Why can t C interfaces contain fields

Question

For example  suppose I want an ICar interface and that all implementations will contain the field Year   Does this mean that every implementation has to separately declare Year   Wouldn t it be nicer to simply define this in the interface

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Eric Lippert nailed it  I ll use a different way to say what he said   All of the members of an interface are virtual and they all need to be overridden by a class that inherits the interface   You don t explicitly write the virtual keyword in the interface declaration  nor use the override keyword in the class  they are implied   The virtual keyword is implemented in  NET with methods and a so-called v-table  an array of method pointers   The override keyword fills the v-table slot with a different method pointer  overwriting the one produced by the base class   Properties  events and indexers are implemented as methods under the hood   But fields are not   Interfaces can therefore not contain fields

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Others have given the  Why   so I ll just add that your interface can define a Control  if you wrap it in a property   public interface IView       Control Year   get        public Form   IView       public Control Year   get   return uxYear        numeric text box or whatever

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Why not just have a Year property  which is perfectly fine   Interfaces don t contain fields because fields represent a specific implementation of data representation  and exposing them would break encapsulation  Thus having an interface with a field would effectively be coding to an implementation instead of an interface  which is a curious paradox for an interface to have   For instance  part of your Year specification might require that it be invalid for ICar implementers to allow assignment to a Year which is later than the current year   1 or before 1900  There s no way to say that if you had exposed Year fields -- far better to use properties instead to do the work here

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An interface defines public instance properties and methods  Fields are typically private  or at the most protected  internal or protected internal  the term  field  is typically not used for anything public    As stated by other replies you can define a base class and define a protected property which will be accessible by all inheritors   One oddity is that an interface can in fact be defined as internal but it limits the usefulness of the interface  and it is typically used to define internal functionality that is not used by other external code

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A lot has been said already  but to make it simple  here s my take  Interfaces are intended to have method contracts to be implemented by the consumers or classes and not to have fields to store values   You may argue that then why properties are allowed  So the simple answer is - properties are internally defined as methods only

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Beginning with C  8 0  an interface may define a default implementation for members  including properties  Defining a default implementation for a property in an interface is rare because interfaces may not define instance data fields  https   docs microsoft com en-us dotnet csharp programming-guide classes-and-structs interface-properties interface IEmployee       string Name               get          set             int Counter               get           public class Employee   IEmployee       public static int numberOfEmployees       private string  name      public string Name     read-write instance property               get   gt   name          set   gt   name   value             private int  counter      public int Counter     read-only instance property               get   gt   counter                constructor     public Employee     gt   counter     numberOfEmployees

User · Answer

Declare it as a property   interface ICar      int Year   get  set

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Though many of the other answers are correct at the semantic level  I find it interesting to also approach these sorts of questions from the implementation details level   An interface can be thought of as a collection of slots  which contain methods  When a class implements an interface  the class is required to tell the runtime how to fill in all the required slots  When you say  interface IFoo   void M       class Foo   IFoo   public void M               the class says  when you create an instance of me  stuff a reference to Foo M in the slot for IFoo M     Then when you do a call   IFoo ifoo   new Foo    ifoo M      the compiler generates code that says  ask the object what method is in the slot for IFoo M  and call that method   If an interface is a collection of slots that contain methods  then some of those slots can also contain the get and set methods of a property  the get and set methods of an indexer  and the add and remove methods of an event   But a field is not a method  There s no  slot  associated with a field that you can then  fill in  with a reference to the field location  And therefore  interfaces can define methods  properties  indexers and events  but not fields

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For this you can have a Car base class that implement the year field  and all other implementations can inheritance from it

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Interfaces do not contain any implementation    Define an interface with a property  Further you can implement that interface in any class and use this class going forward  If required you can have this property defined as virtual in the class so that you can modify its behaviour

User · Answer

Interfaces in C  are intended to define the contract that a class will adhere to - not a particular implementation   In that spirit  C  interfaces do allow properties to be defined - which the caller must supply an implementation for   interface ICar       int Year   get  set        Implementing classes can use auto-properties to simplify implementation  if there s no special logic associated with the property   class Automobile   ICar       public int Year   get  set       automatically implemented

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The short answer is yes  every implementing type will have to create its own backing variable  This is because an interface is analogous to a contract  All it can do is specify particular publicly accessible pieces of code that an implementing type must make available  it cannot contain any code itself   Consider this scenario using what you suggest   public interface InterfaceOne       int myBackingVariable       int MyProperty   get   return myBackingVariable         public interface InterfaceTwo       int myBackingVariable       int MyProperty   get   return myBackingVariable         public class MyClass   InterfaceOne  InterfaceTwo       We have a couple of problems here    Because all members of an interface are--by definition--public  our backing variable is now exposed to anyone using the interface Which myBackingVariable will MyClass use    The most common approach taken is to declare the interface and a barebones abstract class that implements it  This allows you the flexibility of either inheriting from the abstract class and getting the implementation for free  or explicitly implementing the interface and being allowed to inherit from another class  It works something like this   public interface IMyInterface       int MyProperty   get  set       public abstract class MyInterfaceBase   IMyInterface       int myProperty       public int MyProperty               get   return myProperty            set   myProperty   value

[c#] Why can't C# interfaces contain fields?

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