I've been modeling a domain for a couple of days now and not been thinking at all at persistance but instead focusing on domain logic. Now I'm ready to persist my domain objects, some of which contains IEnumerable of child entities. Using RavenDb, the persistance is 'easy', but when loading my objects back again, all of the IEnumerables are empty.
I've realized this is because they don't have any property setters at all, but instead uses a list as a backing field. The user of the domain aggregate root can add child entities through a public method and not directly on the collection.
private readonly List<VeryImportantPart> _veryImportantParts;
public IEnumerable<VeryImportantPart> VeryImportantParts { get { return _veryImportantParts; } }
And the method for adding, nothing fancy...
public void AddVeryImportantPart(VeryImportantPart part)
{
// some logic...
_veryImportantParts.Add(part);
}
I can fix this by adding a private/protected setter on all my IEnumerables with backing fields but it looks... well... not super sexy.
private List<VeryImportantPart> _veryImportantParts;
public IEnumerable<VeryImportantPart> VeryImportantParts
{
get { return _veryImportantParts; }
protected set { _veryImportantParts = value.ToList(); }
}
Now the RavenDb json serializer will populate my objects on load again, but I'm curious if there isn't a cleaner way of doing this?
I've been fiddeling with the JsonContractResolver but haven't found a solution yet...
I think I've found the root cause of this issue and it's probably due to the fact that many of my entities were created using:
protected MyClass(Guid id, string name, string description) : this()
{ .... }
public static MyClass Create(string name, string description)
{
return new MyClass(Guid.NewGuid(), name, description);
}
When deserializing, RavenDb/Json.net couldn't rebuild my entities in a proper way...
Changing to using a public constructor made all the difference.
Do you need to keep a private backing field? Often an automatic property will do.
public IList<VeryImportantPart> VeryImportantParts { get; protected set; }
When doing so, you may want to initialize your list in the constructor:
VeryImportantParts = new List<VeryImportantPart>();
This is optional, of course, but it allows you to create a new class and start adding to the list right away, before it is persisted. When Raven deserializes a class, it will use the setter to overwrite the default blank list, so this just helps with the first store.
You certainly won't be able to use a readonly field, as it couldn't be replaced during deserialization. It might be possible to write a contract resolver or converter that fills an existing list rather than creating a new one, but that seems like a rather complex solution.
Using an automatic property can add clarity to your code anyway - as it is less confusing whether to use the field or the property.
This is such a simple and common scenario I wonder how did I managed until now and why I have problems now.
I have this object (part of the Infrastructure assembly)
public class Queue {}
public class QueueItem
{
public QueueItem(int blogId,string name,Type command,object data)
{
if (name == null) throw new ArgumentNullException("name");
if (command == null) throw new ArgumentNullException("command");
BlogId = blogId;
CommandType = command;
ParamValue = data;
CommandName = name;
AddedOn = DateTime.UtcNow;
}
public Guid Id { get; internal set; }
public int BlogId { get; private set; }
public string CommandName { get; set; }
public Type CommandType { get; private set; }
public object ParamValue { get; private set; }
public DateTime AddedOn { get; private set; }
public DateTime? ExecutedOn { get; private set; }
public void ExecuteIn(ILifetimeScope ioc)
{
throw new NotImplementedException();
}
}
This will be created in another assembly like this
var qi = new QueueItem(1,"myname",typeof(MyCommand),null);
Nothing unusal here. However, this object will be sent t oa repository where it will be persisted.The Queue object will ask the repository for items. The repository should re-create QueueItem objects.
However, as you see, the QueueItem properties are invariable, the AddedOn property should be set only once when the item is created. The Id property will be set by the Queue object (this is not important).
The question is how should I recreate the QueueItem in the repository? I can have another constructor which will require every value for ALL the properties, but I don't want that constructor available for the assembly that will create the queue item initially. The repository is part of a different assembly so internal won't work.
I thought about providing a factory method
class QueueItem
{
/* ..rest of definitions.. */
public static QueueItem Restore(/* list of params*/){}
}
which at least clears the intent, but I don't know why I don't like this approach. I could also enforce the item creation only by the Queue , but that means to pass the Queue as a dependency to the repo which again isn't something I'd like. To have a specific factory object for this, also seems way overkill.
Basically my question is: what is the optimum way to recreate an object in the repository, without exposing that specific creational functionality to another consumer object.
Update
It's important to note that by repository I mean the pattern itself as an abstraction, not a wrapper over an ORM. It doesn't matter how or where the domain objects are persisted. It matters how can be re-created by the repository. Another important thing is that my domain model is different from the persistence model. I do use a RDBMS but I think this is just an implementation detail which should not bear any importance, since I'm looking for way that doesn't depend on a specific storage access.
While this is a specific scenario, it can applied to basically every object that will be restored by the repo.
Update2
Ok I don't know how I could forget about AutoMapper. I was under the wrong impression it can't map private fields/setter but it can, and I think this is the best solution.
In fact I can say the optimum solutions (IMO) are in order:
Directly deserializing, if available.
Automap.
Factory method on the domain object itself.
The first two don't require the object to do anyting in particular, while the third requires the object to provide functionality for that case (a way to enter valid state data). It has clear intent but it pretty much does a mapper job.
Answer Updated
To answer myself, in this case the optimum way is to use a factory method. Initially I opted for the Automapper but I found myself using the factory method more often. Automapper can be useful sometimes but in quite a lot of cases it's not enough.
An ORM framework would take care of that for you. You just have to tell it to rehydrate an object and a regular instance of the domain class will be served to you (sometimes you only have to declare properties as virtual or protected, in NHibernate for instance). The reason is because under the hood, they usually operate on proxy objects derived from your base classes, allowing you to keep these base classes intact.
If you want to implement your own persistence layer though, it's a whole nother story. Rehydrating an object from the database without breaking the scope constraints originally defined in the object is likely to involve reflection. You also have to think about a lot of side concerns : if your object has a reference to another object, you must rehydrate that one before, etc.
You can have a look at that tutorial : Build Your Own dataAccess Layer although I wouldn't recommend reinventing the wheel in most cases.
You talked about a factory method on the object itself. But DDD states that entities should be created by a factory. So you should have a QueueItemFactory that can create new QueueItems and restore existing QueueItems.
Ok I don't know how I could forget about AutoMapper.
I wish I could forget about AutoMapper. Just looking at the hideous API gives me shivers down my spine.
So I've been learning about the Repository model, and it seems that it is expected that Repositories do not do a lot of intricate logic. However I also read that most of the business logic should not be inside of my Controllers. So where do I put it?
I've looked at some sample applications and it seems that they have another layer called Services that do more intricate logic for things. So how does this factor into the MVC pattern?
Do I want to build my services to access my repositories, and then my controllers to access my services? Like this?
interface IMembershipService
{
bool ValidateUser(string username, string password);
MembershipCreateStatus Create(string username, string password);
}
interface IMembershipRepository
{
MembershipCreateStatus Create(string username, string password);
}
class MembershipRepository : IMembershipRepository
{
public MembershipRepository(ISession session)
{
**// this is where I am confused...**
}
}
class MembershipService : IMembershipService
{
private readonly IMembershipRepository membershipRepository;
public MembershipService(IMembershipRepository membershipRepository)
{
this.membershipRepository = membershipRepository;
}
public bool ValidateUser(string username, string password)
{
// validation logic
}
public MembershipCreateStatus Create(string username, string password)
{
return membershipRepository.Create(username, password);
}
}
class MembershipController : Controller
{
private readonly IMembershipService membershipService;
public MembershipController(IMembershipService membershipService)
{
this.membershipService = membershipService
}
}
The marked part of my code is what confuses me. Everything I have read said I should be injecting my ISession into my repositories. This means I could not be injecting ISession into my services, so then how do I do Database access from my Services? I'm not understanding what the appropriate process is here.
When I put ValidateUser in my IMembershipRepository, I was told that was 'bad'. But the IMembershipRepository is where the database access resides. That's the intention, right? To keep the database access very minimal? But if I can't put other logic in them, then what is the point?
Can someone shed some light on this, and show me an example that might be more viable?
I am using Fluent nHibernate, ASP.NET MVC 3.0, and Castle.Windsor.
Should I instead do something like ...
class MembershipService
{
private readonly IMembershipRepository membershipRepository;
public MembershipService(ISession session)
{
membershipRepository = new MembershipRepository(session);
}
}
And never give my Controllers direct access to the Repositories?
Everything I have read said I should be injecting my ISession into my repositories.
That's correct. You need to inject the session into the repository constructor because this is where the data access is made.
This means I could not be injecting ISession into my services, so then how do I do Database access from my Services?
You don't do database access in your services. The service relies on one or more repositories injected into its constructor and uses their respective methods. The service never directly queries the database.
So to recap:
The repository contains the simple CRUD operations on your model. This is where the data access is performed. This data access doesn't necessary mean database. It will depend on the underlying storage you are using. For example you could be calling some remote services on the cloud to perform the data access.
The service relies on one or more repositories to implement a business operation. This business operation might depend on one or more CRUD operations on the repositories. A service shouldn't even know about the existence of a database.
The controller uses the service to invoke the business operation.
In order to decrease the coupling between the different layers, interfaces are used to abstract the operations.
interface IMembershipService
{
bool ValidateUser(string username, string password);
MembershipCreateStatus Create(string username, string password);
}
Creating a service like this an anti-pattern.
How many responsibilities does a service like this have? How many reasons could it have to change?
Also, if you put your logic into services, you are going to end up with an anemic domain. What you will end up with is procedural code in a Transaction Script style. And I am not saying this is necessarily bad.
Perhaps a rich domain model is not appropriate for you, but it should be a conscious decision between the two, and this multiple responsibility service is not appropriate in either case.
This should be a HUGE red flag:
public MembershipCreateStatus Create(string username, string password)
{
return membershipRepository.Create(username, password);
}
What is the point? Layers for the sake of layers? The Service adds no value here, serves no purpose.
There are a lot of concepts missing.
First, consider using a Factory for creating objects:
public interface IMembershipFactory {
MembershipCreateStatus Create(string username, string password);
}
The factory can encapsulate any logic that goes into building an instance or beginning the lifetime of an entity object.
Second, Repositories are an abstraction of a collection of objects. Once you've used a factory to create an object, add it to the collection of objects.
var result = _membershipFactory.Create("user", "pw");
if (result.Failed); // do stuff
_membershipRepository.Add(status.NewMembership); // assumes your status includes the newly created object
Lastly, MyEntityService class that contains a method for every operation that can be performed on an Entity just seems terribly offensive to my senses.
Instead, I try to be more explicit and better capture intent by modeling each operation not as a method on a single Service class, but as individual Command classes.
public class ChangePasswordCommand {
public Guid MembershipId { get; set; }
public string CurrentPassword { get; set; }
public string NewPassword { get; set; }
}
Then, something has to actually do something when this command is sent, so we use handlers:
public interface IHandle<TMessageType> {
void Execute(TMessageType message);
}
public class ChangePasswordCommandHandler : IHandle<ChangePasswordCommand> {
public ChangePasswordCommandHandler(
IMembershipRepository repo
)
{}
public void Execute(ChangePasswordCommand command) {
var membership = repo.Get(command.MembershipId);
membership.ChangePassword(command.NewPassword);
}
}
Commands are dispatched using a simple class that interfaces with our IoC container.
This helps avoids monolithic service classes and makes a project's structure and location of logic much clearer.
I don't know why I started thinking about this, but now I can't seem to stop.
In C# - and probably a lot of other languages, I remember that Delphi used to let you do this too - it's legal to write this syntax:
class WeirdClass
{
private void Hello(string name)
{
Console.WriteLine("Hello, {0}!", name);
}
public string Name
{
set { Hello(name); }
}
}
In other words, the property has a setter but no getter, it's write-only.
I guess I can't think of any reason why this should be illegal, but I've never actually seen it in the wild, and I've seen some pretty brilliant/horrifying code in the wild. It seems like a code smell; it seems like the compiler should be giving me a warning:
CS83417: Property 'Name' appears to be completely useless and stupid. Bad programmer! Consider replacing with a method.
But maybe I just haven't been doing this long enough, or have been working in too narrow a field to see any examples of the effective use of such a construct.
Are there real-life examples of write-only properties that either cannot be replaced by straight method calls or would become less intuitive?
My first reaction to this question was: "What about the java.util.Random#setSeed method?"
I think that write-only properties are useful in several scenarios. For example, when you don't want to expose the internal representation (encapsulation), while allowing to change the state of the object. java.util.Random is a very good example of such design.
Code Analysis (aka FxCop) does give you a diagnostic:
CA1044 : Microsoft.Design : Because
property 'WeirdClass.Name' is write-only,
either add a property getter with an
accessibility that is greater than or
equal to its setter or convert this
property into a method.
Write-only properties are actually quite useful, and I use them frequently. It's all about encapsulation -- restricting access to an object's components. You often need to provide one or more components to a class that it needs to use internally, but there's no reason to make them accessible to other classes. Doing so just makes your class more confusing ("do I use this getter or this method?"), and more likely that your class can be tampered with or have its real purpose bypassed.
See "Why getter and setter methods are evil" for an interesting discussion of this. I'm not quite as hardcore about it as the writer of the article, but I think it's a good thing to think about. I typically do use setters but rarely use getters.
I have code similar to the following in an XNA project. As you can see, Scale is write-only, it is useful and (reasonably) intuitive and a read property (get) would not make sense for it. Sure it could be replaced with a method, but I like the syntax.
public class MyGraphicalObject
{
public double ScaleX { get; set; }
public double ScaleY { get; set; }
public double ScaleZ { get; set; }
public double Scale { set { ScaleX = ScaleY = ScaleZ = value; } }
// more...
}
One use for a write-only property is to support setter dependency injection, which is typically used for optional parameters.
Let's say I had a class:
public class WhizbangService {
public WhizbangProvider Provider { set; private get; }
}
The WhizbangProvider is not intended to be accessed by the outside world. I'd never want to interact with service.Provider, it's too complex. I need a class like WhizbangService to act as a facade. Yet with the setter, I can do something like this:
service.Provider = new FireworksShow();
service.Start();
And the service starts a fireworks display. Or maybe you'd rather see a water and light show:
service.Stop();
service.Provider = new FountainDisplay(new StringOfLights(), 20, UnitOfTime.Seconds);
service.Start();
And so on....
This becomes especially useful if the property is defined in a base class. If you chose construction injection for this property, you'd need to write a constructor overload in any derived class.
public abstract class DisplayService {
public WhizbangProvider Provider { set; private get; }
}
public class WhizbangService : DisplayService { }
Here, the alternative with constructor injection is:
public abstract class DisplayService {
public WhizbangProvider Provider;
protected DisplayService(WhizbangProvider provider) {
Provider = provider ?? new DefaultProvider();
}
}
public class WhizbangService : DisplayService {
public WhizbangService(WhizbangProvider provider)
: base(provider)
{ }
}
This approach is messier in my opinion, because you need to some of the internal workings of the class, specifically, that if you pass null to the constructor, you'll get a reasonable default.
In MVP pattern it is common to write a property with a setter on the view (no need for a getter) - whenever the presenter sets it content the property will use that value to update some UI element.
See here for a small demonstration:
public partial class ShowMeTheTime : Page, ICurrentTimeView
{
protected void Page_Load(object sender, EventArgs e)
{
CurrentTimePresenter presenter = new CurrentTimePresenter(this);
presenter.InitView();
}
public DateTime CurrentTime
{
set { lblCurrentTime.Text = value.ToString(); }
}
}
The presenter InitView method simply sets the property's value:
public void InitView()
{
view.CurrentTime = DateTime.Now;
}
Making something write-only is usefulwhenever you're not supposed to read what you write.
For example, when drawing things onto the screen (this is precisely what the Desktop Window Manager does in Windows):
You can certainly draw to a screen, but you should never need to read back the data (let alone expect to get the same design as before).
Now, whether write-only properties are useful (as opposed to methods), I'm not sure how often they're used. I suppose you could imagine a situation with a "BackgroundColor" property, where writing to it sets the background color of the screen, but reading makes no sense (necessarily).
So I'm not sure about that part, but in general I just wanted to point out that there are use cases for situations in which you only write data, and never read it.
Although the .NET design guidelines recommend using a method ("SetMyWriteOnlyParameter") instead of a write-only property, I find write-only properties useful when creating linked objects from a serialised representation (from a database).
Our application represents oil-field production systems. We have the system as a whole (the "Model" object) and various Reservoir, Well, Node, Group etc objects.
The Model is created and read from database first - the other objects need to know which Model they belong to. However, the Model needs to know which lower object represents the Sales total. It makes sense for this information to be stored a Model property. If we do not want to have to do two reads of Model information, we need to be able to read the name of Sales object before its creation. Then, subsequently, we set the "SalesObject" variable to point to the actual object (so that, e.g., any change by the user of the name of this object does not cause problems)
We prefer to use a write-only property - 'SalesObjectName = "TopNode"' - rather than a method - 'SetSalesObjectName("TopNode") - because it seems to us that the latter suggests that the SalesObject exists.
This is a minor point, but enough to make us want to use a Write-Only property.
As far as I'm concerned, they don't. Every time I've used a write-only property as a quick hack I have later come to regret it. Usually I end up with a constructor or a full property.
Of course I'm trying to prove a negative, so maybe there is something I'm missing.
I can't stop thinking about this, either. I have a use case for a "write-only" property. I can't see good way out of it.
I want to construct a C# attribute that derives from AuthorizeAttribute for an ASP.NET MVC app. I have a service (say, IStore) that returns information that helps decide if the current user should be authorized. Constructor Injection won't work, becuase
public AllowedAttribute: AuthorizeAttribute
{
public AllowedAttribute(IStore store) {...}
private IStore Store { get; set; }
...
}
makes store a positional attribute parameter, but IStore is not a valid attribute parameter type, and the compiler won't build code that is annotated with it. I am forced to fall back on Property Setter Injection.
public AllowedAttribute: AuthorizeAttribute
{
[Inject] public IStore Store { private get; set; }
...
}
Along with all the other bad things about Property Setter instead of Constructor Injection, the service is a write-only property. Bad enough that I have to expose the setter to clients that shouldn't need to know about the implementation detail. It wouldn't do anybody any favors to let clients see the getter, too.
I think that the benefit of Dependency Injection trumps the guidelines against write-only properties for this scenario, unless I am missing something.
I just came across that situation when writing a program that reads data from a JSON database (Firebase). It uses Newtonsoft's Json.NET to populate the objects. The data are read-only, i.e., once loaded they won't change. Also, the objects are only deserialized and won't be serialized again. There may be better ways, but this solution just looks reasonable for me.
using Newtonsoft.Json;
// ...
public class SomeDatabaseClass
{
// JSON object contains a date-time field as string
[JsonProperty("expiration")]
public string ExpirationString
{
set
{
// Needs a custom parser to handle special date-time formats
Expiration = Resources.CustomParseDateTime(value);
}
}
// But this is what the program will effectively use.
// DateTime.MaxValue is just a default value
[JsonIgnore]
public DateTime Expiration { get; private set; } = DateTime.MaxValue;
// ...
}
No, I can' imagine any case where they can't be replaced, though there might people who consider them to be more readable.
Hypothetical case:
CommunicationDevice.Response = "Hello, World"
instead of
CommunicationDevice.SendResponse("Hello, World")
The major job would be to perform IO side-effects or validation.
Interestingly, VB .NET even got it's own keyword for this weird kind of property ;)
Public WriteOnly Property Foo() As Integer
Set(value As Integer)
' ... '
End Set
End Property
even though many "write-only" properties from outside actually have a private getter.
I recently worked on an application that handled passwords. (Note that I'm not claiming that the following is a good idea; I'm just describing what I did.)
I had a class, HashingPassword, which contained a password. The constructor took a password as an argument and stored it in a private attribute. Given one of these objects, you could either acquire a salted hash for the password, or check the password against a given salted hash. There was, of course, no way to retrieve the password from a HashingPassword object.
So then I had some other object, I don't remember what it was; let's pretend it was a password-protected banana. The Banana class had a set-only property called Password, which created a HashingPassword from the given value and stored it in a private attribute of Banana. Since the password attribute of HashingPassword was private, there was no way to write a getter for this property.
So why did I have a set-only property called Password instead of a method called SetPassword? Because it made sense. The effect was, in fact, to set the password of the Banana, and if I wanted to set the password of a Banana object, I would expect to do that by setting a property, not by calling a method.
Using a method called SetPassword wouldn't have had any major disadvantages. But I don't see any significant advantages, either.
I know this has been here for a long time, but I came across it and have a valid (imho) use-case:
When you post parameters to a webapi call from ajax, you can simply try to fill out the parameters class' properties and include validation or whatsoever.
public int MyFancyWepapiMethod([FromBody]CallParams p) {
return p.MyIntPropertyForAjax.HasValue ? p.MyIntPropertyForAjax.Value : 42;
}
public class CallParams
{
public int? MyIntPropertyForAjax;
public object TryMyIntPropertyForAjax
{
set
{
try { MyIntPropertyForAjax = Convert.ToInt32(value); }
catch { MyIntPropertyForAjax = null; }
}
}
}
On JavaScript side you can simply fill out the parameters including validation:
var callparameter = {
TryMyIntPropertyForAjax = 23
}
which is safe in this example, but if you handle userinput it might be not sure that you have a valid intvalue or something similar.
I'm working on a business application which is being developed using DDD philosophy. Database is accessed through NHibernate and data layer is implemented using DAO pattern.
The UML class diagram is shown below.
UML Class Diagram http://img266.imageshack.us/my.php?image=classdiagramhk0.png
http://img266.imageshack.us/my.php?image=classdiagramhk0.png
I don't know the design is good or not. What do you think?
But the problem is not the design is good or not. The problem is after starting up the application an IDaoFactory is instantiated in presentation layer and send as parameter to presenter classes(which is designed using MVC pattern) as below
...
IDaoFactory daoFactory = new NHibernateDaoFactory(); //instantiation in main class
...
SamplePresenterClass s = new SamplePresenterClass(daoFactory);
...
Using just one data provider (which was just one database) was simple. But now we should get data from XML too. And next phases of the development we should connect to different web services and manipulate incoming and outgoing data.
The data from XML is going to be got using a key which is an enum. We add a class named XMLLoader to the data layer and add an interface ILoader to the domain. XMLLoader has a method whose signature is
List<string> LoadData(LoaderEnum key)
If we instantiate ILoader with XMLLoader in presentation layer as below we have to send it to objects which is going to get some XML data from data layer.
ILoader loader = new XMLLoader();
SamplePresenterClass s = new SamplePresenterClass(daoFactory, xmlLoader);
After implementing web service access classes
SamplePresenterClass s = new SamplePresenterClass(daoFactory, xmlLoader, sampleWebServiceConnector1, sampleWebServiceConnector2, ...);
The parameters is going to be grown in time. I think i can hold all instances of data access objects in a class and pass it to required presenters (maybe singleton pattern can helps too). In domain layer there must be a class like this,
public class DataAccessHolder
{
private IDaoFactory daoFactory;
private ILoader loader;
...
public IDaoFactory DaoFactory
{
get { return daoFactory; }
set { daoFactory = value; }
}
...
}
In main class the instantiation can be made with this design as follows
DataAccessHolder dataAccessHolder = new DataAccessHolder();
dataAccessHolder.DaoFactory = new NHibernateDaoFactory();
dataAccessHolder.Loader = new XMLLoader();
...
SamplePresenterClass s = new SamplePresenterClass(dataAccessHolder);
What do you think about this design or can you suggest me a different one?
Thanks for all repliers...
IMO, it would be cleaner to use a "global" or static daoFactory and make it generic.
DaoFactory<SamplePresenterClass>.Create(); // or
DaoFactory<SamplePresenterClass>.Create(id); // etc
Then, you can define DaoFactory<T> to take only, say, IDao's
interface IDao
{
IDaoProvider GetProvider();
}
interface IDaoProvider
{
IDao Create(IDao instance);
void Update(IDao instance);
void Delete(IDao instance);
}
Basically instead of passing every constructor your DaoFactory, you use a static generic DaoFactory. Its T must inherit from IDao. Then the DaoFactory class can look at the T provider at runtime:
static class DaoFactory<T> where T : IDao, new()
{
static T Create()
{
T instance = new T();
IDaoProvider provider = instance.GetProvider();
return (T)provider.Create(instance);
}
}
Where IDaoProvier is a common interface that you would implement to load things using XML, NHibernate, Web Services, etc. depending on the class. (Each IDao object would know how to connect to its data provider).
Overall, not a bad design though. Add a bit more OO and you will have a pretty slick design. For instance, each file for the XmlEnums could be implemented as IDao's
class Cat : IDao
{
IDaoProvider GetProvider()
{
return new XmlLoader(YourEnum.Cat);
}
// ...
}