Given an interface like:
public interface IFoo
{
string Bar();
}
And a class that implements it like:
public class Foo : IFoo
{
public string Bar()
{
returns "Bar";
}
public string SomeOtherMethod()
{
returns "Something else";
}
}
Is there a problem with this code? Should we add all methods to the interface?
One example: imagine a private method that grows complex enough to need unit tests. Could you make it public (so it can be called from the test project) but not add it to the interface (because no clients need to call it)?
If what a class does is simple enough that we can test its public methods and private methods at the same time, then we can just test the public methods.
If the private method grows so complex that between the public and private method we need too many combinations of tests then it's time to separate the private method into its own class. Making the private method public would break the encapsulation of the class. Even if we don't add the method to an interface, making the class method public still adds the method to the public interface of the class itself.
So if we have this:
public class ClassWithComplexPrivateMethod
{
public void DoSomething(int value)
{
PrivateMethodThatNeedsItsOwnTests(value);
}
private string PrivateMethodThatNeedsItsOwnTests(int value)
{
// This method has gotten really complicated!
return value.ToString();
}
}
We might refactor to something like this:
public interface IRepresentsWhatThePrivateMethodDid
{
string MethodThatNeedsItsOwnTests(int value);
}
public class RefactoredClass
{
private readonly IRepresentsWhatThePrivateMethodDid _dependency;
public RefactoredClass(IRepresentsWhatThePrivateMethodDid dependency)
{
_dependency = dependency;
}
public string DoSomething(int value)
{
return _dependency.MethodThatNeedsItsOwnTests(value);
}
}
And now a new class implements IRepresentsWhatThePrivateMethodDid.
Now when we test the refactored class we mock IRepresentsWhatThePrivateMethodDid, and we write separate unit tests for any classes that implement IRepresentsWhatThePrivateMethodDid.
It may seem like a contradiction to say that exposing the private method as public breaks encapsulation, but exposing it as its own separate class doesn't. There are two differences:
The refactored class doesn't depend on the new class containing what used to be the private method. It depends on the new interface.
The interaction between the refactored class and the interface is still hidden within its methods. Other classes that call its public methods don't "know" about how it uses its dependency. (In fact, other classes will likely depend on abstractions instead of directly on the refactored class.)
It's also easy to get carried away with this and introduce the separate dependency too soon, when we could have tested the class - including its private methods - through the public methods. I've done this lots of times, and it can result in lots and lots of unnecessary interfaces and extra classes. There's no perfection, just our best efforts to balance it.
One more thought: We tend to use interfaces to represent dependencies, but we don't have to. If what we're extracting was just a single private method, then perhaps we can represent it with a delegate or a Func instead, like this:
public class RefactoredClass
{
private readonly Func<int, string> _dependency;
public RefactoredClass(Func<int, string> dependency)
{
_dependency = dependency;
}
public string DoSomething(int value)
{
return _dependency(value);
}
}
Or we can use a delegate, which I like better than Func because it indicates what the function does.
Generally when a class implements an interface all members of an interface must be implemented, it does not go the other way around.
Also when implementing an interface member, the corresponding member of the implementing class must be public, non-static, and have the same name and parameter signature as the interface member. You can have methods, even public, in the class that are not defined in the interface.
On top of that, the interface itself provides no functionality that a class or struct can inherit in the way that it can inherit base class functionality. However, if a base class implements an interface, any class that's derived from the base class inherits that implementation.
Interface should represent just the required API without regard to unit-testing and smth else.
To test public-methods of class you should do nothing because Test-project has access to them. You just need to instantiate a class and inject all required dependency using Mock.
Example: testing class without any dependencies
Example: testing class with dependency
To test private methods of a class need to make them visible to test-project:
mark private methods as internal
make internal-method visible to test-project: add to AssemblyInfo.cs the directive [assembly: InternalsVisibleTo("Project.Tests")] (see InternalsVisibleToAttribute)
Related
I have run into this situation many times, and have 'hacked' my way around the situation, but it seems it's a gap in my understanding.
Given the following code:
public class Foo
{
Bar barA = new BarA();
private void Bat() => barA.Baz();
}
public abstract class Bar
{
// Obviously this *can't* be private
private abstract Baz();
}
public class BarA : Bar
{
public void Run() => Baz();
// Again this can't be private
private override Baz() => DoSomething();
}
(caveat: I used expression bodied methods for sake of brevity in the example. Normally I would not unless there was a good reason to.)
Now, in the above example, my logic is that the Baz method needs to be defined by the concrete class, however the reason I want it private (and yes, I understand WHY it can't be, because it would be completely invisible/inaccessible to the concrete class), is logically, I do not want the concrete class to be able to call the method, merely define it.
The way to fix the code is to make the method protected, however, then the method becomes available to executable from within the concrete class.
Ultimately, I want an abstract/base class that handles internals of executing some code that is defined by the concrete class, however never available for the concrete class itself to call/execute. What is the flaw with that logic, or how should I be implementing that logic properly.
** EDIT: A concrete implementation **
Let's say for example this is part of a plugin system within a library (that I am creating, and another developer is implementing). Another part of the library uses derived classes that implement this abstract class (say it's a filter in image processing). The developer define's the method on what the filter does, however the abstract class (essentially handles the only processing that the defined filter provides), and yes the contract (interface) is made for the image processor so it doesn't (need to) know/care about implementation anyhow.
Like I said in the comments, what you're trying to do is a bit of a code smell. If your derived class needs to define how something in the base class works, then consider passing in an interface to the base. That way you're note exposing any public methods you don't need to and making things much more testable. You could do it something like this.
The interface and an implementation:
public interface IBaz
{
void Baz();
}
public class BigBaz : IBaz
{
public void Baz() => Console.WriteLine("Big Baz!");
}
Now your code slightly modified:
public abstract class Bar
{
private readonly IBaz _baz;
public Bar(IBaz baz)
{
_baz = baz;
}
public void DoBaz() => _baz.Baz();
}
public class BarA : Bar
{
//Here I'm passing into the constructor, but you may find it preferable
//to pass the IBaz directly as a parameter of the DoBaz method
public BarA() : base(new BigBaz())
{
}
}
I have a factory object ChallengeManager to generate instances of a Challenge object for a game I'm building. There are many challenges. The constructors for each Challenge class derivation are different, however there is a common interface among them, defined in the base class.
When I call manager.CreateChallenge(), it returns an instance of Challenge, which is one of the derived types.
Ideally, I would like to keep the code for the object construction inside the derived class itself, so all the code related to that object is co-located. Example:
class Challenge {}
class ChallengeA : Challenge {
public static Challenge MakeChallenge() {
return new ChallengeA();
}
}
class ChallengeB : Challenge {
public static Challenge MakeChallenge() {
return new ChallengeB();
}
}
Now, my ChallengeManager.CreateChallenge() call only needs to decide the class to call MakeChallenge() on. The implementation of the construction is contained by the class itself.
Using this paradigm, every derived class must define a static MakeChallenge() method. However, since the method is a static one, I am not able to make use of an Interface here, requiring it.
It's not a big deal, since I can easily remember to add the correct method signature to each derived class. However, I am wondering if there is a more elegant design I should consider.
I really like the pattern you are describing and use it often. The way I like to do it is:
abstract class Challenge
{
private Challenge() {}
private class ChallengeA : Challenge
{
public ChallengeA() { ... }
}
private class ChallengeB : Challenge
{
public ChallengeB() { ... }
}
public static Challenge MakeA()
{
return new ChallengeA();
}
public static Challenge MakeB()
{
return new ChallengeB();
}
}
This pattern has many nice properties. No one can make a new Challenge because it is abstract. No one can make a derived class because Challenge's default ctor is private. No one can get at ChallengeA or ChallengeB because they are private. You define the interface to Challenge and that is the only interface that the client needs to understand.
When the client wants an A, they ask Challenge for one, and they get it. They don't need to worry about the fact that behind the scenes, A is implemented by ChallengeA. They just get a Challenge that they can use.
You're "decentralizing" the factory, such that each subclass is responsible for creating itself.
More commonly you would have a central factory that would know about the possible subtypes and how to construct them (often enough, simply by creating a new instance and returning that instance typed as a common interface or common base class). That approach avoids the issue you currently have. I also see no benefit to your current approach. You are currently gaining no encapsulation or code reuse over the more typical implementation of a factory.
For additional reference, have a look at
http://www.oodesign.com/factory-pattern.html
Not necessarily the answer you are looking for but...
You can use following implementation, if you can move away from static method per class.
using System;
public class Test
{
public static void Main()
{
var c1 = ChallengeManager.CreateChallenge();
var c2 = ChallengeManager.CreateChallenge();
//var c = ChallengeManager.CreateChallenge<Challenage>(); // This statement won't compile
}
}
public class ChallengeManager
{
public static Challenage CreateChallenge()
{
// identify which challenge to instantiate. e.g. Challenage1
var c = CreateChallenge<Challenage1>();
return c;
}
private static Challenage CreateChallenge<T>() where T: Challenage, new()
{
return new T();
}
}
public abstract class Challenage{}
public class Challenage1: Challenage{}
public class Challenage2: Challenage{}
Is this OOP approach doomed to fail or is there some merit in this?
Before I understood abstract classes I was getting more or less the same benefits of code reuse by using an interface class + a regular class that implements certain methods of the interface. For example
public interface IMyService
{
String Helloword1();
String Helloword2();
String Helloword3();
}
public class MyService
{
public String Helloword1(){return "1";}
public String Helloword2(){return "2";}
//Helloworld3 is not here so I would be forced to provide implementation in any subclass
//very similar to calling abstract on a method
}
public class SubClass1: MyService, IMyService
{
public String Helloword3(){return "3";}
}
public class SubClass2: MyService, IMyService
{
public new String Helloword2(){return "override method";}
public String Helloword3(){return "3";}
}
Can anyone see of any advantage of doing this or is this really providing the same advantages as an abstract class?
Can anyone see of any advantage of doing this or is this really providing the same advantages as an abstract class?
There is a big disadvantage to doing this. You allow people to subclass MyService without implementing Helloword3, since that's not part of the contract.
An abstract class would enforce that the type implements that member. Here, you trust that the user will implement the "required" interface.
In your example someone can actually construct an instance of MyService and use it without the use of the abstract method.
Honestly, there isn't even really any need to inherit form MyService in your two sub classes. If MyService can be created concretely, that type can be composed, rather than inherited from.
The real value in an abstract class is where the concrete methods actually use the abstract methods. Being able to write things like this:
public abstract class Foo
{
public abstract Guid CreateUniqueIdentifier();
public void SaveToDatabase()
{
Guid guid = CreateUniqueIdentifier();
//do stuff with guid
}
}
You cannot do something like this using your pattern; the concrete methods can never be dependent on the not-yet-implemented methods.
I have need to use one of two custom file readers classes; one to read a fixed width file and one for a CSV file. Each of these readers will have certain properties, etc. I want to use factory methods and have private constructors so I can run some business logic before creating the objects.
EDIT: better examples
//simple class with it's own factory method
class Class1
{
private Class1()
{
//constructor code
}
public static Class1 CreateClass()
{
//do some business logic here
return new Class1();
}
}
What I want to be able to do is define a base class, then override the factory. I guess the problem is that a static class belongs to the base CLASS, so can never be overriden, even though they ARE inherited. This code works
public class BaseClass
{
//some common properties / fields here
public string SomeField;
//some common methods here
//empty constructor
protected BaseClass() { }
//cannot have a virtual static class!
//Would really like to make this a virtual method
public static BaseClass CreateClass()
{
throw new NotImplementedException("BaseClass is meant to be derived");
}
public static string DoCommonStaticThing(){
return "I don't know why you'd ever do this";
}
}
public class DerivedClass1 : BaseClass
{
//private constructor
private DerivedClass1() {}
//concrete factory method
//would really like to say "override" here
public static BaseClass CreateClass()
{
DerivedClass1 d1 = new DerivedClass1();
d1.SomeField = "I'm a derived class\r\n" + DoCommonStaticThing();
return d1;
}
}
EDIT: To clarify further, what I'm trying to do is put some common functionality in my base class, but define an interface for my file-format-specific methods. Some of the methods are common, but the business logic for the constructor(s) is file format specific. My code above works, but it seems to me it would be better to mark the base class factory method as virtual, and the derived class factory method as "override".
I tried to do this, but got "A static member cannot be marked as override, virtual, or abstract".
What's the right way to achieve my goals?
First, explaining your specific error message: you cannot inherit static members because they belong to the type being defined, not the instance of the type. Inheritance modifiers such as override, virtual, and abstract do not apply to static members.
Second:
Typically when you follow a factory pattern, you have a factory class whose job is to instantiate concrete classes and return those instances cast as a base class or interface. Details vary as to how the factory chooses which concrete class to instantiate, and I won't get into that, but at the fundamental level, that's what a factory does.
So in order to create a factory pattern using the example you provided, you'll need at least four types, which, following your example, could probably be named ReaderBase, ReaderFactory, CsvReader, and FixedWidthReader. Rather than ReaderBase, you might consider IReader -- the choice depends on whether your abstract class pre-implements any functionality that is shared across all Readers.
CsvReader and FixedWidthReader inherit from either IReader or ReaderBase, and ReaderFactory has at least one method called, for example, InstantiateReader, which returns an IReader or ReaderBase. InstantiateReader does the work of determining whether to instantiate a CsvReader or a FixedWidthReader, based on some external criteria.
We define interface as below:
interface IMyInterface
{
void MethodToImplement();
}
And impliments as below:
class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
instead of creating a interface , why can we use the function directly like below :-)
class InterfaceImplementer
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
Any thoughts?
You are not implementing the interface in the bottom example, you are simply creating an object of InterfaceImplementer
EDIT: In this example an interface is not needed. However, they are extremely useful when trying to write loosely coupled code where you don't have to depend on concrete objects. They are also used to define contracts where anything implementing them has to also implement each method that it defines.
There is lots of information out there, here is just a brief intro http://www.csharp-station.com/Tutorials/Lesson13.aspx
If you really want to understand more about interfaces and how they can help to write good code, I would recommend the Head First Design Patterns book. Amazon Link
instead of creating a interface , why
can we use the function directly like
below
Are you asking what the point of the interface is?
Creating an interface allows you to decouple your program from a specific class, and instead code against an abstraction.
When your class is coded against an interface, classes that use your class can inject whichever class they want that implements this interface. This facilitates unit testing since not-easily-testable modules can be substituted with mocks and stubs.
The purpose of the interface is for some other class to be able to use the type without knowing the specific implementation, so long as that type conforms to a set of methods and properties defined in the interface contract.
public class SomeOtherClass
{
public void DoSomething(IMyInterface something)
{
something.MethodToImplement();
}
}
public class Program
{
public static void Main(string[] args)
{
if(args != null)
new SomeOtherClass().DoSomething(new ImplementationOne());
else
new SomeOtherClass().DoSomething(new ImplementationTwo());
}
}
Your example doesn't really follow that pattern, however; if one that one class implements the interface, then there really isn't much of a point. You can call it either way; it just depends on what kind of object hierarchy you have and what you intend to do for us to say whether using an interface is a good choice or not.
To sum: Both snippets you provide are valid code options. We'd need context to determine which is a 'better' solution.
Interfaces are not required, there is nothing wrong with the last section of code you posted. It is simply a class and you call one of it's public methods. It has no knowledge that an interface exists that this class happens to satisfy.
However, there are advantages:
Multiple Inheritance - A class can only extend one parent class, but can implement any number of interfaces.
Freedom of class use - If your code is written so that it only cares that it has an instance of SomethingI, you are not tied to a specific Something class. If tomorrow you decide that your method should return a class that works differently, it can return SomethingA and any calling code will not need to be changed.
The purpose of interfaces isn't found in instantiating objects, but in referencing them. Consider if your example is changed to this:
static void Main()
{
IMyInterface iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
Now the iTmp object is of the type IMyInterface. Its specific implementation is InterfaceImplementer, but there may be times where the implementation is unimportant (or unwanted). Consider something like this:
interface IVehicle
{
void MoveForward();
}
class Car : IVehicle
{
public void MoveForward()
{
ApplyGasPedal();
}
private void ApplyGasPedal()
{
// some stuff
}
}
class Bike : IVehicle
{
public void MoveForward()
{
CrankPedals();
}
private void CrankPedals()
{
// some stuff
}
}
Now say you have a method like this somewhere:
void DoSomething(IVehicle)
{
IVehicle.MoveForward();
}
The purpose of the interface becomes more clear here. You can pass any implementation of IVehicle to that method. The implementation doesn't matter, only that it can be referenced by the interface. Otherwise, you'd need a DoSomething() method for each possible implementation, which can get messy fast.
Interfaces make it possible for an object to work with a variety of objects that have no common base type but have certain common abilities. If a number of classes implement IDoSomething, a method can accept a parameter of type IDoSomething, and an object of any of those classes can be passed to it. The method can then use all of the methods and properties applicable to an IDoSomething without having to worry about the actual underlying type of the object.
The point of the interface is to define a contract that your implementing class abides by.
This allows you to program to a specification rather than an implementation.
Imagine we have the following:
public class Dog
{
public string Speak()
{
return "woof!";
}
}
And want to see what he says:
public string MakeSomeNoise(Dog dog)
{
return dog.Speak();
}
We really don't benefit from the Interface, however if we also wanted to be able to see what kind of noise a Cat makes, we would need another MakeSomeNoise() overload that could accept a Cat, however with an interface we can have the following:
public interface IAnimal
{
public string Speak();
}
public class Dog : IAnimal
{
public string Speak()
{
return "woof!";
}
}
public class Cat : IAnimal
{
public string Speak()
{
return "meow!";
}
}
And run them both through:
public string MakeSomeNoise(IAnimal animal)
{
return animal.Speak();
}