Suppose I have a game, where there are buildings sorted by type. Each type is represented as a separate class, but sometimes I have to do some uncommon logic for the buildings of the same type. How could one implement this kind of behaviour?
For example, I can identify buildings by ID, so I can have a giant switch or command pattern inside the building type class. But I think that something is not right with this approach.
Another approach is to have different class for any divergent logic. But this proposes a lot of small classes.
This is what polymorphism aims to solve, and one of the big differences between procedural and oop programming. You can achieve it through extending a base class, or by implementing an interface. Here is extending a base class:
public abstract class Building {
abstract void destroy();
}
public BrickBuilding extends Building {
#Override
public void destroy() {
bricks.fallToGround();
}
}
public HayBuilding extends Building {
#Override
public void destroy() {
straw.blowInWind();
}
}
In places in your code where you would have used a switch statement to switch on building type, just hold a reference to the abstract Building type, and call method destroy() on it:
public class BuildingDestroyer {
public void rampage() {
for(Building building : allTheBuildings) {
// Could be a BrickBuilding, or a HayBuilding
building.destroy();
}
}
}
Or, to address your concern about having a lot of small types, you can 'inject' a destroy behaviour you want into a common building type, like so...albeing, you will end up with a lot of different destroy behaviour classes too...so, this might not be a solution.
public interface DestroyBehaviour {
void destroy(Building building);
}
public class Building {
private int id;
public DestroyBehaviour destroyBehaviour;
public Building(int id, DestroyBehaviour destroyBehaviour) {
this.id = id;
this.destroyBehaviour = destroyBehaviour;
}
public void destroy() {
destroyBehaviour.destroy(this); // or something along those lines;
}
}
You can get rid of the giant switch by having a BuildingFactory class which exposes a registerBuildingType(typeName, instanceCreatorFunc) method, that each building class calls (from a static initialize method for example) and that gets called with a unique string for that class (class name would suffice) and a static "create" method that returns a new instance.
This approach also has the advantage of being able to load new buildings from dynamically linked libraries.
if I have the interface interfaceA
public interface IInterfaceA
{
void MethodA();
void MethodB();
}
and I have the classA
class ClassA:IInterfaceA
{
public void MethodA()
{
}
public void MethodB()
{
}
}
it's ok that I use ninject's bind,but when it comes that I have a method that called MethodC,I think the method should only exists in classA(just for classA) and should not be defined in InterfaceA,so how to use ninject'bind when just calling like this:
var a = _kernel.get<IInterfaceA>()
should I convert the result into ClassA ? (is that a bad habbit?) or there are another solution
Usually this is needed when you want interface separation but need both interfaces to be implemented by the same object since it holds data relevant to both interfaces. If that is not the case you would be able to separate interfaces and implementation completely - and then you should do so.
For simplicitys sake i'm going to asume Singleton Scope, but you could also use any other scope.
Create two interfaces instead:
public interface IInterfaceA {
{
void MethodA();
}
public interface IInterfaceC {
void MethodC();
}
public class SomeClass : IInterfaceA, IInterfaceC {
....
}
IBindingRoot.Bind<IInterfaceA, IInterfaceB>().To<SomeClass>()
.InSingletonScope();
var instanceOfA = IResolutionRoot.Get<IInterfaceA>();
var instanceOfB = IResolutionRoot.Get<IInterfaceB>();
instanceOfA.Should().Be(instanceOfB);
Does this answer your question?
Look at this very simple example: Calling CreateCar it works, calling GetCar it fails, saying "Error activating ICar: No matching bindings are available, and the type is not self-bindable".
public interface ICar { }
public class Car : ICar
{
public Car(string carType) { }
}
public interface ICarFactory
{
ICar CreateCar(string carType); // this is fine
ICar GetCar(string carType); // this is bad
}
public class CarModule : NinjectModule
{
public override void Load()
{
Bind<ICarFactory>().ToFactory();
Bind<ICar>().To<Car>();
}
}
public class Program
{
public static void Main()
{
using (var kernel = new StandardKernel(new FuncModule(), new CarModule()))
{
var factory = kernel.Get<ICarFactory>();
var car1 = factory.CreateCar("a type");
var car2 = factory.GetCar("another type");
}
}
}
Is assume it must be related to some kind of convention with Get*ClassName* (something like the NamedLikeFactoryMethod stuff). Is there any way to avoid this convention to be applied? I don't need it and I don't want it (I already wasted too much time trying to figure out why the binding was failing, it was just luck that I made a typo in 1 of my 10 factories and I noticed it to work just because the factory method name was "Ger" instead of "Get").
Thanks!
Yes, there is a convention, where the Get is used to obtain instances using a named binding. The factory extension generates code for you so you don't have to create boilerplate code for factories. You don't need to use it, if you don't want to.
But if you do, you are bound to its conventions. Use Create to build instances and Get to retrieve instances via a named binding.
All this is documented in the wiki.
i have a question regarding design patterns.
suppose i want to design pig killing factory
so the ways will be
1) catch pig
2)clean pig
3) kill pig
now since these pigs are supplied to me by a truck driver
now if want to design an application how should i proceed
what i have done is
public class killer{
private Pig pig ;
public void catchPig(){ //do something };
public void cleanPig(){ };
public void killPig(){};
}
now iam thing since i know that the steps will be called in catchPig--->cleanPig---->KillPig manner so i should have an abstract class containing these methods and an execute method calling all these 3 methods.
but i can not have instance of abstract class so i am confused how to implement this.
remenber i have to execute this process for all the pigs that comes in truck.
so my question is what design should i select and which design pattern is best to solve such problems .
I would suggest a different approach than what was suggested here before.
I would do something like this:
public abstract class Killer {
protected Pig pig;
protected abstract void catchPig();
protected abstract void cleanPig();
protected abstract void killPig();
public void executeKillPig {
catchPig();
cleanPig();
killPig();
}
}
Each kill will extend Killer class and will have to implement the abstract methods. The executeKillPig() is the same for every sub-class and will always be performed in the order you wanted catch->clean->kill. The abstract methods are protected because they're the inner implementation of the public executeKillPig.
This extends Avi's answer and addresses the comments.
The points of the code:
abstract base class to emphasize IS A relationships
Template pattern to ensure the steps are in the right order
Strategy Pattern - an abstract class is as much a interface (little "i") as much as a Interface (capital "I") is.
Extend the base and not use an interface.
No coupling of concrete classes. Coupling is not an issue of abstract vs interface but rather good design.
public abstract Animal {
public abstract bool Escape(){}
public abstract string SaySomething(){}
}
public Wabbit : Animal {
public override bool Escape() {//wabbit hopping frantically }
public override string SaySomething() { return #"What's Up Doc?"; }
}
public abstract class Killer {
protected Animal food;
protected abstract void Catch(){}
protected abstract void Kill(){}
protected abstract void Clean(){}
protected abstract string Lure(){}
// this method defines the process: the methods and the order of
// those calls. Exactly how to do each individual step is left up to sub classes.
// Even if you define a "PigKiller" interface we need this method
// ** in the base class ** to make sure all Killer's do it right.
// This method is the template (pattern) for subclasses.
protected void FeedTheFamily(Animal somethingTasty) {
food = somethingTasty;
Catch();
Kill();
Clean();
}
}
public class WabbitHunter : Killer {
protected override Catch() { //wabbit catching technique }
protected override Kill() { //wabbit killing technique }
protected override Clean() { //wabbit cleaning technique }
protected override Lure() { return "Come here you wascuhwy wabbit!"; }
}
// client code ********************
public class AHuntingWeWillGo {
Killer hunter;
Animal prey;
public AHuntingWeWillGo (Killer aHunter, Animal aAnimal) {
hunter = aHunter;
prey = aAnimal;
}
public void Hunt() {
if ( !prey.Escape() ) hunter.FeedTheFamily(prey)
}
}
public static void main () {
// look, ma! no coupling. Because we pass in our objects vice
// new them up inside the using classes
Killer ElmerFudd = new WabbitHunter();
Animal BugsBunny = new Wabbit();
AHuntingWeWillGo safari = new AHuntingWeWillGo( ElmerFudd, BugsBunny );
safari.Hunt();
}
The problem you are facing refer to part of OOP called polymorphism
Instead of abstract class i will be using a interface, the difference between interface an abstract class is that interface have only method descriptors, a abstract class can have also method with implementation.
public interface InterfaceOfPigKiller {
void catchPig();
void cleanPig();
void killPig();
}
In the abstract class we implement two of three available methods, because we assume that those operation are common for every future type that will inherit form our class.
public abstract class AbstractPigKiller implements InterfaceOfPigKiller{
private Ping pig;
public void catchPig() {
//the logic of catching pigs.
}
public void cleanPig() {
// the logic of pig cleaning.
}
}
Now we will create two new classes:
AnimalKiller - The person responsible for pig death.
AnimalSaver - The person responsible for pig release.
public class AnimalKiller extends AbstractPigKiller {
public void killPig() {
// The killing operation
}
}
public class AnimalSaver extends AbstractPigKiller {
public void killPing() {
// The operation that will make pig free
}
}
As we have our structure lets see how it will work.
First the method that will execute the sequence:
public void doTheRequiredOperation(InterfaceOfPigKiller killer) {
killer.catchPig();
killer.cleanPig();
killer.killPig();
}
As we see in the parameter we do not use class AnimalKiller or AnimalSever. Instead of that we have the interface. Thank to this operation we can operate on any class that implement used interface.
Example 1:
public void test() {
AnimalKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `AnimalKilleraKiller `
AnimalSaver aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
Example 2:
public void test() {
InterfaceOfPigKiller aKiller = new AnimalKiller();// We create new instance of class AnimalKiller and assign to variable aKiller with is type of `InterfaceOfPigKiller `
InterfaceOfPigKiller aSaver = new AnimalSaver(); //
doTheRequiredOperation(aKiller);
doTheRequiredOperation(aSaver);
}
The code example 1 and 2 are equally in scope of method doTheRequiredOperation. The difference is that in we assign once type to type and in the second we assign type to interface.
Conclusion
We can not create new object of abstract class or interface but we can assign object to interface or class type.
I have a proxy for a lazy entity which has been created in the session by loading a child entity. A subsequent fetch on the parent entity only returns the NH proxy. I need the actual instance to check the type (the entity has joined subclasses). I must be missing something, but I can't find a way to do this. Session.Refresh(proxy) does not appear to help, nor does any flavour of HQL that I've tried.
Can anyone help?
In my opinion, rather then solving this problem, you should rather rethink your design. Are you absolutely sure, that you can't use polymorphism in this situation - either directly make entity responsible for operation you're trying to perform or use visitor pattern. I came across this issue few times and always decided to change design - it resulted in clearer code. I suggest you do the same, unless you're absolutely sure that relying on type is the best solution.
The problem
In order to have example with at least some resemblance to the real world, let's suppose you have following entities:
public abstract class Operation
{
public virtual DateTime PerformedOn { get; set; }
public virtual double Ammount { get; set; }
}
public class OutgoingTransfer : Operation
{
public virtual string TargetAccount { get; set; }
}
public class AtmWithdrawal : Operation
{
public virtual string AtmAddress { get; set; }
}
It'd naturally be a small part of much larger model. And now you're facing a problem: for each concrete type of Operation, there's a different way to display it:
private static void PrintOperation(Operation operation)
{
Console.WriteLine("{0} - {1}", operation.PerformedOn,
operation.Ammount);
}
private static void PrintOperation(OutgoingTransfer operation)
{
Console.WriteLine("{0}: {1}, target account: {2}",
operation.PerformedOn, operation.Ammount,
operation.TargetAccount);
}
private static void PrintOperation(AtmWithdrawal operation)
{
Console.WriteLine("{0}: {1}, atm's address: {2}",
operation.PerformedOn, operation.Ammount,
operation.AtmAddress);
}
Simple, overloaded methods will work in simple case:
var transfer = new OutgoingTransfer
{
Ammount = -1000,
PerformedOn = DateTime.Now.Date,
TargetAccount = "123123123"
};
var withdrawal = new AtmWithdrawal
{
Ammount = -1000,
PerformedOn = DateTime.Now.Date,
AtmAddress = "Some address"
};
// works as intended
PrintOperation(transfer);
PrintOperation(withdrawal);
Unfortunately, overloaded methods are bound at compile time, so as soon as you introduce an array/list/whatever of operations, only a generic (Operation operation) overload will be called.
Operation[] operations = { transfer, withdrawal };
foreach (var operation in operations)
{
PrintOperation(operation);
}
There are two solutions to this problem, and both have downsides. You can introduce an abstract/virtual method in Operation to print information to selected stream. But this will mix UI concerns into your model, so that's not acceptable for you (I'll show you how can you improve this solution to meet your expectations in a moment).
You can also create lots of ifs in form of:
if(operation is (ConcreteType))
PrintOperation((ConcreteType)operation);
This solution is ugly and error prone. Every time you add/change/remove type of operation, you have to go through every place you used these hack and modify it. And if you miss one place, you'll probably only be able to catch that runtime - no strict compile-time checks for some of errors (like missing one subtype).
Furthermore, this solution will fail as soon as you introduce any kind of proxy.
How proxy works
The code below is VERY simple proxy (in this implementation it's same as decorator pattern - but those patterns are not the same in general. It'd take some additional code to distinguish those two patterns).
public class OperationProxy : Operation
{
private readonly Operation m_innerOperation;
public OperationProxy(Operation innerOperation)
{
if (innerOperation == null)
throw new ArgumentNullException("innerOperation");
m_innerOperation = innerOperation;
}
public override double Ammount
{
get { return m_innerOperation.Ammount; }
set { m_innerOperation.Ammount = value; }
}
public override DateTime PerformedOn
{
get { return m_innerOperation.PerformedOn; }
set { m_innerOperation.PerformedOn = value; }
}
}
As you can see - there is only one proxy class for whole hierarchy. Why? Because you should write your code in a way that doesn't depend on concrete type - only on provided abstraction. This proxy could defer entity loading in time - maybe you won't use it at all? Maybe you'll use just 2 out of 1000 entities? Why load them all then?
So NHibernate uses proxy like on above (much more sophisticated, though) to defer entity loading. It could create 1 proxy per sub-type, but it would destroy whole purpose of lazy loading. If you look carefuly at how NHibernate stores subclasses you'll see, that in order to determine what type entity is, you have to load it. So it is impossible to have concrete proxies - you can only have the most abstract, OperationProxy.
Altough the solution with ifs it's ugly - it was a solution. Now, when you introduced proxies to your problem - it's no longer working. So that just leaves us with polymorphic method, which is unacceptable because of mixing UI responsibility to your model. Let's fix that.
Dependency inversion and visitor pattern
First, let's have a look at how the solution with virtual methods would look like (just added code):
public abstract class Operation
{
public abstract void PrintInformation();
}
public class OutgoingTransfer : Operation
{
public override void PrintInformation()
{
Console.WriteLine("{0}: {1}, target account: {2}",
PerformedOn, Ammount, TargetAccount);
}
}
public class AtmWithdrawal : Operation
{
public override void PrintInformation()
{
Console.WriteLine("{0}: {1}, atm's address: {2}",
PerformedOn, Ammount, AtmAddress);
}
}
public class OperationProxy : Operation
{
public override void PrintInformation()
{
m_innerOperation.PrintInformation();
}
}
And now, when you call:
Operation[] operations = { transfer, withdrawal, proxy };
foreach (var operation in operations)
{
operation.PrintInformation();
}
all works as a charm.
In order to remove this UI dependency in model, let's create an interface:
public interface IOperationVisitor
{
void Visit(AtmWithdrawal operation);
void Visit(OutgoingTransfer operation);
}
Let's modify model to depend on this interface:
And now create an implementation - ConsoleOutputOperationVisitor (I have deleted PrintInformation methods):
public abstract class Operation
{
public abstract void Accept(IOperationVisitor visitor);
}
public class OutgoingTransfer : Operation
{
public override void Accept(IOperationVisitor visitor)
{
visitor.Visit(this);
}
}
public class AtmWithdrawal : Operation
{
public override void Accept(IOperationVisitor visitor)
{
visitor.Visit(this);
}
}
public class OperationProxy : Operation
{
public override void Accept(IOperationVisitor visitor)
{
m_innerOperation.Accept(visitor);
}
}
What happens here? When you call Accept on operation and pass a visitor, implementation of accept will be called, where appropriate overload of Visit method will be invoked (compiler can determine type of "this"). So you combine "power" of virtual methods and overloads to get appropriate method called. As you can see - now UI reference here, model only depends on an interface, which can be included in model layer.
So now, to get this working, an implementation of the interface:
public class ConsoleOutputOperationVisitor : IOperationVisitor
{
#region IOperationVisitor Members
public void Visit(AtmWithdrawal operation)
{
Console.WriteLine("{0}: {1}, atm's address: {2}",
operation.PerformedOn, operation.Ammount,
operation.AtmAddress);
}
public void Visit(OutgoingTransfer operation)
{
Console.WriteLine("{0}: {1}, target account: {2}",
operation.PerformedOn, operation.Ammount,
operation.TargetAccount);
}
#endregion
}
And code:
Operation[] operations = { transfer, withdrawal, proxy };
foreach (var operation in operations)
{
operation.Accept(visitor);
}
I'm well aware that this isn't a perfect solution. You'll still have to modify the interface and visitors as you add new types. But you get compile time checking and will never miss anything. One thing that would be really hard to achieve using this method is to get pluggable subtypes - but I'm not convinced this is a valid scenario anyway. You'll also have to modify this pattern to meet your needs in concrete scenario, but I'll leave this to you.
To force a proxy to be fetched from the database, you can use the NHibernateUtil.Initialize(proxy) method, or access a method/property of the proxy.
var foo = session.Get<Foo>(id);
NHibernateUtil.Initialize(foo.Bar);
To check if an object is initialized or not, you can use the NHibernateUtil.IsInitialized(proxy) method.
Update:
To remove an object from the session cache, use the Session.Evict(obj) method.
session.Evict(myEntity);
Info about Evict and other methods for managing the session cache can be found in chapter 14.5 of the NHibernate docs.
Disabling lazy loading will force the actual instance to be returned instead of the NHibernate Proxy.
eg..
mapping.Not.LazyLoad();
or
<class name="OrderLine" table="OrderLine" lazy="false" >
Since the proxy is derived from the entity class, you can probably just check entity.GetType().BaseType to get your defined type.