A class or struct can implement multiple interfaces, but a class can only inherit from a single class. Interfaces can contain instance methods, properties, events, indexers, or any combination of those four member types. Interfaces may contain static constructors, fields, constants, or operators. An interface can't contain instance fields, instance constructors, or finalizers.
Interface members are public by default, and you can explicitly specify accessibility modifiers, such as public , protected , internal , private , protected internal , or private protected. A private member must have a default implementation. To implement an interface member, the corresponding member of the implementing class must be public, non-static, and have the same name and signature as the interface member.
When a class or struct implements an interface, the class or struct must provide an implementation for all of the members that the interface declares but doesn't provide a default implementation for. However, if a base class implements an interface, any class that's derived from the base class inherits that implementation.
The implementing class, Car , must provide an implementation of the Equals method. Properties and indexers of a class can define extra accessors for a property or indexer that's defined in an interface.
For example, an interface might declare a property that has a get accessor. The class that implements the interface can declare the same property with both a get and set accessor.
However, if the property or indexer uses explicit implementation, the accessors must match. For more information about explicit implementation, see Explicit Interface Implementation and Interface Properties.
Interfaces can inherit from one or more interfaces. The derived interface inherits the members from its base interfaces. A class that implements a derived interface must implement all members in the derived interface, including all members of the derived interface's base interfaces.
That class may be implicitly converted to the derived interface or any of its base interfaces. A class might include an interface multiple times through base classes that it inherits or through interfaces that other interfaces inherit. You want to build new buttons, textboxes, listviews, grids, whatnot and you'd like them all to have certain features unique to your set of controls. For instance you might want a common way to handle theming, or a common way to handle localization.
In this case you can't "just create a base class" because if you do that, you have to reimplement everything that relates to controls. But this poses a problem, how can you now identify which controls are "yours", how can you build some code that says "for all the controls on the form that are mine, set the theme to X".
Interfaces are a way to look at an object, to determine that the object adheres to a certain contract. You would create "YourButton", descend from Button, and add support for all the interfaces you need.
In this case, you could and probably would just define a Pizza base class and inherit from them. However, there are two reasons where Interfaces allow you to do things that cannot be achieved in other ways:. A class can implement multiple interfaces. It just defines features that the class must have.
Implementing a range of interfaces means that a class can fulfil multiple functions in different places. An interface can be defined in a hogher scope than the class or the caller. This means that you can separate the functionality, separate the project dependency, and keep the functionality in one project or class, and the implementation of this elsewhere.
One implication of 2 is that you can change the class that is being used, just requiring that it implements the appropriate interface. I did a search for the word "composition" on this page and didn't see it once.
This answer is very much in addition to the answers aforementioned. One of the absolutely crucial reasons for using interfaces in an Object Oriented Project is that they allow you to favour composition over inheritance. By implementing interfaces you can decouple your implementations from the various algorithms you are applying to them. This superb "Decorator Pattern" tutorial by Derek Banas which - funnily enough - also uses pizza as an example is a worthwhile illustration:.
So, I will create an interface, which will abstract my implementation from what you call. Then, you have a contract, so you know what functions you can expect in MyInterface. So, you can call i. This becomes more important if you are using Dependency Injection, as then you can, in an XML file, swap implementations out.
So, you may have one crypto library that can be exported that is for general use, and another that is for sale only to American companies, and the difference is in that you change a config file, and the rest of the program isn't changed.
This is used a great deal with collections in. As long as you code to the interface then the developer can change the actual implementation and the rest of the program is left unchanged. This is also useful when unit testing, as you can mock out entire interfaces, so, I don't have to go to a database, but to a mocked out implementation that just returns static data, so I can test my method without worrying if the database is down for maintenance or not. Interfaces are for applying connection between different classes.
But each class have their own ways to burn. An interface is really a contract that the implementing classes must follow, it is in fact the base for pretty much every design pattern I know. In your example, the interface is created because then anything that IS A Pizza, which means implements the Pizza interface, is guaranteed to have implemented. I'm surprised that not many posts contain the one most important reason for an interface: Design Patterns.
It's the bigger picture into using contracts, and although it's a syntax decoration to machine code to be honest, the compiler probably just ignores them , abstraction and interfaces are pivotal for OOP, human understanding, and complex system architectures. Let's expand the pizza analogy to say a full fledge 3 course meal. Based on these specifications, we could implement the Abstract Factory pattern to conceptualise the whole process, but use interfaces to ensure that only the foundations were concrete.
Everything else could become flexible or encourage polymorphism, yet maintain encapsulation between the different classes of Course that implement the ICourse interface. If I had more time, I'd like to draw up a full example of this, or someone can extend this for me, but in summary, a C interface would be the best tool in designing this type of system.
You will get interfaces, when you will need them : You can study examples, but you need the Aha! Now that you know what interfaces are, just code without them. Sooner or later you will run into a problem, where the use of interfaces will be the most natural thing to do.
An interface defines a contract between the provider of a certain functionality and the correspondig consumers. It decouples the implementation from the contract interface. You should have a look at object oriented architecture and design. Interfaces are basically a contract that all the classes implementing the Interface should follow. They looks like a class but has no implementation.
In C Interface names by convention is defined by Prefixing an 'I' so if you want to have an interface called shapes, you would declare it as IShapes. Lets say you want to draw Circle , Triangle. Now when you add them there is a great chance that you might break other parts of your code. If you were asked add Square and Rectangle to it, all you have to do is create the implentation for it in class Square: IShapes and in Main add to list shapes.
Add new Square ;. There are a lot of good answers here but I would like to try from a slightlt different perspective. Following the SOLID principles helps to produce code that is clean, well factored, cohesive and loosely coupled.
Given that:. Interfaces and the Dependency Inversion Principle really help to decouple code from dependencies on concrete classes, so code can be written and reasoned about in terms of behaviours rather than implementations. This helps to break the code into components which can be composed at runtime rather than compile time and also means those components can be quite easily plugged in and out without having to alter the rest of the code.
Interfaces help in particular with the Dependency Inversion Principle, where code can be componentized into a collection of services, with each service being described by an interface.
Services can then be "injected" into classes at runtime by passing them in as a constructor parameter. This technique really becomes critical if you start to write unit tests and use test driven development. Try it! You will quickly understand how interfaces help to break apart the code into manageable chunks that can be individually tested in isolation. I know I am very late..
In simple words, you use Interface when you know what an Object can do or what function we are going to implement on an object.. Example Insert,Update, and Delete. So to begin, yes you could have used a concrete base and derived class here. In that case, you would have to do an empty or useless implementation for the Prepare method in the base class also making this method virtual and then the derived classes would override this Prepare method for themselves.
This case, the implementation of Prepare in Base class is useless. The reason why you chose to use an Interface is because you had to define a contract, not an implementation. There is a IPizza type and it provides a functionality to Prepare. This is contract. How it is prepared is the implementation and it is not your lookout.
It is responsibility of the various Pizza implementations. An interface or an abstract class is preferred here over a concrete base class because you had to create an abstraction, i. You cannot create an abstract method in a concrete base class. But when you need some abstraction along with a concrete implementation, go with abstract class. It means. Example: Lets say all your pizzas will have a base and base preparation will be the same process.
However, all pizza types and toppings will vary. In this case you could create an Abstract class with an abstract method Prepare and a concrete method PreparePizzaBase. The main purpose of the interfaces is that it makes a contract between you and any other class that implement that interface which makes your code decoupled and allows expandability.
Another, in the case of a switch statement, you no longer have the need to maintain and switch every time you want rio perform a task in a specific way. In your pizza example, if want to make a pizza, the interface is all you need, from there each pizza takes care of it's own logic. This helps to reduce coupling and cyclomatic complexity. You have to still implement the logic but there will be less you have to keep track of in the broader picture.
For each pizza you can then keep track of information specific to that pizza. What other pizzas have doesn't matter because only the other pizzas need to know.
The simplest way to think about interfaces is to recognize what inheritance means. If class CC inherits class C, it means both that:. Those two function of inheritance are in some sense independent; although inheritance applies both simultaneously, it is also possible to apply the second without the first.
This is useful because allowing an object to inherit members from two or more unrelated classes is much more complicated than allowing one type of thing to be substitutable for multiple types. An interface is somewhat like an abstract base class, but with a key difference: an object which inherits a base class cannot inherit any other class. By contrast, an object may implement an interface without affecting its ability to inherit any desired class or implement any other interfaces.
One nice feature of this underutilized in the. Some objects, for example, will want data-source object from which they can retrieve things by index as is possible with a List , but they won't need to store anything there. Other routines will need a data-depository object where they can store things not by index as with Collection. Add , but they won't need to read anything back. Some data types will allow access by index, but won't allow writing; others will allow writing, but won't allow access by index.
Some, of course, will allow both. If ReadableByIndex and Appendable were unrelated base classes, it would be impossible to define a type which could be passed both to things expecting a ReadableByIndex and things expecting an Appendable.
One could try to mitigate this by having ReadableByIndex or Appendable derive from the other; the derived class would have to make available public members for both purposes, but warn that some public members might not actually work. Some of Microsoft's classes and interfaces do that, but that's rather icky.
A cleaner approach is to have interfaces for the different purposes, and then have objects implement interfaces for the things they can actually do. If one had an interface IReadableByIndex and another interface IAppendable, classes which could do one or the other could implement the appropriate interfaces for the things they can do. Interfaces can also be daisy chained to create yet another interface.
This ability to implement multiple Interfaces give the developer the advantage of adding functionality to their classes without having to change current class functionality SOLID Principles. Since you can only inherit 1 abstract class but you can implement multiple interfaces, changes to a system that inherits an abstract class in many places becomes problematic. If it is inherited in places, a change requires changes to all But, with the interface, you can place the new change in a new interface and just use that interface where its needed Interface Seq.
Additionally, the memory usage seems like it would be less with the interface as an object in the interface example is used just once in memory despite how many places implement the interface. Interfaces are used to drive consistency,in a manner that is loosely coupled which makes it different to abstract class which is tightly coupled. That's why its also commonly defined as a contract.
Imagine in a factory there are 3 types of machines. A rectangle machine,a triangle machine and a polygon machine. Times are competitive and you want to streamline operator training. You just want to train them in one methodology of starting and stopping machines so you have a green start button and red stop button. So now across 3 different machines you have a consistent way of starting and stopping 3 different types of machines.
Now imagine these machines are classes and the classes need to have start and stop methods,how you going to drive consistency across these classes which can be very different? Interface is the answer.
A simple example to help you visualize,one might ask why not use abstract class? With an interface the objects don't have to be directly related or inherited and you can still drive consistency across different classes. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. An interface can contain declarations for functions, events, and properties. All interface members have public accessibility. An interface can also contain static data members, functions, events, and properties, and these static members must be defined in the interface. An interface defines how a class may be implemented.
An interface is not a class and classes can only implement interfaces. When a class defines a function declared in an interface, the function is implemented, not overridden. Therefore, name lookup does not include interface members. A class or struct that derives from an interface must implement all members of the interface.
Interface Static Constructor. For more information, see Compiler Support for Type Traits.
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