Lets assume I've got a class Called StreetModel that implements an interface called IStreetModel:
public class StreetModel: IStreetModel {
// Properties go here
}
Said IStreetModel looks like so:
public interface IStreetModel {
// Properties go here
}
Now, say I have another interface called ILocationModel that contains a property of type IStreetModel:
public interface ILocationModel {
IStreetModel Street { get; }
}
And this ILocationModel interface is implemented by a class called LocationModel:
public class LocationModel: ILocationModel {
public StreetModel Street { get; } // This is where my query is
}
My Question: Within LocationModel, why can't I implement StreetModel even though it implements IStreetModel. Why does the compiler want IStreetModel specifically? It's confusing for me because in almost every other scenario within my program (except for collections) I can interchange usage of the two. Why not here?
Error Message:
'LocationModel' does not implement interface member
'ILocationModel.Street'. 'LocationModel.Street' cannot implement
'ILocationModel.Street' because it does not have the matching return
type of 'IStreetModel'.
The answer to your actual question ("why doesn't it compile?") is: The c# language requires that a return type for an implemented interface member matches exactly. That's the way the language was designed.
This was fixed by c# 9 in some cases, but not for interface implementations. See the section following The remainder of the draft specification below proposes a further extension to covariant returns of interface methods to be considered later in this documentation.
In the meantime, a possible workaround is to make your ILocationModel generic like so:
public interface IStreetModel
{
// Properties go here
}
public interface ILocationModel<out T> where T: IStreetModel
{
T Street { get; }
}
public class StreetModel : IStreetModel
{
// Properties go here
}
public class LocationModel : ILocationModel<StreetModel>
{
public StreetModel Street { get; } // This is where my query is
}
Of course the most obvious solution is simply to declare the return type of LocationModel.Street as IStreetModel.
Another alternative is to use explicit interface implementation, like so:
public class LocationModel : ILocationModel
{
public StreetModel Street { get; } // This is where my query is
IStreetModel ILocationModel.Street => Street;
}
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 am trying to find the right way to use a Generic List of Generic Interfaces as a variable.
Here is an example. It is probably not the best, but hopefully you will get the point:
public interface IPrimitive<T>
{
T Value { get; }
}
and then in another class, I want to be able to declare a variable that holds a list of objects that implement IPrimitive<T> for arbitrary T.
// I know this line will not compile because I do not define T
List<IPrimitive<T>> primitives = new List<IPrimitives<T>>;
primitives.Add(new Star()); // Assuming Star implements IPrimitive<X>
primitives.Add(new Sun()); // Assuming Sun implements IPrimitive<Y>
Note that the T in IPrimitive<T> could be different for each entry in the list.
Any ideas on how I could setup such a relationship? Alternative Approaches?
public interface IPrimitive
{
}
public interface IPrimitive<T> : IPrimitive
{
T Value { get; }
}
public class Star : IPrimitive<T> //must declare T here
{
}
Then you should be able to have
List<IPrimitive> primitives = new List<IPrimitive>;
primitives.Add(new Star()); // Assuming Star implements IPrimitive
primitives.Add(new Sun()); // Assuming Sun implements IPrimitive
John is correct.
Might I also suggest (if you are using C# 4) that you make your interface covariant?
public interface IPrimitive<out T>
{
T Value { get; }
}
This could save you some trouble later when you need to get things out of the list.
You say it won't work because you don't define T. So define it:
public class Holder<T>
{
public List<IPrimitive<T>> Primitives {get;set;}
}
This is one of the most complicated elements of the c# language though it is incredibly important for building well defined components. As such, c# falls short. However it is definitely possible to make this work.
The trick is to have 3 parts:
A non generic interface that contains all requirements of the interface.
A generic abstract class that implements the non generic interface and performs the type conversions as necessary.
A class that implements the generic abstract class with the appropriately typed results
For example:
public interface INonGenericInterface{
void Execute(object input);
object GetModel();
}
public abstract class IGenericInterfaceBase<T> : INonGenericInterface{
void INonGenericInterface.Execute(object input){
Execute((T) input);
}
object INonGenericInterface.GetModel(){
return GetModel();
}
protected abstract void Execute(T input);
protected abstract T GetModel();
}
public class ImplementingClass : IGenericInterfaceBase<ModelClass>{
protected override void Execute(ModelClass input){ /*Do something with the input */ }
protected override ModelClass GetModel(){ return new ModelClass();}
}
//Extras for demo
public class ModelClass { }
public class ModelClass2 { }
public class ImplementingClass2 : IGenericInterfaceBase<ModelClass2>
{
protected override void Execute(ModelClass2 input) { /*Do something with the input */ }
protected override ModelClass2 GetModel() { return new ModelClass2(); }
}
var agi = new INonGenericInterface[] { new ImplementingClass(), new ImplementingClass2() };
agi[0].Execute(); var model = agi[0].GetModel();
agi[1].Execute(); var model2 = agi[1].GetModel();
//Check the types of the model and model2 objects to see that they are appropriately typed.
This structure is incredibly useful when coordinating classes w/ one another because you're able to indicate that an implementing class will make use of multiple classes and have type checking validate that each class follows established type expectations. In addition, you might consider using an actual class instead of object for the non-generic class so that you can execute functions on the result of the various non-generic calls. Using this same design you can have those classes be generic classes w/ their own implementations and thus create incredibly complex applications.
To OP: Please consider changing the accepted answer to this to raise awareness of the correct approach as all previously stated answers fall short for various reasons and have probably left readers with more questions. This should handle all future questions related to generic classes in a collection.
according to below definitions
interface myin
{
int id { get; set; }
}
class myclass:myin
{
public int id { get; set; }
}
[Database]
public sealed class SqlDataContext : DataContext, IDataContext
{
public SqlDataContext(string connectionString) : base(connectionString){}
public ITable<IUrl> Urls
{
get { return base.GetTable<Url>(); } //how to cast Table<Url> to ITable<IUrl>?
}
...
}
Update:
public IEnumerable<IUrl> Urls
{
get { return base.GetTable<Url>(); }
}
so by use above approach, i haven't Table class associated methods and abilities. this is good solution or not? and why?
In C# 3.0 and odler this is not easily possible - see also covariance and contravariance in C# 4.0.
The problem is that Table<Url> implements the ITable<Url> interface - this part of the casting is easy. The tricky bit is casting ITable<Url> to ITable<IUrl>, because these two types aren't actually related in any way...
In C# before 4.0, there is no easy way to do this - you'll explicitly need to create a new implementation of ITable<..> for the right generic type (e.g. by delegation). In C# 4.0, this conversion can be done as long as ITable is a covariant interface.
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"; }
}
}