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?
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.
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
}
Declare it as a property:
interface ICar {
int Year { get; set; }
}
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.
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.
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; }
}
}
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
}
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.
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.
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://learn.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 => _name;
set => _name = value;
}
private int _counter;
public int Counter // read-only instance property
{
get => _counter;
}
// constructor
public Employee() => _counter = ++numberOfEmployees;
}
For this you can have a Car base class that implement the year field, and all other implementations can inheritance from it.
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.
I've the following class:
namespace Warnings
{
public abstract class BaseWarningIntField : IWarningInnerDataField
{
public string PropName;
public string HeaderCaption;
public sealed WarningInnerDataType DataType
{
get { return WarningInnerDataType.Integer; }
}
}
}
I want the last property DataType to be not overridable, since that's the base class for a warning-detail field of type Integer, so it needs to always return the correct type WarningInnerDataType.Integer.
Anyway, the compiler give me the following error:
'Warnings.BaseWarningIntField.DataType' cannot be sealed because it is not an override
But, as far as I know the override does exactly the opposite of what I'm trying to achieve.
in C# all methods by default are non-virtual. You can't override non-virtual method in sub-classes. So leaving property as usual will safe you from subclass overriding it.
Sealed is a keyword used in class declaration for inheritance restrictions or is used to stop virtual chain of members of a class hierarchy. But again - this relates to virtual methods and properties.
Trying to override "normal" property in sub-class will result in compile error
'WarningIntField.DataType.get': cannot override inherited
member 'BaseWarningIntField.DataType.get' because it is not
marked virtual, abstract, or override
To answer you comment, I'll present some code examples to illustrate my point. You can't actually restrict derived classes from hiding a method or property. So next situation is legal and there is no way to overcome it (this related to virtual method and methods denoted with new keyword as well)
class BaseClass
{
public string Property {get; set;}
}
class DerivedClass : BaseClass
{
//compiler will give you a hint here, that you are hiding a base class prop
public string Property {get; set;}
}
The same way you can't restrict of hiding a field in a class by local variable, so this situation is also valid. Note that compiler will also help you to note, that you are hiding class field in by a local variable. This also related to readonly const and simple static fields as well.
int field = 0; //class field
void Foo()
{
int field = 0; //local variable
}
This question is similar to Can I make a type "sealed except for internal types" but with interfaces instead of classes.
I want to make a library with something like:
public class SomeClass {
ISupporter Supporter {get; set;}
}
public interface ISupporter {/*Some public methods*/}
internal interface ISupporterInternal {/*Some secret methods*/}
public class SupporterA : ISupporterInternal {/*Includes some explicit interface impls of ISupporterInternal*/}
public class SupporterB : ISupporterInternal {/*Includes some explicit interface impls of ISupporterInternal*/}
The user of the library should be able to set a supporter object for instances of SomeClass. The user should also be able to use the methods from ISupporter but I don't want the user to create his own implementations of ISupporter and have him assign instances of those implementations.
Is there any way besides throwing an exception when the type of the assigned supporter is not derived from ISupporterInternal.
If my interface has the signature only for getter such as:
public interface IInterface
{
object Id{get;}
}
So the interface only dictates a public getter for Id on any implemented class
now when i have the class :
public class Simple : IInterface
{
object Id
{
get{return something;}
set{ do something else;}
}
}
the compiler complains about the setter as the setter is not defined in the interface. However I didnt dictate anything on the interface contract for a setter; why does the interface insist on the setter on the derived classes ?
You just need to make Id public. For example, this compiles fine:
public interface IInterface
{
object Id { get; }
}
public class Simple : IInterface
{
private int something;
public object Id
{
get { return something; }
set{ something = (int)value;}
}
}
In designing .net, Microsoft decided to make there be three non-interchangeable types of properties: read-only, write-only, and read-write. In C#, if one declares a read-write property with the same name as one or more interface properties one is supposed to implement, the compiler can automatically create not only the read-write property the programmer actually specified, but read-only and/or write-only properties as needed to satisfy the interfaces. For example, if interface IReadableFoo implements a read-only property Foo, IWritableFoo implements a write-only property Foo, and IReadWriteFoo inherits IReadableFoo and IWritablefoo, and implements a "new" read-write property Foo, and a class ReadWriteFoo implements IReadWriteFoo and declares a public read-write property Foo, the compiler will have ReadWriteFoo generate interface implementations of read-only property IReadableFoo.Foo, write-only property IWritableFoo.Foo, and read-write property IReadWriteFoo.Foo.
I have been refactoring the codebase of the project that I am currently on so that classes/interfaces which are not useful beyond the confines of the assembly should be declared as internal (rather than public). But I've run into a problem with the following code:
internal interface IFirstInterface
{
...
}
internal interface ISecondInterface
{
IFirstInterface First{ get; }
...
}
public class Implementer : ISecondInterface
{
public IFirstInterface First {get; private set;}
...
}
My questions:
Why do members of internal interfaces have to be publicly implemented? If you implement the interface on an internal class, shouldn't the implemented members be internal? This is not a big issue since the interface members won't be publicly accessible anyway, given the class is internal. It just seems counter intuitive.
The main problem is with the scenario above since I cannot have a public getter for IFirstInterface since it is purportedly an internal interface i.e. I get the following error from the compiler:
Inconsistent accessibility: property
type 'IFirstInterface' is less
accessible than property
'Implementer.First'
Is there any way around this?
Note: I realise that there is probably little value in this refactoring exercise but I thought it would be a good way for me to understand more deeply the implications of the internal modifier.
Just to note - the code you've actually provided does compile, because Implementer is an internal class. The problem comes when Implementer is public.
The way round this is to use explicit interface implementation:
public class Implementer : ISecondInferface
{
private IFirstInterface first;
IFirstInterface ISecondInterface.First { get { return first; } }
}
You can't have the setter in there, because you're explicitly implementing the interface which doesn't define the setter. You could do this as an alternative:
public class Implementer : ISecondInterface
{
internal IFirstInterface First { get; private set; }
IFirstInterface ISecondInterface.First { get { return First; } }
}
It's unfortunate that internal interfaces have public members - it does complicate things like this. It would be strange for a public interface to have an internal member (what would it be internal to - the implementer or the declarer?) but for internal interfaces it makes a lot more sense.
Why do members of internal interfaces have to be publicly implemented?
When you define an interface, you do not define access level for the members, since all interface members are public. Even if the interface as such is internal, the members are still considered public. When you make an implicit implementation of such a member the signature must match, so it needs to be public.
Regarding exposing the getter, I would suggest making an explicit implementation of the interface instead, and creating an internal property to expose the value:
internal IFirstInterface First { get; private set; }
IFirstInterface ISecondInterface.First
{
get { return this.First; }
}
I know this post is a few years old but i think it’s worth noting that you can implement an internal interface on a public class, see the following links:
http://forums.create.msdn.com/forums/p/29808/167820.aspx
http://msdn.microsoft.com/en-us/library/aa664591%28VS.71%29.aspx
An example from the first link:
internal interface ISecretInterface
{
string Property1 { get; }
}
public class PublicClass : ISecretInterface
{
// class property
public string Property1
{
get { return "Foo"; }
}
// interface property
string ISecretInterface.Property1
{
get { return "Secret"; }
}
}