Suppose that I have a class BaseClass that has two child classes ChildClassA : BaseClass and ChildClassB : BaseClass. For each child class, I want to provide some implementation for two per-class constants, Zero and One, so that the following code would work:
var valueOfZeroA = ChildClassA.Zero;
var valueOfOneB = ChildClassB.Zero;
I should also be able to create a generic class that has a method that selects the appropriate value based on the generic type.
class GenericUsingClass<T> where T : BaseClass
{
public void DoStuff()
{
var valueOfZero = T.Zero;
var valueOfOne = T.One;
}
}
For instance, if I created a new GenericUsingClass<ChildClassA>(), then, the value of valueOfZero would be the same as ChildClassA.Zero.
Here is my problem: I need some programming pattern to implement something like this. I know that the syntax in my code examples will not be possible in the solution; it is just for illustrative purposes. I have provided a possible solution but I find it kind of ugly and am wondering if anyone can provide me with a better solution or explain why mine might be as good as it gets.
So far, I have come up with a factory pattern that seems like it would work decently. It goes as follows:
For each child class of BaseClass, there needs to be a factory class such as ChildClassAFactory and ChildClassBFactory each deriving from a class BaseClassFactory.
Then, BaseClassFactory would be defined as:
abstract class BaseClassFactory<T> where T : BaseClass
{
public abstract T Zero() { ... }
public abstract T One() { ... }
}
and the child factories would implement the methods Zero() and One().
The GenericUsingClass would have to then be refactored into:
class GenericUsingClass<T> where T : BaseClass
{
public void DoStuff()
{
BaseClassFactory<T> factory = new BaseClassFactory<T>();
var valueOfZero = factory.Zero();
var valueOfOne = factory.One();
}
}
or alternatively, using dependency injection:
class GenericUsingClass<T> where T : BaseClass
{
public void DoStuff(BaseClassFactory<T> factory)
{
var valueOfZero = factory.Zero();
var valueOfOne = factory.One();
}
}
I dislike this approach because
It requires twice as many classes to implement the functionality of a single child class although this is my lesser concern.
I have to either create or inject an object factory in my generic using class which seems to add clutter to the code and make the implementation less easy to use.
I also know that reflection would be a possibility but it seems like overkill for this problem and would greatly messy things up. I'm just wondering what the best software architecture idea would be.
Thanks!
I have a class using a group of properties in similar ways (only two shown in the example, for brevity).
The general behavior is defined on a base class, while the specific behavior is defined in specific interfaces
The problem is: If I declare them as base class, I have to cast them to interface to call interface methods. Now if I declare them as interface, I have to cast them to base class when I want to call base methods.
My goal when using interfaces here is to improve testability (with dependency injection, later), and to cultivate the habit of "programming to the interface", but I cannot decide which way is best, or even if the whole rationale is good in the first place.
public class Conductor
{
// These properties inherit from base class
// and implement one specific interface each:
// declared as interface:
IPlotterHelper _plotter_helper = new PlotterHelper();
// declared as base class:
Helper _file_writer_helper = new FileWriterHelper();
// When using handlers defined in specific interfaces:
// have to cast this:
this.NewFrame += ((IPlotterHelper)_file_writer_helper).ProcessFrame();
// but not this:
this.NewSamples += _plotter_helper.ProcessSamples();
// While when using handlers from the base class
// have to cast this to the base class (since it is an interface):
this.CommandSent += ((Helper)_plotter_helper).RunCommand;
// but not this:
this.CommandSent += _file_writer_helper.RunCommand;
}
internal class FileWriterHelper : Helper, IFileWriterHelper
{
IFileWriterHelper.ProcessFrame()
{
// ...
}
// ...
}
internal class PlotterHelper : Helper, IPlotterHelper
{
IPlotterHelper.ProcessSamples ()
{
///
}
// ...
}
internal class Helper
{
internal void RunCommand()
{
// ...
}
}
When I am faced with the desire to have default behavior in an interface, I would generally consider using an abstract base class either with protected helper methods and a set of abstract interface methods or a default implementations of the "interface" methods. This may be the case even if I start with only a single concrete implementation.
Many people treat abstract classes and interfaces as being in the same broad category of implementation options.
The problem with abstract classes is single inheritance, so we should only use an abstract class if it really is to be the base of a class hierarchy (even a shallow one). Interfaces can be used to decorate classes (from diverse hierarchies) with common behavior.
For testing, I don't see much difference between faking with an interface and faking with an abstract class - but that might depend on your testing infrastructure.
In this case, I would use an abstract class and forget about the interface (unless it already exists, in which case you don't have any choice anyway).
It's hard to exactly see what you're trying to do, but it seems like this might be a more suitable design:
public class Conductor
{
private IPlotterHelper _plotter_helper = new PlotterHelper();
private IFileWriterHelper _file_writer_helper = new FileWriterHelper();
public void Conduct()
{
_file_writer_helper.ProcessFrame();
_file_writer_helper.RunCommand();
_plotter_helper.ProcessSamples();
_plotter_helper.RunCommand();
}
}
internal interface IHelper
{
void RunCommand();
}
internal interface IFileWriterHelper : IHelper
{
void ProcessFrame();
}
internal interface IPlotterHelper : IHelper
{
void ProcessSamples();
}
internal class FileWriterHelper : Helper, IFileWriterHelper
{
public void ProcessFrame()
{
}
}
internal class PlotterHelper : Helper, IPlotterHelper
{
public void ProcessSamples()
{
}
}
internal class Helper : IHelper
{
public void RunCommand()
{
}
}
Interfaces and abstract classes have the same purpose: provide abstraction. Make that abstraction coherent, and if the base class has public members, make sure they're also on the interface.
But then, why would I need the abstract class or interface for? - right. get rid of either the base class or the interface - you likely don't really need both. I'd drop the base class.
I am designing a class that uses an abstract property to provide a point of access to a field. Here is a snippet of my code:
public abstract class PageBroker : WebBroker
{
public abstract IPageProvider Provider { get; }
}
public class ArticleBroker : PageBroker
{
public override IPageProvider Provider { get; } = new ArticleProvider();
}
I realized I could refactor this to use DI instead, like so:
public class PageBroker : WebBroker
{
public PageBroker(IPageProvider provider)
{
this.Provider = provider;
}
public IPageProvider Provider { get; private set; }
// implementation...
}
// no derived class, just new PageBroker(new ArticleProvider())
In what situation(s) would one technique be appropriate over the other?
I agree with BJ Safdie's answer.
That said, I think the main difference is that in the first implementation, ArticleBroker is coupled to a specific implementation of IPageProvider (namely ArticleProvider). If this is intended, I see no problem with it. However, if you want to provide an implementation of IPageProvider at runtime (either in production code or in a unit test via mocks/fakes), then injecting the dependency is the better choice.
This is about favoring composition over inheritance, which is a good guideline. I would suggest that since the PageBroker has an IPageProvider, you should use DI. Users of the class can provide alternate implementations without inheritance, say to inject a test IPageProvider. For this reason alone, I would go with DI.
If there is an existential difference based on IPageProvider such that using a different provider semantically needs to reflect an is a relationship (a subtype of PageBroker), then I would go with an abstract class.
As already stated I would populate your Provider with DI. That said what are you looking to gain via the abstract class? If it is simply to inject the IPageProvider then there is really nothing more to say.
If however you have other common methods in mind that are very specific to WebBrokers or PageBrokers then why not use both?
For example:
public abstract class WebBroker
{
private IPageProvider _pageProvider;
protected WebBroker(IPageProvider pageProvider)
{
_pageProvider = pageProvider;
}
public override void CommonThing(int someData)
{
var result = _pageProvider.CommonMethod(someData);
//do something with result
}
}
Now all your WebBrokers have the CommonThing method which uses the _pageProvider that DI passed in.
I am making a payment system for my site. Users can select one of several payment providers to pay, but all should behave in the same way. I thought to represent this behavior like this:
public abstract class PaymentProvider {
private static var methods = Dictionary<String,PaymentProvider>
{
{"paypal",new PaymentProviderPaypal()},
{"worldpay",new PaymentProviderWorldpay()}
}
public static Dictionary<String,PaymentProvider> AllPaymentProviders
{
get {return methods;}
}
public abstract pay();
}
public class PaymentProviderPaypal : PaymentProvider {
public override pay() {
}
}
public class PaymentProviderWorldpay : PaymentProvider {
public override pay() {
}
}
You are supposed to use this by writing PaymentProvider.AllPaymentProviders["key"].pay(). The idea is that the functions using this class don't need to know about how the underlying payment provider is implemented, they just need to know the key.
However, at the moment, if you have access to the PaymentProvider class, you also have access to the inheriting classes. Its possible to instantiate a new copy of the inheriting classes, and make use of them in an unexpected way. I want to encapsulate the inheriting classes so that only the abstract PaymentProvider knows about them.
How should I do this? Different protection levels like protected don't work here - In Java, protected means that only other classes in the namespace can use that class, but in C# it means something else.
Do I have the right idea here? Or should I use a different method?
A couple of options spring to mind:
Put this in a separate assembly from the client code, and make the implementations abstract
Put the implementations inside the PaymentProvider class as private nested classes. You can still separate the source code by making PaymentProvider a partial class - use one source file per implementation
The first option is likely to be the cleanest if you don't mind separating the clients from the implementation in terms of assemblies.
Note that both of these are still valid options after the change proposed by Jamiec's answer - the "visibility" part is somewhat orthogonal to the inheritance part.
(As an aside, I hope the method is really called Pay() rather than pay() :)
Your inheritance heirachy is a bit wonky, I would be tempted to do it a similar but crucially different way.
public interface IPaymentProvider
{
void Pay()
}
// Implementations of IPaymentProvider for PaypalPaymentProvider & WorldpayPaymentProvider
public static class PaymentHelper
{
private static var providers = Dictionary<String,IPaymentProvider>
{
{"paypal",new PaymentProviderPaypal()},
{"worldpay",new PaymentProviderWorldpay()}
}
public static void Pay(string provider)
{
if(!providers.Containskey(provider))
throw new InvalidOperationException("Invalid provider: " + provider);
providers[provider].Pay();
}
}
Then the usage would be something like PaymentHelper.Pay("paypal").
Obviously if there is more data to provide to the Pay method this can be added to both the interface, and the helper. for example:
public interface IPaymentProvider
{
void Pay(double amount);
}
public static void Pay(string provider, double amount)
{
if(!providers.Containskey(provider))
throw new InvalidOperationException("Invalid provider: " + provider);
providers[provider].Pay(amount);
}
I'm still trying to get a better understanding of Interfaces. I know about what they are and how to implement them in classes.
What I don't understand is when you create a variable that is of one of your Interface types:
IMyInterface somevariable;
Why would you do this? I don't understand how IMyInterface can be used like a class...for example to call methods, so:
somevariable.CallSomeMethod();
Why would you use an IMyInterface variable to do this?
You are not creating an instance of the interface - you are creating an instance of something that implements the interface.
The point of the interface is that it guarantees that what ever implements it will provide the methods declared within it.
So now, using your example, you could have:
MyNiftyClass : IMyInterface
{
public void CallSomeMethod()
{
//Do something nifty
}
}
MyOddClass : IMyInterface
{
public void CallSomeMethod()
{
//Do something odd
}
}
And now you have:
IMyInterface nifty = new MyNiftyClass()
IMyInterface odd = new MyOddClass()
Calling the CallSomeMethod method will now do either something nifty or something odd, and this becomes particulary useful when you are passing in using IMyInterface as the type.
public void ThisMethodShowsHowItWorks(IMyInterface someObject)
{
someObject.CallSomeMethod();
}
Now, depending on whether you call the above method with a nifty or an odd class, you get different behaviour.
public void AnotherClass()
{
IMyInterface nifty = new MyNiftyClass()
IMyInterface odd = new MyOddClass()
// Pass in the nifty class to do something nifty
this.ThisMethodShowsHowItWorks(nifty);
// Pass in the odd class to do something odd
this.ThisMethodShowsHowItWorks(odd);
}
EDIT
This addresses what I think your intended question is - Why would you declare a variable to be of an interface type?
That is, why use:
IMyInterface foo = new MyConcreteClass();
in preference to:
MyConcreteClass foo = new MyConcreteClass();
Hopefully it is clear why you would use the interface when declaring a method signature, but that leaves the question about locally scoped variables:
public void AMethod()
{
// Why use this?
IMyInterface foo = new MyConcreteClass();
// Why not use this?
MyConcreteClass bar = new MyConcreteClass();
}
Usually there is no technical reason why the interface is preferred. I usually use the interface because:
I typically inject dependencies so the polymorphism is needed
Using the interface clearly states my intent to only use members of the interface
The one place where you would technically need the interface is where you are utilising the polymorphism, such as creating your variable using a factory or (as I say above) using dependency injection.
Borrowing an example from itowlson, using concrete declaration you could not do this:
public void AMethod(string input)
{
IMyInterface foo;
if (input == "nifty")
{
foo = new MyNiftyClass();
}
else
{
foo = new MyOddClass();
}
foo.CallSomeMethod();
}
Because this:
public void ReadItemsList(List<string> items);
public void ReadItemsArray(string[] items);
can become this:
public void ReadItems(IEnumerable<string> items);
Edit
Think of it like this:
You have to be able to do this.
rather than:
You have to be this.
Essentially this is a contract between the method and it's callers.
Using interface variables is the ONLY way to allow handler methods to be written which can accept data from objects that have different base classes.
This is about as clear as anyone is going to get.
An interface is used so you do not need to worry about what class implements the interface. An example of this being useful is when you have a factory method that returns a concrete implementation that may be different depending on the environment you are running in. It also allows an API designer to define the API while allowing 3rd parties to implement the API in any way they see fit. Sun does this with it's cryptographic API's for Java.
public interface Foo {
}
public class FooFactory {
public static Foo getInstance() {
if(os == 'Windows') return new WinFoo();
else if(os == 'OS X') return new MacFoo();
else return new GenricFoo();
}
}
Your code that uses the factory only needs to know about Foo, not any of the specific implementations.
I was in same position and took me few days to figure out why do we have to use interface variable.
IDepartments rep = new DepartmentsImpl();
why not
DepartmentsImpl rep = new DepartmentsImpl();
Imagine If a class implements two interfaces that contain a member with the same signature, then implementing that member on the class will cause both interfaces to use that member as their implementation.
class Test
{
static void Main()
{
SampleClass sc = new SampleClass();
IControl ctrl = (IControl)sc;
ISurface srfc = (ISurface)sc;
// The following lines all call the same method.
sc.Paint();
ctrl.Paint();
srfc.Paint();
}
}
interface IControl
{
void Paint();
}
interface ISurface
{
void Paint();
}
class SampleClass : IControl, ISurface
{
// Both ISurface.Paint and IControl.Paint call this method.
public void Paint()
{
Console.WriteLine("Paint method in SampleClass");
}
}
// Output:
// Paint method in SampleClass
// Paint method in SampleClass
// Paint method in SampleClass
If the two interface members do not perform the same function, however, this can lead to an incorrect implementation of one or both of the interfaces.
public class SampleClass : IControl, ISurface
{
void IControl.Paint()
{
System.Console.WriteLine("IControl.Paint");
}
void ISurface.Paint()
{
System.Console.WriteLine("ISurface.Paint");
}
}
The class member IControl.Paint is only available through the IControl interface, and ISurface.Paint is only available through ISurface. Both method implementations are separate, and neither is available directly on the class. For example:
IControl c = new SampleClass();
ISurface s = new SampleClass();
s.Paint();
Please do correct me if i am wrong as i am still learning this Interface concept.
Lets say you have class Boat, Car, Truck, Plane.
These all share a common method TakeMeThere(string destination)
You would have an interface:
public interface ITransportation
{
public void TakeMeThere(string destination);
}
then your class:
public class Boat : ITransportation
{
public void TakeMeThere(string destination) // From ITransportation
{
Console.WriteLine("Going to " + destination);
}
}
What you're saying here, is that my class Boat will do everything ITransportation has told me too.
And then when you want to make software for a transport company. You could have a method
Void ProvideServiceForClient(ITransportation transportationMethod, string whereTheyWantToGo)
{
transportationMethod.TakeMeThere(whereTheyWantToGo); // Cause ITransportation has this method
}
So it doesn't matter which type of transportation they want, because we know it can TakeMeThere
This is not specific to C#,so i recommend to move to some othere flag.
for your question,
the main reason why we opt for interface is to provide a protocol between two components(can be a dll,jar or any othere component).
Please refer below
public class TestClass
{
static void Main()
{
IMyInterface ob1, obj2;
ob1 = getIMyInterfaceObj();
obj2 = getIMyInterfaceObj();
Console.WriteLine(ob1.CallSomeMethod());
Console.WriteLine(obj2.CallSomeMethod());
Console.ReadLine();
}
private static bool isfirstTime = true;
private static IMyInterface getIMyInterfaceObj()
{
if (isfirstTime)
{
isfirstTime = false;
return new ImplementingClass1();
}
else
{
return new ImplementingClass2();
}
}
}
public class ImplementingClass1 : IMyInterface
{
public ImplementingClass1()
{
}
#region IMyInterface Members
public bool CallSomeMethod()
{
return true;
}
#endregion
}
public class ImplementingClass2 : IMyInterface
{
public ImplementingClass2()
{
}
#region IMyInterface Members
public bool CallSomeMethod()
{
return false;
}
#endregion
}
public interface IMyInterface
{
bool CallSomeMethod();
}
Here the main method does not know about the classes still it is able to get different behaviour using the interface.
The purpose of the Interface is to define a contract between several objects, independent of specific implementation.
So you would usually use it when you have an Intrace ISomething, and a specific implementation
class Something : ISomething
So the Interface varialbe would come to use when you instantiate a contract:
ISomething myObj = new Something();
myObj.SomeFunc();
You should also read interface C#
Update:
I will explaing the logic of using an Interface for the variable and not the class itself by a (real life) example:
I have a generic repositor interace:
Interface IRepository {
void Create();
void Update();
}
And i have 2 seperate implementations:
class RepositoryFile : interface IRepository {}
class RepositoryDB : interface IRepository {}
Each class has an entirely different internal implementation.
Now i have another object, a Logger, that uses an already instansiated repository to do his writing. This object, doesn't care how the Repository is implemented, so he just implements:
void WriteLog(string Log, IRepository oRep);
BTW, this can also be implemented by using standard classes inheritance. But the difference between using interfaces and classes inheritance is another discussion.
For a slightly more details discussion on the difference between abstract classes and interfaces see here.
Say, for example, you have two classes: Book and Newspaper. You can read each of these, but it wouldn't really make sense for these two to inherit from a common superclass. So they will both implement the IReadable interface:
public interface IReadable
{
public void Read();
}
Now say you're writing an application that will read books and newspapers for the user. The user can select a book or newspaper from a list, and that item will be read to the user.
The method in your application that reads to the user will take this Book or Newspaper as a parameter. This might look like this in code:
public static void ReadItem(IReadable item)
{
item.Read();
}
Since the parameter is an IReadable, we know that the object has the method Read(), thus we call it to read it to the user. It doesn't matter whether this is a Book, Newspaper, or anything else that implements IReadable. The individual classes implement exactly how each item will be read by implementing the Read() method, since it will most likely be different for the different classes.
Book's Read() might look like this:
public void Read()
{
this.Open();
this.TurnToPage(1);
while(!this.AtLastPage)
{
ReadText(this.CurrentPage.Text);
this.TurnPage();
}
this.Close();
}
Newspaper's Read() would likely be a little different:
public void Read()
{
while(!this.OnBackPage)
{
foreach(Article article in this.CurrentPage.Articles)
{
ReadText(article.Text);
}
}
}
The point is that the object contained by a variable that is an interface type is guaranteed to have a specific set of methods on it, even if the possible classes of the object are not related in any other way. This allows you to write code that will apply to a variety of classes that have common operations that can be performed on them.
No, it is not possible. Designers did not provide a way. Of course, it is of common sense also. Because interface contains only abstract methods and as abstract methods do not have a body (of implementation code), we cannot create an object..
Suppose even if it is permitted, what is the use. Calling the abstract method with object does not yield any purpose as no output. No functionality to abstract methods.
Then, what is the use of interfaces in Java design and coding. They can be used as prototypes from which you can develop new classes easily. They work like templates for other classes that implement interface just like a blue print to construct a building.
I believe everyone is answering the polymorphic reason for using an interface and David Hall touches on partially why you would reference it as an interface instead of the actual object name. Of course, being limited to the interface members etc is helpful but the another answer is dependency injection / instantiation.
When you engineer your application it is typically cleaner, easier to manage, and more flexible if you do so utilizing dependency injection. It feels backwards at first if you've never done it but when you start backtracking you'll wish you had.
Dependency injection normally works by allowing a class to instantiate and control the dependencies and you just rely on the interface of the object you need.
Example:
Layer the application first. Tier 1 logic, tier 2 interface, tier 3 dependency injection. (Everyone has their own way, this is just for show).
In the logic layer you reference the interfaces and dependency layer and then finally you create logic based on only the interfaces of foreign objects.
Here we go:
public IEmployee GetEmployee(string id)
{
IEmployee emp = di.GetInstance<List<IEmployee>>().Where(e => e.Id == id).FirstOrDefault();
emp?.LastAccessTimeStamp = DateTime.Now;
return emp;
}
Notice above how we use di.GetInstance to get an object from our dependency. Our code in that tier will never know or care about the Employee object. In fact if it changes in other code it will never affect us here. If the interface of IEmployee changes then we may need to make code changes.
The point is, IEmployee emp = never really knows what the actual object is but does know the interface and how to work with it. With that in mind, this is when you want to use an interface as opposed to an object becase we never know or have access to the object.
This is summarized.. Hopefully it helps.
This is a fundamental concept in object-oriented programming -- polymorphism. (wikipedia)
The short answer is that by using the interface in Class A, you can give Class A any implementation of IMyInterface.
This is also a form of loose coupling (wikipedia) -- where you have many classes, but they do not rely explicitly on one another -- only on an abstract notion of the set of properties and methods that they provide (the interface).