I have the following interface:
public interface IAgable {
int Age { get; internal set; }
}
I want the Age property, to be read-only for external assemblies that reference this interface, but also I want the Age property to be set on the interface's same assembly, hence the internal modifier.
This, however, seems to throw a compilation error, as accessibility modifiers may not be used on accessors in an interface.
I want the property to be called from an interface, and I want to be able to set it at an internal level. At the same time, if referenced from an outside project, I want it to be readonly.
Is this possible?
Have an internal interface that provides both a get and a set, and a public interface that provides only a get. Have the public interface extend the internal interface:
public interface IAgable
{
int Age { get; }
}
internal interface IAgableInternal : IAgable
{
int Age { set; }
}
This is not possible according to Microsoft
Interfaces declared directly within a namespace can be declared as public or internal and, just like classes and structs, interfaces default to internal access. Interface members are always public because the purpose of an interface is to enable other types to access a class or struct. No access modifiers can be applied to interface members.
http://msdn.microsoft.com/en-us/library/ms173121.aspx
Try this:
public interface IAgeable {
MyAge {get;set}
}
public class MyAge:IAgeable{
public MyAge(int age){
MyAge = age;
}
int MyAge { get; internal set; }
}
Related
When I have a public interface
public interface IPub { string Foo { get; set; } }
then I can implement this interface either by doing it explicitly:
public class CFoo : IPub { string IPub.Foo { get; set; } }
or implicitly by using the public modifier:
public class CFoo : IPub { public string Foo { get; set; } }
. Makes sense: The modifier must be public because the interface is public.
But when I have an internal interface
internal interface IInt { string Bar { get; set; } }
then I can only implement it explicitly:
public class CBar : IInt { string IInt.Bar { get; set; } }
or implicitly with the public modifier:
public class CBar : IInt { public string Bar { get; set; } }
, but NOT with the internal modifier:
public class CBar : IInt { internal string Bar { get; set; } }
// compiler error:
// 'CBar' does not implement interface member 'IInt.Bar'.
// 'CBar.Bar' cannot implement an interface member
// because it is not public.
This make no sense. Why do I need a public modifier when the interface is only internal? Are there any technical reasons why public must always be used in implicit interface implementations, or could the C# developers have made it differently (without needing to change a lot of things in the language)?
The modifier must be public because the interface is public.
While that would have been a way of determining it: that isn't what the compiler wants. For implicit interface implementation (regardless of the visibility of the interface type), a member must be declared as public, no "ifs", "buts" or "maybes" (the implementing type, however, can be any visibility level)
Absolutely the language designers could have looked at more complex rules, but: since there is also an option for explicit interface implementation, they presumably didn't feel that it was necessary to do so.
Specifically, this is §18.6.5 ("Interface mapping") in the specification (v5) - emphasis mine ("I"=interface type, "M"=member, "S"=implementing type):
If S contains a declaration of an explicit interface member implementation that matches I and M, then
this member is the implementation of I.M.
Otherwise, if S contains a declaration of a non-static public member that matches M, then this member
is the implementation of I.M. If more than one member matches, it is unspecified which member is
the implementation of I.M. This situation can only occur if S is a constructed type where the two
members as declared in the generic type have different signatures, but the type arguments make their
signatures identical.
I have an interface that declares some properties (shortened to Id only in the example) with only a get method. Classes implementing this interface do not have to provide a public setter for this property:
public interface IMetadataColumns
{
Int32 Id { get; }
}
Now I need another interface with the same properties, but now they must all be writable as well. So I added a new interface inheriting from the old one where each property also has a setter:
public interface IMetadataColumnsWritable : IMetadataColumns
{
Int32 Id { get; set; }
}
Visual Studio now underlines this and warns me to use the new keyword if hiding the old properties was intended.
What shall I do now? I have classes implementing the IMetadataColumns interface which need some of the properties to be read-only, but I also have other classes where exactly those same properties must be writable as well.
I guess hiding a property sounds somehow not like the way to go...
When interfaces are involved, the new keyword doesn't mean you are hiding the property in the same sense as with classes.
Your class implementing IMetadataColumnsWritable will only have one Id, regardless of whether you cast it to the base IMetadataColumns interface or not (unless you add an explicit implementation for the readonly property - but this is not needed and would only allow for errors in this case).
In other words, you might have:
// this interface is public, and it allows everyone to read the Id
public interface IMetadataColumns
{
int Id { get; }
}
// this one is writeable, but it's internal
internal interface IMetadataColumnsWritable : IMetadataColumns
{
// we need to use 'new' here, but this doesn't mean
// you will end up with two getters
new int Id { get; set; }
}
// internal implementation is writeable, but you can pass it
// to other assemblies through the readonly IMetadataColumns
// interface
internal class MetadataColumns : IMetadataColumnsWritable
{
public int Id { get; set; }
}
So, unless you explicitly implement the readonly property as a separate one, you will have only a single property, whichever interface you use to access it:
var columns = new MetadataColumns { Id = 5 };
var x = (columns as IMetadataColumns).Id;
var y = (columns as IMetadataColumnsWritable).Id;
Debug.Assert(x == y);
Hiding a member inside the class using the new keyword, on the other hand, is what I would not recommend in most cases, because it basically creates a completely new member, which just happens to be called the same as a different member in the base class.
You should indeed use the new keyword here. In your class implementation, you have to implement both properties (since you are not really hiding the property). If they have different return types, at least one of them has to be implemented explicitly.
public class X : IMetadataColumnsWritable
{
public int Id
{
get;
set;
}
// only necessary if you have to differentiate on the implementation.
int IMetadataColumns.Id
{
get
{
return this.Id; // this will call the public `Id` property
}
}
}
Does using explicit implementation of the getter only version forwarding to a public version work?
int IMetadataColumns.Id {
get {
return this.Id;
}
}
public Id {
get { … }
set { … }
}
You might find the public version does need to be new, but the explicit version will be picked up by references of type IMetadataColumns.
is there any diffrence between this 3 auto properties ?
interface MyInterface {
public int p1 { get; set; }
public int p2 { get; }
public int p3 { set; }
}
also why we can write this code in an interface but not in a class ?
public int p { get; }
For the same reason you can write this in an interface:
interface IFace {
void Test();
}
Also, your interface is invalid, as public isn't valid in an interface. The point being, different things are legal in interfaces and classes.
When you do public int P1 { get; set; } in a class, that turns into a auto property. However, you can't do public int P1 { get; }, because what would you want that to mean? Should it always return 0? There is no way to set it. So if you want a read only property you have to define the getter yourself like this:
int _p1;
public int P1 {
get { return _p1; }
}
Also. Another way to achieve more or less the same is this:
public int P1 { get; private set; }
There are differences between those properties. Firstly, you should remove the public modifier from your declaration. Secondly, by putting get or set within the block you define what properties in derived classes should look like. For example, public int p1 { get; set; } requires getter and setter in a derived class, public int p2 { get; } only getter, and public int p3 { set; } requires only setter to be implemented.
You can't use access modifiers inside interfaces because interfaces are guidelines for other developers that force them to go in a certain direction when developing the implementing classes.
Look at this post for more information about that.
Keep in mind interface does NOT contain any implementation data. When you add property in an interface, it merely says that a class implementing this interface needs to have said property with get, set or both methods, depending on what you wrote. So any class implementing your interface has to implement (or have auto-generated) p1 property with get and set method, p2 with get method, and p3 with set method. Interface doesn't care whether these will be auto-generated or your own custom implementations, they just have to be in an implementing class.
Therefore, you can write
int p { get; }
in an interface as all it does is telling that any class implementing this interface has to have property p with getter, again, not caring about its actual implementation - you could write a getter that does some computations, returns some constant, etc. OTOH in a class writing the same would mean that you want a property with auto-generated backing field, except since it would have no setter, you couldn't actually change its value, so it would always have its default value 0.
And as noted, you cannot write access modifiers in an interface, as all interface members are implicitly public.
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 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"; }
}
}