, then the only rule I need to be concerned about to prevent my domain object from becoming invalid is to ensure that an e-mail address has been provided to the ChangeEmail method. The other rules are higher level concerns regarding the validity of the value itself and really have no affect on the validity of the domain entity itself.
This has been the source of a lot of 'discussions' with fellow developers but when most take a broader view and investigate the role validation really serves, they tend to see the light.
Finally, there is also a place for UI validation (and by UI I mean whatever serves as the interface to the application be it a screen, service endpoint or whatever). I find it perfectly reasonably to duplicate some of the logic in the UI to provide better interactivity for the user. But it is because this validation serves that single purpose why I allow such duplication. However, using injected validator/specification objects promotes reuse in this way without the negative implications of having these rules defined in multiple locations.
Not sure if that helps or not...
I wouldn't suggest trowing big pieces of code into your domain for validation. We eliminated most of our awkward placed validations by seeing them as a smell of missing concepts in our domain. In your sample code you write I see validation for an e-mail address. A Customer doesn't have anything to do with email validation.
Why not make an ValueObject called Email that does this validation at construct?
My experience is that awkward placed validations are hints to missed concepts in your domain. You can catch them in Validator objects, but I prefer value object because you make the related concept part of your domain.
I am at the beginning of a project and I am going to implement my validation outside my domain entities. My domain entities will contain logic to protect any invariants (such as missing arguments, null values, empty strings, collections, etc). But the actual business rules will live in validator classes. I am of the mindset of #SonOfPirate...
I am using FluentValidation that will essentially give me bunch of validators that act on my domain entities: aka, the specification pattern. Also, in accordance with the patterns described in Eric's blue book, I can construct the validators with any data they may need to perform the validations (be it from the database or another repository or service). I would also have the option to inject any dependencies here too. I can also compose and reuse these validators (e.g. an address validator can be reused in both an Employee validator and Company validator). I have a Validator factory that acts as a "service locator":
public class ParticipantService : IParticipantService
{
public void Save(Participant participant)
{
IValidator<Participant> validator = _validatorFactory.GetValidator<Participant>();
var results = validator.Validate(participant);
//if the participant is valid, register the participant with the unit of work
if (results.IsValid)
{
if (participant.IsNew)
{
_unitOfWork.RegisterNew<Participant>(participant);
}
else if (participant.HasChanged)
{
_unitOfWork.RegisterDirty<Participant>(participant);
}
}
else
{
_unitOfWork.RollBack();
//do some thing here to indicate the errors:generate an exception (or fault) that contains the validation errors. Or return the results
}
}
}
And the validator would contain code, something like this:
public class ParticipantValidator : AbstractValidator<Participant>
{
public ParticipantValidator(DateTime today, int ageLimit, List<string> validCompanyCodes, /*any other stuff you need*/)
{...}
public void BuildRules()
{
RuleFor(participant => participant.DateOfBirth)
.NotNull()
.LessThan(m_today.AddYears(m_ageLimit*-1))
.WithMessage(string.Format("Participant must be older than {0} years of age.", m_ageLimit));
RuleFor(participant => participant.Address)
.NotNull()
.SetValidator(new AddressValidator());
RuleFor(participant => participant.Email)
.NotEmpty()
.EmailAddress();
...
}
}
We have to support more than one type of presentation: websites, winforms and bulk loading of data via services. Under pinning all these are a set of services that expose the functionality of the system in a single and consistent way. We do not use Entity Framework or ORM for reasons that I will not bore you with.
Here is why I like this approach:
The business rules that are contained in the validators are totally unit testable.
I can compose more complex rules from simpler rules
I can use the validators in more than one location in my system (we support websites and Winforms, and services that expose functionality), so if there is a slightly different rule required for a use case in a service that differs from the websites, then I can handle that.
All the vaildation is expressed in one location and I can choose how / where to inject and compose this.
You put validation in the wrong place.
You should use ValueObjects for such things.
Watch this presentation http://www.infoq.com/presentations/Value-Objects-Dan-Bergh-Johnsson
It will also teach you about Data as Centers of Gravity.
There also a sample of how to reuse data validation, like for example using static validation methods ala Email.IsValid(string)
I would not call a class which inherits from EntityBase my domain model since it couples it to your persistence layer. But that's just my opinion.
I would not move the email validation logic from the Customer to anything else to follow the Open/Closed principle. To me, following open/closed would mean that you have the following hierarchy:
public class User
{
// some basic validation
public virtual void ChangeEmail(string email);
}
public class Employee : User
{
// validates internal email
public override void ChangeEmail(string email);
}
public class Customer : User
{
// validate external email addresses.
public override void ChangeEmail(string email);
}
You suggestions moves the control from the domain model to an arbitrary class, hence breaking the encapsulation. I would rather refactor my class (Customer) to comply to the new business rules than doing that.
Use domain events to trigger other parts of the system to get a more loosely coupled architecture, but don't use commands/events to violate the encapsulation.
Exceptions
I just noticed that you throw DomainException. That's a way to generic exception. Why don't you use the argument exceptions or the FormatException? They describe the error much better. And don't forget to include context information helping you to prevent the exception in the future.
Update
Placing the logic outside the class is asking for trouble imho. How do you control which validation rule is used? One part of the code might use SomeVeryOldRule when validating while another using NewAndVeryStrictRule. It might not be on purpose, but it can and will happen when the code base grows.
It sounds like you have already decided to ignore one of the OOP fundamentals (encapsulation). Go ahead and use a generic / external validation framework, but don't say that I didn't warn you ;)
Update2
Thanks for your patience and your answers, and that's the reason why I posted this question, I feel the same an entity should be responsible to guarantee it's in a valid state (and I have done it in previous projects) but the benefits of placing it in individual objects is huge and like I posted there's even a way to use individual objects and keep the encapsulation but personally I am not so happy with design but on the other hand it is not out of the table, consider this ChangeEmail(IEnumerable> validators, string email) I have not thought in detail the imple. though
That allows the programmer to specify any rules, it may or may not be the currently correct business rules. The developer could just write
customer.ChangeEmail(new IValidator<Customer>[] { new NonValidatingRule<Customer>() }, "notAnEmail")
which accepts everything. And the rules have to be specified in every single place where ChangeEmail is being called.
If you want to use a rule engine, create a singleton proxy:
public class Validator
{
IValidatorEngine _engine;
public static void Assign(IValidatorEngine engine)
{
_engine = engine;
}
public static IValidatorEngine Current { get { return _engine; } }
}
.. and use it from within the domain model methods like
public class Customer
{
public void ChangeEmail(string email)
{
var rules = Validator.GetRulesFor<Customer>("ChangeEmail");
rules.Validate(email);
// valid
}
}
The problem with that solution is that it will become a maintenance nightmare since the rule dependencies are hidden. You can never tell if all rules have been specified and working unless you test every domain model method and each rule scenario for every method.
The solution is more flexible but will imho take a lot more time to implement than to refactor the method who's business rules got changed.
I cannot say what I did is the perfect thing to do for I am still struggling with this problem myself and fighting one fight at a time. But I have been doing so far the following thing :
I have basic classes for encapsulating validation :
public interface ISpecification<TEntity> where TEntity : class, IAggregate
{
bool IsSatisfiedBy(TEntity entity);
}
internal class AndSpecification<TEntity> : ISpecification<TEntity> where TEntity: class, IAggregate
{
private ISpecification<TEntity> Spec1;
private ISpecification<TEntity> Spec2;
internal AndSpecification(ISpecification<TEntity> s1, ISpecification<TEntity> s2)
{
Spec1 = s1;
Spec2 = s2;
}
public bool IsSatisfiedBy(TEntity candidate)
{
return Spec1.IsSatisfiedBy(candidate) && Spec2.IsSatisfiedBy(candidate);
}
}
internal class OrSpecification<TEntity> : ISpecification<TEntity> where TEntity : class, IAggregate
{
private ISpecification<TEntity> Spec1;
private ISpecification<TEntity> Spec2;
internal OrSpecification(ISpecification<TEntity> s1, ISpecification<TEntity> s2)
{
Spec1 = s1;
Spec2 = s2;
}
public bool IsSatisfiedBy(TEntity candidate)
{
return Spec1.IsSatisfiedBy(candidate) || Spec2.IsSatisfiedBy(candidate);
}
}
internal class NotSpecification<TEntity> : ISpecification<TEntity> where TEntity : class, IAggregate
{
private ISpecification<TEntity> Wrapped;
internal NotSpecification(ISpecification<TEntity> x)
{
Wrapped = x;
}
public bool IsSatisfiedBy(TEntity candidate)
{
return !Wrapped.IsSatisfiedBy(candidate);
}
}
public static class SpecsExtensionMethods
{
public static ISpecification<TEntity> And<TEntity>(this ISpecification<TEntity> s1, ISpecification<TEntity> s2) where TEntity : class, IAggregate
{
return new AndSpecification<TEntity>(s1, s2);
}
public static ISpecification<TEntity> Or<TEntity>(this ISpecification<TEntity> s1, ISpecification<TEntity> s2) where TEntity : class, IAggregate
{
return new OrSpecification<TEntity>(s1, s2);
}
public static ISpecification<TEntity> Not<TEntity>(this ISpecification<TEntity> s) where TEntity : class, IAggregate
{
return new NotSpecification<TEntity>(s);
}
}
and to use it, I do the following :
command handler :
public class MyCommandHandler : CommandHandler<MyCommand>
{
public override CommandValidation Execute(MyCommand cmd)
{
Contract.Requires<ArgumentNullException>(cmd != null);
var existingAR= Repository.GetById<MyAggregate>(cmd.Id);
if (existingIntervento.IsNull())
throw new HandlerForDomainEventNotFoundException();
existingIntervento.DoStuff(cmd.Id
, cmd.Date
...
);
Repository.Save(existingIntervento, cmd.GetCommitId());
return existingIntervento.CommandValidationMessages;
}
the aggregate :
public void DoStuff(Guid id, DateTime dateX,DateTime start, DateTime end, ...)
{
var is_date_valid = new Is_dateX_valid(dateX);
var has_start_date_greater_than_end_date = new Has_start_date_greater_than_end_date(start, end);
ISpecification<MyAggregate> specs = is_date_valid .And(has_start_date_greater_than_end_date );
if (specs.IsSatisfiedBy(this))
{
var evt = new AgregateStuffed()
{
Id = id
, DateX = dateX
, End = end
, Start = start
, ...
};
RaiseEvent(evt);
}
}
the specification is now embedded in these two classes :
public class Is_dateX_valid : ISpecification<MyAggregate>
{
private readonly DateTime _dateX;
public Is_data_consuntivazione_valid(DateTime dateX)
{
Contract.Requires<ArgumentNullException>(dateX== DateTime.MinValue);
_dateX= dateX;
}
public bool IsSatisfiedBy(MyAggregate i)
{
if (_dateX> DateTime.Now)
{
i.CommandValidationMessages.Add(new ValidationMessage("datex greater than now"));
return false;
}
return true;
}
}
public class Has_start_date_greater_than_end_date : ISpecification<MyAggregate>
{
private readonly DateTime _start;
private readonly DateTime _end;
public Has_start_date_greater_than_end_date(DateTime start, DateTime end)
{
Contract.Requires<ArgumentNullException>(start == DateTime.MinValue);
Contract.Requires<ArgumentNullException>(start == DateTime.MinValue);
_start = start;
_end = end;
}
public bool IsSatisfiedBy(MyAggregate i)
{
if (_start > _end)
{
i.CommandValidationMessages.Add(new ValidationMessage(start date greater then end date"));
return false;
}
return true;
}
}
This allows me to reuse some validations for different aggregate and it is easy to test. If you see any flows in it. I would be real happy to discuss it.
yours,
From my OO experience (I am not a DDD expert) moving your code from the entity to a higher abstraction level (into a command handler) will cause code duplication. This is because every time a command handler gets an email address, it has to instantiate email validation rules. This kind of code will rot after a while, and it will smell very badly. In the current example it might not, if you don't have another command which changes the email address, but in other situations it surely will...
If you don't want to move the rules back to a lower abstraction level, like the entity or an email value object, then I strongly suggest you to reduce the pain by grouping the rules. So in your email example the following 3 rules:
if(string.IsNullOrWhitespace(email)) throw new DomainException(“...”);
if(!email.IsEmail()) throw new DomainException();
if(email.Contains(“#mailinator.com”)) throw new DomainException();
can be part of an EmailValidationRule group which you can reuse easier.
From my point of view there is no explicit answer to the question where to put the validation logic. It can be part of every object depending on the abstraction level. In you current case the formal checking of the email address can be part of an EmailValueObject and the mailinator rule can be part of a higher abstraction level concept in which you state that your user cannot have an email address pointing on that domain. So for example if somebody wants to contact with your user without registration, then you can check her email against formal validation, but you don't have to check her email against the mailinator rule. And so on...
So I completely agree with #pjvds who claimed that this kind of awkward placed validation is a sign of a bad design. I don't think you will have any gain by breaking encapsulation, but it's your choice and it will be your pain.
The validation in your example is validation of a value object, not an entity (or aggregate root).
I would separate the validation into distinct areas.
Validate internal characteristics of the Email value object internally.
I adhere to the rule that aggregates should never be in an invalid state. I extend this principal to value objects where practical.
Use createNew() to instantiate an email from user input. This forces it to be valid according to your current rules (the "user#email.com" format, for example).
Use createExisting() to instantiate an email from persistent storage. This performs no validation, which is important - you don't want an exception to be thrown for a stored email that was valid yesterday but invalid today.
class Email
{
private String value_;
// Error codes
const Error E_LENGTH = "An email address must be at least 3 characters long.";
const Error E_FORMAT = "An email address must be in the 'user#email.com' format.";
// Private constructor, forcing the use of factory functions
private Email(String value)
{
this.value_ = value;
}
// Factory functions
static public Email createNew(String value)
{
validateLength(value, E_LENGTH);
validateFormat(value, E_FORMAT);
}
static public Email createExisting(String value)
{
return new Email(value);
}
// Static validation methods
static public void validateLength(String value, Error error = E_LENGTH)
{
if (value.length() < 3)
{
throw new DomainException(error);
}
}
static public void validateFormat(String value, Error error = E_FORMAT)
{
if (/* regular expression fails */)
{
throw new DomainException(error);
}
}
}
Validate "external" characteristics of the Email value object externally, e.g., in a service.
class EmailDnsValidator implements IEmailValidator
{
const E_MX_MISSING = "The domain of your email address does not have an MX record.";
private DnsProvider dnsProvider_;
EmailDnsValidator(DnsProvider dnsProvider)
{
dnsProvider_ = dnsProvider;
}
public void validate(String value, Error error = E_MX_MISSING)
{
if (!dnsProvider_.hasMxRecord(/* domain part of email address */))
{
throw new DomainException(error);
}
}
}
class EmailDomainBlacklistValidator implements IEmailValidator
{
const Error E_DOMAIN_FORBIDDEN = "The domain of your email address is blacklisted.";
public void validate(String value, Error error = E_DOMAIN_FORBIDDEN)
{
if (/* domain of value is on the blacklist */))
{
throw new DomainException(error);
}
}
}
Advantages:
Use of the createNew() and createExisting() factory functions allow control over internal validation.
It is possible to "opt out" of certain validation routines, e.g., skip the length check, using the validation methods directly.
It is also possible to "opt out" of external validation (DNS MX records and domain blacklisting). E.g., a project I worked on initially validated the existance of MX records for a domain, but eventually removed this because of the number of customers using "dynamic IP" type solutions.
It is easy to query your persistent store for email addresses that do not fit the current validation rules, but running a simple query and treating each email as "new" rather than "existing" - if an exception is thrown, there's a problem. From there you can issue, for example, a FlagCustomerAsHavingABadEmail command, using the exception error message as guidance for the user when they see the message.
Allowing the programmer to supply the error code provides flexibility. For example, when sending a UpdateEmailAddress command, the error of "Your email address must be at least 3 characters long" is self explanatory. However, when updating multiple email addresses (home and work), the above error message does not indicate WHICH email was wrong. Supplying the error code/message allows you to provide richer feedback to the end user.
I wrote a blog post on this topic a while back. The premise of the post was that there are different types of validation. I called them Superficial Validation and Domain Based Command Validation.
This simple version is this. Validating things like 'is it a number' or 'email address' are more often than not just superficial. These can be done before the command reaches the domain entities.
However, where the validation is more tied to the domain then it's right place is in the domain. For example, maybe you have some rules about the weight and type of cargo a certain lorry can take. This sounds much more like domain logic.
Then you have the hybrid types. Things like set based validation. These need to happen before the command is issued or injected into the domain (try to avoid that if at all possible - limiting dependencies is a good thing).
Anyway, you can read the full post here: How To Validate Commands in a CQRS Application
I'm still experimenting with this concept but you can try Decorators. If you use SimpleInjector you can easily inject your own validation classes that run ahead of your command handler. Then the command can assume it is valid if it got that far. However, This means all validation should be done on the command and not the entities. The entities won't go into an invalid state. But each command must implement its own validation fully so similar commands may have duplication of rules but you could either abstract common rules to share or treat different commands as truly separate.
You can use a message based solution with Domain Events as explained here.
Exceptions are not the right method for all validation errors, is not said that a not valid entity is an exceptional case.
If the validation is not trivial, the logic to validate the aggregate can be executed directly on the server and while you are trying to set new input you can raise a Domain Event to tell to the user (or the application that is using your domain) why the input is not correct.
I have several entities that have calculated fields on them such as TotalCost. Right now I have them all as properties but I'm wondering if they should actually be methods. Is there a C# standard for this?
public class WorkOrder
{
public int LaborHours { get; set; }
public decimal LaborRate { get; set; }
// Should this be LaborCost()?
public decimal LaborCost
{
get
{
return LaborHours * LaborRate;
}
}
}
It's OK to use calculated properties rather than methods, as long as the calculation doesn't take a noticeable time
See Property usage guidelines
I think methods should perform actions on the object, typically change the state of the object. Properties should reflect the current state of the object even if the property is calculated. So you should keep your properties IMO.
I think they should all be properties. As long as it doesn't change the state of the object, I'm cool with it as a property.
Additionally, if I'm using your class for data binding (WPF, etc.), then I can bind directly to your property without having to modify/extend the class.
If they are a) lightweight and b) have no side effects, I would make them Properties.
Lightweight is a bit fuzzy of course, but the rule of thumb is: If I ever have to worry calling a Property (be it in a loop or anywhere else), it should possibly be a method.
I would leave them as properties. But there's not "standard" reason to do things one way or another. If you're by yourself, do whatever you like best. If you're on a team, then follow conventions the rest of your team are following.
If a property is particularly expensive to calculate, I might change it to a GetWhatever() method. This serves as a hint to whoever uses my class that this value requires some significant work to arrive at, and the caller should cache the value rather than calling the method multiple times.
Trivial calculations are perfectly appropriate inside of properties.
In my opinion, it's a preference; it's what you want to do. I do propreties in most cases, unless there is logic involved. Additionally, if you need to pass in parameters to change the functionality then obviously a method would apply...
Depends, if your "properties" become mammoths and require a whole slew of business logic they shouldn't be properties, there should be a method.
The example you posted looks ok to be a property. No standard way of doing it, go with your gut instinct; if it looks like it needs to do a lot you probably need a method.
It's largely just syntactic sugar anyway, so do want you is convention in your team, or what you prefer, as long as it is just returning information about the object and not changing it or interacting with other objects.
MSDN gives information about this here
Class library designers often must
decide between implementing a class
member as a property or a method. In
general, methods represent actions and
properties represent data.
Which one do you think it is? An action calculate/getLaborCost or data?
WorkOrder workOrder = new WorkOrder();
workOrder.LaborHours = 8;
workOrder.LaborRate = 20;
decimal cost = workOrder.LaborCost; // This is OK here
but if you are going to do this for the same object also:
worOrder.LaborHours = 18;
decimal newCost = workOrder.LaborCost
Now this cannot be a property. It would be a lot better to be a method.
Sometimes, you have to consider also what you're modeling... On some domains, the calculated values are often or expected to be an attribute of the model -- a Property. If this were the case, then write it as a Property even though the calculation is not at all trivial or a little bit expensive to compute. Just document it on your API or implement some caching mechanism to minimize recomputation for this property.