I know that an abstract class is a special kind of class that cannot be instantiated. An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but, it cannot be instantiated. The advantage is that it can enforce certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.
Also I know that An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn’t support multiple inheritance, interfaces are used to implement multiple inheritance.
When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one from inheritance hierarchy and one from the interface.
When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract. If all the methods of an abstract class are uncompleted then it is same as an interface.
BUT
BUT
BUT
I noticed that we will have Default Interface Methods in C# 8.0
Maybe I'm asking it because I have only 1-2 years of experience in programming, but what would be main difference between abstract class and interface now?
I know that we can't make state in interface, will it be only one difference between them?
Conceptual
First of all, there is a conceptual difference between a class and an interface.
A class should describe an "is a" relationship. E.g. a Ferrari is a Car
An interface should describe a contract of a type. E.g. A Car has a steering wheel.
Currently abstract classes are sometimes used for code reuse, even when there is no "is a" relationship. This pollutes the OO design. E.g. FerrariClass inherits from CarWithSteeringWheel
Benefits
So from above, you could reuse code without introducing a (conceptually wrong) abstract class.
You could inherit from multiple interfaces, while an abstract class is only single inheritance
There is co- and contravariance on interfaces and not on classes in C#
It's easier to implement an interface because some methods have default implementations. This could save a lot of work for an implementer of the interface, but the user won't see the difference :)
But most important for me (as I'm a library maintainer), you could add new methods to an interface without making a breaking change! Before C# 8, if an interface was publicly published, it should be fixed. Because changing the interface could break a lot.
The logger interface
This example shows some of the benefits.
You could describe a (oversimplified) logger interface as follows:
interface ILogger
{
void LogWarning(string message);
void LogError(string message);
void Log(LogLevel level, string message);
}
Then a user of that interface could log easily as warning and error using LogWarning and LogError. But the downside is that an implementer must implement all the methods.
An better interface with defaults would be:
interface ILogger
{
void LogWarning(string message) => Log(LogLevel.Warning, message);
void LogError(string message) => Log(LogLevel.Error, message);
void Log(LogLevel level, string message);
}
Now a user could still use all the methods, but the implementer only needs to implement Log. Also, he could implement LogWarning and LogError.
Also, in the future you might like to add the logLevel "Catastrophic". Before C#8 you could not add the method LogCatastrophic to ILogger without breaking all current implementations.
There is not a lot of difference between the two apart from the obvious fact that abstract classes can have state and interfaces cannot. Default methods or also known as virtual extension methods have actually been available in Java for a while. The main drive for default methods is interface evolution which means being able to add methods to an interface in future versions without breaking source or binary compatibility with existing implementations of that interface.
another couple of good points mentioned by this post:
The feature enables C# to interoperate with APIs targeting Android
(Java) and iOs (Swift), which support similar features.
As it turns out, adding default interface implementations provides
the elements of the "traits" language feature
(https://en.wikipedia.org/wiki/Trait_(computer_programming)). Traits
have proven to be a powerful programming technique
(http://scg.unibe.ch/archive/papers/Scha03aTraits.pdf).
Another thing which still makes the interface unique is covariance / contravariance.
To be honest, never found myself in situation where a default impl. in interface was the solution. I am a bit sceptical about it.
Both abstract classes and the new default interface methods have their appropriate uses.
A. Reasons
Default interface methods have not been introduced to substitute abstract classes.
What's new in C# 8.0 states:
This language feature enables API authors to add methods to an interface in later versions without breaking source or binary compatibility with existing implementations of that interface. Existing implementations inherit the default implementation.
This feature also enables C# to interoperate with APIs that target Android or Swift, which support similar features. Default interface methods also enable scenarios similar to a "traits" language feature.
B. Functional differences
There are still significant differences between an abstract class and an interface (even with default methods).
Here are a few things that an interface still cannot have/do while an abstract class can:
have a constructor,
keep state,
inherit from non abstract class,
have private methods.
C. Design
While default interface methods make interfaces even more powerful, abstract/base classes and interfaces still represent fundamentally different relationships.
(From When should I choose inheritance over an interface when designing C# class libraries?)
Inheritance describes an is-a relationship.
Implementing an interface describes a can-do relationship.
The only main difference coming to my mind is that you can still overload the default constructor for abstract classes which interfaces will never have.
abstract class LivingEntity
{
public int Health
{
get;
protected set;
}
protected LivingEntity(int health)
{
this.Health = health;
}
}
class Person : LivingEntity
{
public Person() : base(100)
{ }
}
class Dog : LivingEntity
{
public Dog() : base(50)
{ }
}
Two main differences:
Abstract classes can have state, but interfaces cannot.
A type can derive from a single abstract class, but can implement multiple interfaces.
There are some other, smaller, differences when it comes to default modifiers.
When should I use an interface and when should I use a base class?
Should it always be an interface if I don't want to actually define a base implementation of the methods?
If I have a Dog and Cat class. Why would I want to implement IPet instead of PetBase? I can understand having interfaces for ISheds or IBarks (IMakesNoise?), because those can be placed on a pet by pet basis, but I don't understand which to use for a generic Pet.
Let's take your example of a Dog and a Cat class, and let's illustrate using C#:
Both a dog and a cat are animals, specifically, quadruped mammals (animals are waaay too general). Let us assume that you have an abstract class Mammal, for both of them:
public abstract class Mammal
This base class will probably have default methods such as:
Feed
Mate
All of which are behavior that have more or less the same implementation between either species. To define this you will have:
public class Dog : Mammal
public class Cat : Mammal
Now let's suppose there are other mammals, which we will usually see in a zoo:
public class Giraffe : Mammal
public class Rhinoceros : Mammal
public class Hippopotamus : Mammal
This will still be valid because at the core of the functionality Feed() and Mate() will still be the same.
However, giraffes, rhinoceros, and hippos are not exactly animals that you can make pets out of. That's where an interface will be useful:
public interface IPettable
{
IList<Trick> Tricks{get; set;}
void Bathe();
void Train(Trick t);
}
The implementation for the above contract will not be the same between a cat and dog; putting their implementations in an abstract class to inherit will be a bad idea.
Your Dog and Cat definitions should now look like:
public class Dog : Mammal, IPettable
public class Cat : Mammal, IPettable
Theoretically you can override them from a higher base class, but essentially an interface allows you to add on only the things you need into a class without the need for inheritance.
Consequently, because you can usually only inherit from one abstract class (in most statically typed OO languages that is... exceptions include C++) but be able to implement multiple interfaces, it allows you to construct objects in a strictly as required basis.
Well, Josh Bloch said himself in Effective Java 2d:
Prefer interfaces over abstract classes
Some main points:
Existing classes can be easily retrofitted to implement a new
interface. All you have to do is add
the required methods if they don’t yet
exist and add an implements clause to
the class declaration.
Interfaces are ideal for defining mixins. Loosely speaking, a
mixin is a type that a class can
implement in addition to its “primary
type” to declare that it provides
some optional behavior. For example,
Comparable is a mixin interface that
allows a class to declare that its
instances are ordered with respect to
other mutually comparable objects.
Interfaces allow the construction of nonhierarchical type
frameworks. Type hierarchies are
great for organizing some things, but
other things don’t fall neatly into a
rigid hierarchy.
Interfaces enable safe, powerful functionality enhancements via the
wrap- per class idiom. If you use
abstract classes to define types, you
leave the programmer who wants to add
functionality with no alternative but
to use inheritance.
Moreover, you can combine the virtues
of interfaces and abstract classes by
providing an abstract skeletal
implementation class to go with each
nontrivial interface that you export.
On the other hand, interfaces are very hard to evolve. If you add a method to an interface it'll break all of it's implementations.
PS.: Buy the book. It's a lot more detailed.
Interfaces and base classes represent two different forms of relationships.
Inheritance (base classes) represent an "is-a" relationship. E.g. a dog or a cat "is-a" pet. This relationship always represents the (single) purpose of the class (in conjunction with the "single responsibility principle").
Interfaces, on the other hand, represent additional features of a class. I'd call it an "is" relationship, like in "Foo is disposable", hence the IDisposable interface in C#.
Modern style is to define IPet and PetBase.
The advantage of the interface is that other code can use it without any ties whatsoever to other executable code. Completely "clean." Also interfaces can be mixed.
But base classes are useful for simple implementations and common utilities. So provide an abstract base class as well to save time and code.
Interfaces
Most languages allow you to implement multiple interfaces
Modifying an interface is a breaking change. All implementations need to be recompiled/modified.
All members are public. Implementations have to implement all members.
Interfaces help in Decoupling. You can use mock frameworks to mock out anything behind an interface
Interfaces normally indicate a kind of behavior
Interface implementations are decoupled / isolated from each other
Base classes
Allows you to add some default implementation that you get for free by derivation (From C# 8.0 by interface you can have default implementation)
Except C++, you can only derive from one class. Even if could from multiple classes, it is usually a bad idea.
Changing the base class is relatively easy. Derivations do not need to do anything special
Base classes can declare protected and public functions that can be accessed by derivations
Abstract Base classes can't be mocked easily like interfaces
Base classes normally indicate type hierarchy (IS A)
Class derivations may come to depend on some base behavior (have intricate knowledge of parent implementation). Things can be messy if you make a change to the base implementation for one guy and break the others.
In general, you should favor interfaces over abstract classes. One reason to use an abstract class is if you have common implementation among concrete classes. Of course, you should still declare an interface (IPet) and have an abstract class (PetBase) implement that interface.Using small, distinct interfaces, you can use multiples to further improve flexibility. Interfaces allow the maximum amount of flexibility and portability of types across boundaries. When passing references across boundaries, always pass the interface and not the concrete type. This allows the receiving end to determine concrete implementation and provides maximum flexibility. This is absolutely true when programming in a TDD/BDD fashion.
The Gang of Four stated in their book "Because inheritance exposes a subclass to details of its parent's implementation, it's often said that 'inheritance breaks encapsulation". I believe this to be true.
This is pretty .NET specific, but the Framework Design Guidelines book argues that in general classes give more flexibility in an evolving framework. Once an interface is shipped, you don't get the chance to change it without breaking code that used that interface. With a class however, you can modify it and not break code that links to it. As long you make the right modifications, which includes adding new functionality, you will be able to extend and evolve your code.
Krzysztof Cwalina says on page 81:
Over the course of the three versions of the .NET Framework, I have talked about this guideline with quite a few developers on our team. Many of them, including those who initially disagreed with the guidelines, have said that they regret having shipped some API as an interface. I have not heard of even one case in which somebody regretted that they shipped a class.
That being said there certainly is a place for interfaces. As a general guideline always provide an abstract base class implementation of an interface if for nothing else as an example of a way to implement the interface. In the best case that base class will save a lot of work.
Juan,
I like to think of interfaces as a way to characterize a class. A particular dog breed class, say a YorkshireTerrier, may be a descended of the parent dog class, but it is also implements IFurry, IStubby, and IYippieDog. So the class defines what the class is but the interface tells us things about it.
The advantage of this is it allows me to, for example, gather all the IYippieDog's and throw them into my Ocean collection. So now I can reach across a particular set of objects and find ones that meet the criteria I am looking at without inspecting the class too closely.
I find that interfaces really should define a sub-set of the public behavior of a class. If it defines all the public behavior for all the classes that implement then it usually does not need to exist. They do not tell me anything useful.
This thought though goes counter to the idea that every class should have an interface and you should code to the interface. That's fine, but you end up with a lot of one to one interfaces to classes and it makes things confusing. I understand that the idea is it does not really cost anything to do and now you can swap things in and out with ease. However, I find that I rarely do that. Most of the time I am just modifying the existing class in place and have the exact same issues I always did if the public interface of that class needs changing, except I now have to change it in two places.
So if you think like me you would definitely say that Cat and Dog are IPettable. It is a characterization that matches them both.
The other piece of this though is should they have the same base class? The question is do they need to be broadly treated as the same thing. Certainly they are both Animals, but does that fit how we are going to use them together.
Say I want to gather all Animal classes and put them in my Ark container.
Or do they need to be Mammals? Perhaps we need some kind of cross animal milking factory?
Do they even need to be linked together at all? Is it enough to just know they are both IPettable?
I often feel the desire to derive a whole class hierarchy when I really just need one class. I do it in anticipation someday I might need it and usually I never do. Even when I do, I usually find I have to do a lot to fix it. That’s because the first class I am creating is not the Dog, I am not that lucky, it is instead the Platypus. Now my entire class hierarchy is based on the bizarre case and I have a lot of wasted code.
You might also find at some point that not all Cats are IPettable (like that hairless one). Now you can move that Interface to all the derivative classes that fit. You will find that a much less breaking change that all of a sudden Cats are no longer derived from PettableBase.
Here is the basic and simple definiton of interface and base class:
Base class = object inheritance.
Interface = functional inheritance.
cheers
It is explained well in this Java World article.
Personally, I tend to use interfaces to define interfaces - i.e. parts of the system design that specify how something should be accessed.
It's not uncommon that I will have a class implementing one or more interfaces.
Abstract classes I use as a basis for something else.
The following is an extract from the above mentioned article JavaWorld.com article, author Tony Sintes, 04/20/01
Interface vs. abstract class
Choosing interfaces and abstract classes is not an either/or proposition. If you need to change your design, make it an interface. However, you may have abstract classes that provide some default behavior. Abstract classes are excellent candidates inside of application frameworks.
Abstract classes let you define some behaviors; they force your subclasses to provide others. For example, if you have an application framework, an abstract class may provide default services such as event and message handling. Those services allow your application to plug in to your application framework. However, there is some application-specific functionality that only your application can perform. Such functionality might include startup and shutdown tasks, which are often application-dependent. So instead of trying to define that behavior itself, the abstract base class can declare abstract shutdown and startup methods. The base class knows that it needs those methods, but an abstract class lets your class admit that it doesn't know how to perform those actions; it only knows that it must initiate the actions. When it is time to start up, the abstract class can call the startup method. When the base class calls this method, Java calls the method defined by the child class.
Many developers forget that a class that defines an abstract method can call that method as well. Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.
Class vs. interface
Some say you should define all classes in terms of interfaces, but I think recommendation seems a bit extreme. I use interfaces when I see that something in my design will change frequently.
For example, the Strategy pattern lets you swap new algorithms and processes into your program without altering the objects that use them. A media player might know how to play CDs, MP3s, and wav files. Of course, you don't want to hardcode those playback algorithms into the player; that will make it difficult to add a new format like AVI. Furthermore, your code will be littered with useless case statements. And to add insult to injury, you will need to update those case statements each time you add a new algorithm. All in all, this is not a very object-oriented way to program.
With the Strategy pattern, you can simply encapsulate the algorithm behind an object. If you do that, you can provide new media plug-ins at any time. Let's call the plug-in class MediaStrategy. That object would have one method: playStream(Stream s). So to add a new algorithm, we simply extend our algorithm class. Now, when the program encounters the new media type, it simply delegates the playing of the stream to our media strategy. Of course, you'll need some plumbing to properly instantiate the algorithm strategies you will need.
This is an excellent place to use an interface. We've used the Strategy pattern, which clearly indicates a place in the design that will change. Thus, you should define the strategy as an interface. You should generally favor interfaces over inheritance when you want an object to have a certain type; in this case, MediaStrategy. Relying on inheritance for type identity is dangerous; it locks you into a particular inheritance hierarchy. Java doesn't allow multiple inheritance, so you can't extend something that gives you a useful implementation or more type identity.
I recommend using composition instead of inheritence whenever possible. Use interfaces but use member objects for base implementation. That way, you can define a factory that constructs your objects to behave in a certain way. If you want to change the behavior then you make a new factory method (or abstract factory) that creates different types of sub-objects.
In some cases, you may find that your primary objects don't need interfaces at all, if all of the mutable behavior is defined in helper objects.
So instead of IPet or PetBase, you might end up with a Pet which has an IFurBehavior parameter. The IFurBehavior parameter is set by the CreateDog() method of the PetFactory. It is this parameter which is called for the shed() method.
If you do this you'll find your code is much more flexible and most of your simple objects deal with very basic system-wide behaviors.
I recommend this pattern even in multiple-inheritence languages.
Also keep in mind not to get swept away in OO (see blog) and always model objects based on behavior required, if you were designing an app where the only behavior you required was a generic name and species for an animal then you would only need one class Animal with a property for the name, instead of millions of classes for every possible animal in the world.
I have a rough rule-of-thumb
Functionality: likely to be different in all parts: Interface.
Data, and functionality, parts will be mostly the same, parts different: abstract class.
Data, and functionality, actually working, if extended only with slight changes: ordinary (concrete) class
Data and functionality, no changes planned: ordinary (concrete) class with final modifier.
Data, and maybe functionality: read-only: enum members.
This is very rough and ready and not at all strictly defined, but there is a spectrum from interfaces where everything is intended to be changed to enums where everything is fixed a bit like a read-only file.
Source: http://jasonroell.com/2014/12/09/interfaces-vs-abstract-classes-what-should-you-use/
C# is a wonderful language that has matured and evolved over the last 14 years. This is great for us developers because a mature language provides us with a plethora of language features that are at our disposal.
However, with much power becomes much responsibility. Some of these features can be misused, or sometimes it is hard to understand why you would choose to use one feature over another. Over the years, a feature that I have seen many developers struggle with is when to choose to use an interface or to choose to use an abstract class. Both have there advantages and disadvantages and the correct time and place to use each. But how to we decide???
Both provide for reuse of common functionality between types. The most obvious difference right away is that interfaces provide no implementation for their functionality whereas abstract classes allow you to implement some “base” or “default” behavior and then have the ability to “override” this default behavior with the classes derived types if necessary.
This is all well and good and provides for great reuse of code and adheres to the DRY (Don’t Repeat Yourself) principle of software development. Abstract classes are great to use when you have an “is a” relationship.
For example: A golden retriever “is a” type of dog. So is a poodle. They both can bark, as all dogs can. However, you might want to state that the poodle park is significantly different than the “default” dog bark. Therefor, it could make sense for you to implement something as follows:
public abstract class Dog
{
public virtual void Bark()
{
Console.WriteLine("Base Class implementation of Bark");
}
}
public class GoldenRetriever : Dog
{
// the Bark method is inherited from the Dog class
}
public class Poodle : Dog
{
// here we are overriding the base functionality of Bark with our new implementation
// specific to the Poodle class
public override void Bark()
{
Console.WriteLine("Poodle's implementation of Bark");
}
}
// Add a list of dogs to a collection and call the bark method.
void Main()
{
var poodle = new Poodle();
var goldenRetriever = new GoldenRetriever();
var dogs = new List<Dog>();
dogs.Add(poodle);
dogs.Add(goldenRetriever);
foreach (var dog in dogs)
{
dog.Bark();
}
}
// Output will be:
// Poodle's implementation of Bark
// Base Class implementation of Bark
//
As you can see, this would be a great way to keep your code DRY and allow for the base class implementation be called when any of the types can just rely on the default Bark instead of a special case implementation. The classes like GoldenRetriever, Boxer, Lab could all could inherit the “default” (bass class) Bark at no charge just because they implement the Dog abstract class.
But I’m sure you already knew that.
You are here because you want to understand why you might want to choose an interface over an abstract class or vice versa. Well one reason you may want to choose an interface over an abstract class is when you don’t have or want to prevent a default implementation. This is usually because the types that are implementing the interface not related in an “is a” relationship. Actually, they don’t have to be related at all except for the fact that each type “is able” or has “the ablity” to do something or have something.
Now what the heck does that mean? Well, for example: A human is not a duck…and a duck is not a human. Pretty obvious. However, both a duck and a human have “the ability” to swim (given that the human passed his swimming lessons in 1st grade :) ). Also, since a duck is not a human or vice versa, this is not an “is a” realationship, but instead an “is able” relationship and we can use an interface to illustrate that:
// Create ISwimable interface
public interface ISwimable
{
public void Swim();
}
// Have Human implement ISwimable Interface
public class Human : ISwimable
public void Swim()
{
//Human's implementation of Swim
Console.WriteLine("I'm a human swimming!");
}
// Have Duck implement ISwimable interface
public class Duck: ISwimable
{
public void Swim()
{
// Duck's implementation of Swim
Console.WriteLine("Quack! Quack! I'm a Duck swimming!")
}
}
//Now they can both be used in places where you just need an object that has the ability "to swim"
public void ShowHowYouSwim(ISwimable somethingThatCanSwim)
{
somethingThatCanSwim.Swim();
}
public void Main()
{
var human = new Human();
var duck = new Duck();
var listOfThingsThatCanSwim = new List<ISwimable>();
listOfThingsThatCanSwim.Add(duck);
listOfThingsThatCanSwim.Add(human);
foreach (var something in listOfThingsThatCanSwim)
{
ShowHowYouSwim(something);
}
}
// So at runtime the correct implementation of something.Swim() will be called
// Output:
// Quack! Quack! I'm a Duck swimming!
// I'm a human swimming!
Using interfaces like the code above will allow you to pass an object into a method that “is able” to do something. The code doesn’t care how it does it…All it knows is that it can call the Swim method on that object and that object will know which behavior take at run-time based on its type.
Once again, this helps your code stay DRY so that you would not have to write multiple methods that are calling the object to preform the same core function (ShowHowHumanSwims(human), ShowHowDuckSwims(duck), etc.)
Using an interface here allows the calling methods to not have to worry about what type is which or how the behavior is implemented. It just knows that given the interface, each object will have to have implemented the Swim method so it is safe to call it in its own code and allow the behavior of the Swim method be handled within its own class.
Summary:
So my main rule of thumb is use an abstract class when you want to implement a “default” functionality for a class hierarchy or/and the classes or types you are working with share a “is a” relationship (ex. poodle “is a” type of dog).
On the other hand use an interface when you do not have an “is a” relationship but have types that share “the ability” to do something or have something (ex. Duck “is not” a human. However, duck and human share “the ability” to swim).
Another difference to note between abstract classes and interfaces is that a class can implement one to many interfaces but a class can only inherit from ONE abstract class (or any class for that matter). Yes, you can nest classes and have an inheritance hierarchy (which many programs do and should have) but you cannot inherit two classes in one derived class definition (this rule applies to C#. In some other languages you are able to do this, usually only because of the lack of interfaces in these languages).
Also remember when using interfaces to adhere to the Interface Segregation Principle (ISP). ISP states that no client should be forced to depend on methods it does not use. For this reason interfaces should be focused on specific tasks and are usually very small (ex. IDisposable, IComparable ).
Another tip is if you are developing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class.
Hope this clears things up for some people!
Also if you can think of any better examples or want to point something out, please do so in the comments below!
Interfaces should be small. Really small. If you're really breaking down your objects, then your interfaces will probably only contain a few very specific methods and properties.
Abstract classes are shortcuts. Are there things that all derivatives of PetBase share that you can code once and be done with? If yes, then it's time for an abstract class.
Abstract classes are also limiting. While they give you a great shortcut to producing child objects, any given object can only implement one abstract class. Many times, I find this a limitation of Abstract classes, and this is why I use lots of interfaces.
Abstract classes may contain several interfaces. Your PetBase abstract class may implement IPet (pets have owners) and IDigestion (pets eat, or at least they should). However, PetBase will probably not implement IMammal, since not all pets are mammals and not all mammals are pets. You may add a MammalPetBase that extends PetBase and add IMammal. FishBase could have PetBase and add IFish. IFish would have ISwim and IUnderwaterBreather as interfaces.
Yes, my example is extensively over-complicated for the simple example, but that's part of the great thing about how interfaces and abstract classes work together.
The case for Base Classes over Interfaces was explained well in the Submain .NET Coding Guidelines:
Base Classes vs. Interfaces
An interface type is a partial
description of a value, potentially
supported by many object types. Use
base classes instead of interfaces
whenever possible. From a versioning
perspective, classes are more flexible
than interfaces. With a class, you can
ship Version 1.0 and then in Version
2.0 add a new method to the class. As long as the method is not abstract,
any existing derived classes continue
to function unchanged.
Because interfaces do not support
implementation inheritance, the
pattern that applies to classes does
not apply to interfaces. Adding a
method to an interface is equivalent
to adding an abstract method to a base
class; any class that implements the
interface will break because the class
does not implement the new method.
Interfaces are appropriate in the
following situations:
Several unrelated classes want to support the protocol.
These classes already have established base classes (for
example,
some are user interface (UI) controls,
and some are XML Web services).
Aggregation is not appropriate or practicable. In all other
situations,
class inheritance is a better model.
One important difference is that you can only inherit one base class, but you can implement many interfaces. So you only want to use a base class if you are absolutely certain that you won't need to also inherit a different base class. Additionally, if you find your interface is getting large then you should start looking to break it up into a few logical pieces that define independent functionality, since there's no rule that your class can't implement them all (or that you can define a different interface that just inherits them all to group them).
When I first started learning about object-oriented programming, I made the easy and probably common mistake of using inheritance to share common behavior - even where that behavior was not essential to the nature of the object.
To further build on an example much used in this particular question, there are lots of things that are petable - girlfriends, cars, fuzzy blankets... - so I might have had a Petable class that provided this common behavior, and various classes inheriting from it.
However, being petable is not part of the nature of any of these objects. There are vastly more important concepts that are essential to their nature - the girlfriend is a person, the car is a land vehicle, the cat is a mammal...
Behaviors should be assigned first to interfaces (including the default interface of the class), and promoted to a base class only if they are (a) common to a large group of classes that are subsets of a larger class - in the same sense that "cat" and "person" are subsets of "mammal".
The catch is, after you understand object-oriented design sufficiently better than I did at first, you'll normally do this automatically without even thinking about it. So the bare truth of the statement "code to an interface, not an abstract class" becomes so obvious you have a hard time believing anyone would bother to say it - and start trying to read other meanings into it.
Another thing I'd add is that if a class is purely abstract - with no non-abstract, non-inherited members or methods exposed to child, parent, or client - then why is it a class? It could be replaced, in some cases by an interface and in other cases by Null.
Prefer interfaces over abstract classes
Rationale,
the main points to consider [two already mentioned here] are :
Interfaces are more flexible, because a class can implement multiple
interfaces. Since Java does not have multiple inheritance, using
abstract classes prevents your users from using any other class
hierarchy. In general, prefer interfaces when there are no default
implementations or state. Java collections offer good examples of
this (Map, Set, etc.).
Abstract classes have the advantage of allowing better forward
compatibility. Once clients use an interface, you cannot change it;
if they use an abstract class, you can still add behavior without
breaking existing code. If compatibility is a concern, consider using
abstract classes.
Even if you do have default implementations or internal state,
consider offering an interface and an abstract implementation of it.
This will assist clients, but still allow them greater freedom if
desired [1].
Of course, the subject has been discussed at length
elsewhere [2,3].
[1] It adds more code, of course, but if brevity is your primary concern, you probably should have avoided Java in the first place!
[2] Joshua Bloch, Effective Java, items 16-18.
[3] http://www.codeproject.com/KB/ar...
Previous comments about using abstract classes for common implementation is definitely on the mark. One benefit I haven't seen mentioned yet is that the use of interfaces makes it much easier to implement mock objects for the purpose of unit testing. Defining IPet and PetBase as Jason Cohen described enables you to mock different data conditions easily, without the overhead of a physical database (until you decide it's time to test the real thing).
Don't use a base class unless you know what it means, and that it applies in this case. If it applies, use it, otherwise, use interfaces. But note the answer about small interfaces.
Public Inheritance is overused in OOD and expresses a lot more than most developers realize or are willing to live up to. See the Liskov Substitutablity Principle
In short, if A "is a" B then A requires no more than B and delivers no less than B, for every method it exposes.
Another option to keep in mind is using the "has-a" relationship, aka "is implemented in terms of" or "composition." Sometimes this is a cleaner, more flexible way to structure things than using "is-a" inheritance.
It may not make as much sense logically to say that Dog and Cat both "have" a Pet, but it avoids common multiple inheritance pitfalls:
public class Pet
{
void Bathe();
void Train(Trick t);
}
public class Dog
{
private Pet pet;
public void Bathe() { pet.Bathe(); }
public void Train(Trick t) { pet.Train(t); }
}
public class Cat
{
private Pet pet;
public void Bathe() { pet.Bathe(); }
public void Train(Trick t) { pet.Train(t); }
}
Yes, this example shows that there is a lot of code duplication and lack of elegance involved in doing things this way. But one should also appreciate that this helps to keep Dog and Cat decoupled from the Pet class (in that Dog and Cat do not have access to the private members of Pet), and it leaves room for Dog and Cat to inherit from something else--possibly the Mammal class.
Composition is preferable when no private access is required and you don't need to refer to Dog and Cat using generic Pet references/pointers. Interfaces give you that generic reference capability and can help cut down on the verbosity of your code, but they can also obfuscate things when they are poorly organized. Inheritance is useful when you need private member access, and in using it you are committing yourself to highly coupling your Dog and Cat classes to your Pet class, which is a steep cost to pay.
Between inheritance, composition, and interfaces there is no one way that is always right, and it helps to consider how all three options can be used in harmony. Of the three, inheritance is typically the option that should be used the least often.
Conceptually, an interface is used to formally and semi-formally define a set of methods that an object will provide. Formally means a set of method names and signatures, and semi-formally means human readable documentation associated with those methods.
Interfaces are only descriptions of an API (after all, API stands for application programming interface), they can't contain any implementation, and it's not possible to use or run an interface. They only make explicit the contract of how you should interact with an object.
Classes provide an implementation, and they can declare that they implement zero, one or more Interfaces. If a class is intended to be inherited, the convention is to prefix the class name with "Base".
There is a distinction between a base class and an abstract base classes (ABC). ABCs mix interface and implementation together. Abstract outside of computer programming means "summary", that is "abstract == interface". An abstract base class can then describe both an interface, as well as an empty, partial or complete implementation that is intended to be inherited.
Opinions on when to use interfaces versus abstract base classes versus just classes is going to vary wildly based on both what you are developing, and which language you are developing in. Interfaces are often associated only with statically typed languages such as Java or C#, but dynamically typed languages can also have interfaces and abstract base classes. In Python for example, the distinction is made clear between a Class, which declares that it implements an interface, and an object, which is an instance of a class, and is said to provide that interface. It's possible in a dynamic language that two objects that are both instances of the same class, can declare that they provide completely different interfaces. In Python this is only possible for object attributes, while methods are shared state between all objects of a class. However, in Ruby, objects can have per-instance methods, so it's possible that the interface between two objects of the same class can vary as much as the programmer desires (however, Ruby doesn't have any explicit way of declaring Interfaces).
In dynamic languages the interface to an object is often implicitly assumed, either by introspecting an object and asking it what methods it provides (look before you leap) or preferably by simply attempting to use the desired interface on an object and catching exceptions if the object doesn't provide that interface (easier to ask forgiveness than permission). This can lead to "false positives" where two interfaces have the same method name, but are semantically different. However, the trade-off is that your code is more flexible since you don't need to over specify up-front to anticipate all possible uses of your code.
It depends on your requirements. If IPet is simple enough, I would prefer to implement that. Otherwise, if PetBase implements a ton of functionality you don't want to duplicate, then have at it.
The downside to implementing a base class is the requirement to override (or new) existing methods. This makes them virtual methods which means you have to be careful about how you use the object instance.
Lastly, the single inheritance of .NET kills me. A naive example: Say you're making a user control, so you inherit UserControl. But, now you're locked out of also inheriting PetBase. This forces you to reorganize, such as to make a PetBase class member, instead.
I usually don't implement either until I need one. I favor interfaces over abstract classes because that gives a little more flexibility. If there's common behavior in some of the inheriting classes I move that up and make an abstract base class. I don't see the need for both, since they essentially server the same purpose, and having both is a bad code smell (imho) that the solution has been over-engineered.
Regarding C#, in some senses interfaces and abstract classes can be interchangeable. However, the differences are: i) interfaces cannot implement code; ii) because of this, interfaces cannot call further up the stack to subclass; and iii) only can abstract class may be inherited on a class, whereas multiple interfaces may be implemented on a class.
By def, interface provides a layer to communicate with other code. All the public properties and methods of a class are by default implementing implicit interface. We can also define an interface as a role, when ever any class needs to play that role, it has to implement it giving it different forms of implementation depending on the class implementing it. Hence when you talk about interface, you are talking about polymorphism and when you are talking about base class, you are talking about inheritance. Two concepts of oops !!!
I've found that a pattern of Interface > Abstract > Concrete works in the following use-case:
1. You have a general interface (eg IPet)
2. You have a implementation that is less general (eg Mammal)
3. You have many concrete members (eg Cat, Dog, Ape)
The abstract class defines default shared attributes of the concrete classes, yet enforces the interface. For example:
public interface IPet{
public boolean hasHair();
public boolean walksUprights();
public boolean hasNipples();
}
Now, since all mammals have hair and nipples (AFAIK, I'm not a zoologist), we can roll this into the abstract base class
public abstract class Mammal() implements IPet{
#override
public walksUpright(){
throw new NotSupportedException("Walks Upright not implemented");
}
#override
public hasNipples(){return true}
#override
public hasHair(){return true}
And then the concrete classes merely define that they walk upright.
public class Ape extends Mammal(){
#override
public walksUpright(return true)
}
public class Catextends Mammal(){
#override
public walksUpright(return false)
}
This design is nice when there are lots of concrete classes, and you don't want to maintain boilerplate just to program to an interface. If new methods were added to the interface, it would break all of the resulting classes, so you are still getting the advantages of the interface approach.
In this case, the abstract could just as well be concrete; however, the abstract designation helps to emphasize that this pattern is being employed.
An inheritor of a base class should have an "is a" relationship. Interface represents An "implements a" relationship.
So only use a base class when your inheritors will maintain the is a relationship.
Use Interfaces to enforce a contract ACROSS families of unrelated classes. For example, you might have common access methods for classes that represent collections, but contain radically different data i.e. one class might represent a result set from a query, while the other might represent the images in a gallery. Also, you can implement multiple interfaces, thus allowing you to blend (and signify) the capabilities of the class.
Use Inheritance when the classes bear a common relationship and therefore have a similair structural and behavioural signature, i.e. Car, Motorbike, Truck and SUV are all types of road vehicle that might contain a number of wheels, a top speed
Right now, I am learning OOP, mainly in c#. I am interested in what are the main reasons to make a class that can't be instantiated. What would be the correct example of when to make an abstract class?
I found myself using the abstract class in inheritance way too enthusiastically. Are there some rules when class is abstract in system and when class should not be abstract?
For instance, I made doctor and patient classes which are similar in some way so I derived them both from abstract class Person (since both have name and surname). Was that wrong?
Sorry if the question is stupid, I am very new at this.
There are a couple of things no one has pointed out so far, so I would just like to point them out.
You can only inherit from one base class (which could be abstract) but you can implement many interfaces. So in this sense inheriting an abstract class is a closer relationship than implementing an interface.
So if you later on realize that you have a need for a class which implements two different abstract classes you are in deep shit :)
To answer your question "when to make an abstract class" I'd say never, avoid it if possible, it will never pay off in the long run, if the main class is not suitable as a ordinary class, it probably isn't really needed as abstract either, use an interface. If you ever get in the situation where you are duplicating code it might be suitable with an abstract class, but always have a look at interfaces and behavioral patterns first (ex the strategy pattern solves a lot of issues people wrongly use inheritance to solve, always prefer composition over inheritance). Use abstract classes as a last hand solution, not as a design.
To get a better understanding of OOP in general, I'd recommend you to have a look at Design Patterns: Elements of Reusable Object-Oriented Software (a book) which gives a good overview of OO-design and reusability of OO-components. OO-design is about so much more than inheritance :)
For Example: you have a scenario where you need to pull data from different sources, like "Excel File,XML,any Database etc" and save in one common destination. It may be any database. So in this situation you can use abstract classes like this.
abstract class AbstractImporter
{
public abstract List<SoldProduct> FetchData();
public bool UploadData(List<SoldProduct> productsSold)
{
// here you can do code to save data in common destination
}
}
public class ExcelImporter : AbstractImporter
{
public override List<SoldProduct> FetchData()
{
// here do code to get data from excel
}
}
public class XMLImporter : AbstractImporter
{
public override List<SoldProduct> FetchData()
{
// here do code to get data from XML
}
}
public class AccessDataImporter : AbstractImporter
{
public override List<SoldProduct> FetchData()
{
// here do code to get data from Access database
}
}
and calling can be like this
static class Program
{
static void Main()
{
List<SoldProduct> lstProducts;
ExcelImporter excelImp = new ExcelImporter();
lstProducts = excelImp.FetchData();
excelImp.UploadData(lstProducts);
XMLImporter xmlImp = new XMLImporter ();
lstProducts = xmlImp.FetchData();
xmlImp.UploadData(lstProducts);
AccessDataImporterxmlImp accImp = new AccessDataImporter();
lstProducts = accImp .FetchData();
accImp.UploadData(lstProducts);
}
}
So, in Above example, implementation of data import functionality is separated in extended (derived) class but data upload functionality is common for all.
This is probably a non-academic definition, but an abstract class should represent an entity that is so "abstract" that make no sense to instantiate it.
It is often used to create "templates" that must be extended by concrete classes. So an abstract class can implement common features, for example implementing some methods of an interface, an delegate to concrete classes implementation of specific behaviors.
In essence what you have done is fine if you never want to instantiate a Person class, however as I'm guessing you may want to instantiate a Person class at some point in the future then it should not be abstract.
Although there is an argument that you code to fix current issues, not to cater for issues which may never arise, so if you need to instantiate Person class do not mark it as abstract.
Abstract classes are incomplete and must be implemented in a derived class... Generally speaking I tend to prefer abstract base classes over interfaces.
Look into the difference between abstract classes and interfaces...
"The difference between an abstract class and an interface is that an abstract class can have a default implementation of methods, so if you don't override them in a derived class, the abstract base class implementation is used. Interfaces cannot have any implementation." Taken from this SO post
As already stated, noone will force you to use abstract classes, it is just a methodology to abstract certain functionality which is common among a number of classes.
Your case is a good example where to use abstract classes, because you have common properties among two different types. But of cause it restricts you to use Person as a type by itself. If you want to have this restriction is basically up to you.
In general, I would not use abstract classes for Model like classes as you have unless you want to prevent Person from being instantiated.
Usually I use abstract classes if I also have defined an interface and I need to code different implementations for this interface but also want to have a BaseClass which already covers some common functionality for all implementations.
Deriving both 'Doctor' and 'Patient' from an abstract class 'Person' is fine, but you should probably make Person just a regular class. It depends on the context in which 'Person' is being used, though.
For example, you might have an abstract class named 'GameObject'. Every object in the game (e.g. Pistol, OneUp) extends 'GameObject'. But you can't have a 'GameObject' by itself, as 'GameObject' describes what a class should have, but doesn't go into detail as to what they are.
For example, GameObject might say something like: "All GameObjects must look like something'. A Pistol might extend on what GameObject said, and it says "All Pistols must look like a long barrel with a grip on one end and a trigger."
The key is whether instantiation of that class ever makes sense. If it will never be appropriate to instantiate that class, then it should be abstract.
A classic example is a Shape base class, with Square, Circle and Triangle child classes. A Shape should never be instantiated because by definition, you don't know what shape you want it to be. Therefore, it makes sense to make Shape an abstract class.
Incidentally, another issue which hasn't yet been mentioned is that it is possible to add members to an abstract class, have existing implementations automatically support them, and allow consumers to use implementations which know about the new members and implementations which don't, interchangeably. While there are some plausible mechanisms by which a future .NET runtime could allow interfaces to work that way as well, at present they do not.
For example, if IEnumerable had been an abstract class (there are of course good many reasons why it isn't), something like a Count method could have been added when its usefulness became apparent; its default implementation of Count could behave much like the IEnumerable<T>.Count extension method, but implementations which knew about the new method could implement it more efficiently (although IEnumerable<T>.Count will try to take advantage of implementations of ICollection<T>.Count or ICollection.Count, it first has to determine whether they exist; by contrast, any override would know that it has code to handle Count directly).
It would have been possible to add an ICountableEnumerable<T> interface which inherited from IEnumerable<T> but included Count, and existing code would continue to work just fine with IEnumerable<T> as it always had, but any time an ICountableEnumerable<T> was passed through existing code, the recipient would have to recast it to ICountableEnumerable<T> to use the Count method. Far less convenient than having a directly-dispatched Count method which could simply act directly on IEnumerable<T> [the Count extension method isn't horrible, but it's far less efficient than would be a directly-dispatched virtual method].
If there were a means by which an interface could include static methods, and if the class loader, upon finding that a class Boz which claimed to implement IFoo, was missing method string IFoo.Bar(int), would automatically add to that class:
stringIFoo.Bar(int p1) { return IFoo.classHelper_Bar(Boz this, int p1); }
[assuming the interface contains that static method], then it would be possible to have interfaces add members without breaking existing implementations, provided that they also included static methods that could be called by default implementations. Unfortunately, I know of no plans to add any such functionality.
first question here, so hopefully you'll all go gently on me!
I've been reading an awful lot over the past few days about polymorphism, and trying to apply it to what I do in c#, and it seems there are a few different ways to implement it. I hope I've gotten a handle on this, but I'd be delighted even if I haven't for clarification.
From what I can see, I've got 3 options:
I can just inherit from a base
class and use the keyword
'virtual' on any methods that I
want my derived classes to
override.
I could implement an abstract class with virtual methods
and do it that way,
I could use an interface?
From what I can see, if I don't require any implementation logic in the base, then an interface gives me the most flexibility (as I'm then not limiting myself with regards multiple inheritance etc.), but if I require the base to be able to do something on top of whatever the derived classes are doing, then going with either 1 or 2 would be the better solution?
Thanks for any input on this guys - I have read so much this weekend, both on this site and elsewhere, and I think I understand the approaches now, yet I just want to clarify in a language specific way if I'm on the right track. Hopefully also I've tagged this correctly.
Cheers,
Terry
An interface offers the most abstraction; you aren't tied to any specific implementation (useful if the implementation must, for other reasons, have a different base class).
For true polymorphism, virtual is a must; polymorphism is most commonly associated with type subclassing...
You can of course mix the two:
public interface IFoo {
void Bar();
}
class Foo : IFoo {
public virtual void Bar() {...}
}
class Foo2 : Foo {
public override ...
}
abstract is a separate matter; the choice of abstract is really: can it be sensibly defined by the base-class? If there is there no default implementation, it must be abstract.
A common base-class can be useful when there is a lot of implementation details that are common, and it would be pointless to duplicate purely by interface; but interestingly - if the implementation will never vary per implementation, extension methods provide a useful way of exposing this on an interface (so that each implementation doesn't have to do it):
public interface IFoo {
void Bar();
}
public static class FooExtensions {
// just a silly example...
public static bool TryBar(this IFoo foo) {
try {
foo.Bar();
return true;
} catch {
return false;
}
}
}
All three of the above are valid, and useful in their own right.
There is no technique which is "best". Only programming practice and experience will help you to choose the right technique at the right time.
So, pick a method that seems appropriate now, and implement away.
Watch what works, what fails, learn your lessons, and try again.
Interfaces are usually favored, for several reasons :
Polymorphisme is about contracts, inheritance is about reuse
Inheritance chains are difficult to get right (especially with single inheritance, see for instance the design bugs in the Windows Forms controls where features like scrollability, rich text, etc. are hardcoded in the inheritance chain
Inheritance causes maintenance problems
That said, if you want to leverage common functionnality, you can use interfaces for polymorphism (have your methods accept interfaces) but use abstract base classes to share some behavior.
public interface IFoo
{
void Bar();
enter code here
}
will be your interface
public abstract class BaseFoo : IFoo
{
void Bar
{
// Default implementation
}
}
will be your default implementation
public class SomeFoo : BaseFoo
{
}
is a class where you reuse your implementation.
Still, you'll be using interfaces to have polymorphism:
public class Bar
{
int DoSometingWithFoo(IFoo foo)
{
foo.Bar();
}
}
notice that we're using the interface in the method.
The first thing you should ask is "why do I need to use polymorphism?", because polymorphism is not and end by itself, but a mean to reach an end. Once you have your problem well defined, it should be more clear which approach to use.
Anyway, those three aproaches you commented are not exclusive, you still can mix them if you need to reuse logic between just some classes but not others, or need some distinct interfaces...
use abstract classes to enforce a class structure
use interfaces for describing behaviors
It really depends on how you want to structure your code and what you want to do with it.
Having a base class of type Interface is good from the point of view of testing as you can use mock objects to replace it.
Abstract classes are really if you wish to implement code in some functions and not others, as if an abstract class has nothing other than abstract functions it is effectively an Interface.
Remember that an abstract class cannot be instantiated and so for working code you must have a class derived from it.
In practice all are valid.
I tend to use an abstract class if I have a lot of classes which derive from it but on a shallow level (say only 1 class down).
If I am expecting a deep level of inheritence then I use a class with virtual functions.
Eitherway it's best to keep classes simple, along with their inheritence as the more complex they become the more likelyhood of introducing bugs.
I'm a bit confused about the usage of Abstract classes in C#. In C++, it makes sense to define a template which classes inheriting the abstract class can follow. But, in C# doesn't Interface serve the same purpose?
True that abstract classes can have default implementation which is not provided by Interfaces. So if implementation doesn't need to be included in base class, is it better to go for Interfaces?
I still like to provide a default abstract implementation of an interface, assuming it's a substantial interface (and it makes sense). You never know when you might add something to the interface that has an easy default implementation that could be included and given "for free" to anyone who inherits from the abstract base class.
This CodeProject article has a lot of information on the difference between the two including a table comparing and contrasting the features of each.
Interfaces define the contract between classes - the ways classes call each other. A class can implement multiple interfaces, but can only inherit from one abstract class.
True that abstract classes can have default implementation which is not provided by Interfaces. So if implementation doesn't need to be included in base class, is it better to go for Interfaces?
Yes :). If it makes sense to implement some methods in the base class which will be common to all inhereted class you should use an abstract class. If the base class would only be used to define an interface but there is no common logic between the inherited classes, use an interface.
Interfaces and abstract classes serve different goals. Interfaces are used to declare contracts for classes while abstract classes are used to share a common implementation.
If you only use abstract classes, your classes cannot inherit from other classes because C# does not support multiple inheritance. If you only use interfaces, your classes cannot share common code.
public interface IFoo
{
void Bar();
}
public abstract class FooBase : IFoo
{
public abstract void Bar()
{
// Do some stuff usually required for IFoo.
}
}
Now we can use the interface and base implementation in various situations.
public class FooOne : FooBase
{
public override void Bar()
{
base.Bar(); // Use base implementation.
// Do specialized stuff.
}
}
public class FooTwo : FooBase
{
public override void Bar()
{
// Do other specialized stuff.
base.Bar(); // Use base implementation.
// Do more specialized stuff.
}
}
// This class cannot use the base implementation from FooBase because
// of inheriting from OtherClass but it can still implement IFoo.
public class FooThree : OtherClass, IFoo
{
public virtual void Bar()
{
// Do stuff.
}
}
For your first question, Yes.
For your second answer i'll give you some tips I've followed.
Use abstract classes and interfaces in combination to optimize your design trade-offs.
Use an abstract class
When creating a class library which will be widely distributed or reused—especially to clients, use an abstract class in preference to an interface; because, it simplifies versioning.
Use an abstract class to define a common base class for a family of types.
Use an abstract class to provide default behavior.
Subclass only a base class in a hierarchy to which the class logically belongs.
Use an interface
When creating a standalone project which can be changed at will, use an interface in preference to an abstract class; because, it offers more design flexibility.
Use interfaces to introduce polymorphic behavior without subclassing and to model multiple inheritance—allowing a specific type to support numerous behaviors.
Use an interface to design a polymorphic hierarchy for value types.
Use an interface when an immutable contract is really intended.
A well-designed interface defines a very specific range of functionality. Split up interfaces that contain unrelated functionality.
You can implement any number of Interfaces, but can only inherit one Class. So Classes and Interfaces are quite different beasts in C# and you cannot use them interchangeably. In C# abstract classes are still classes, not interfaces.
If you don't have any default/common code, then go with an interface.
An abstract class can also serve as a template, where it defines the steps of some algorithm and the order in which they are called, and derived classes provide the implementation of these steps:
public abstract class Processor
{
// this is the only public method
// implements the order of the separate steps
public void Process()
{
Step1();
Step2();
//...
}
// implementation is provided by derived classes
protected abstract void Step1();
protected abstract void Step2();
}
Whilst it's true that an abstract class with no implementation is equivalent to an interface, interfaces and abstract classes are used for different things.
Interfaces can be used for polymorphism in the most general sense. For example, ICollection is used to define the interface for all collections (there are quite a few). Here it is defining the operations that you want to perform on a certain kind of type. There are many other uses (such as testability, dependency injection etc). Also, interfaces can be mixed and this works both conceptually and technically.
Abstract classes are more to do with templateable behaviour, where virtual methods are a place to 'fill in the gaps'. Obviously you can't mix abstract classes (at least, not in C#).
In C# a large deterrent for the use of abstract classes is that you can only use one. With interfaces you have the advantage of not limiting the base class for the implementation. To this end, I always use an interface even if I create an abstract base class to aid with the implementation.
Often another annoyance of base abstract classes is that they tend to rely on template arguments. This can make it very difficult for the rest of your code to utilize. The easy answer for this is to provide an interface to talk to the abstract class without knowing the type argument of the template class.
Others seem to be typing their answer faster, but allow me to summarize...
Use an interface. If you need to share implementation, you can also create an abstract base class that provides common implementation details.
Note that with C#3, you can provide default behavior for interfaces through the use of extension methods. There are some limitations, though, and abstract classes still have their place.
The rule I follow when modeling is:
Classes(abstract included) and structs model entities.Interfaces model behavior.
Entities implementing an interface can be considered as exhibiting behaviors that the interface(contract) exposes.
This is hinted at in a few of the answers but not explicitly stated.
The fact that you can implement multiple interfaces and only inherit from one base class, as if they were two sides of the same coin, isn't a good way to look at it.
Don't think of interfaces as part of an object hierarchy. They are usually just small parts of functionality (or at least specific if not small) that your real object heirarchy can declare as implementing. Take IDisposable for instance. If you were the one writing that, would you ask yourself whether it should have been an abstract class or an interface? It seems obvious that in this case they are two completely different things. I want to BE disposable. Think ICloneable and IEnumerable. You can implement those in your class without having to try and make your class derive from some unrelated classes like List or Array. Or take IEnumerator. Simply gives a MoveNext type of view to an object. My class can provide that functionality without having to awkwardly be derived from some other sequential collection data type that has nothing to do with my class.
I always prefer interfaces as long as the base class don't have some really "heavy duty" implementation that will save lots of time to the implementers.
giving that .net allows only one base class inheritance, forcing your users to inherit is a huge limitation.
You should always prefer programming to interfaces than to concrete classes.
If you also want to have a default implementation you can still create a base class which implements your interface(s).