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C# interface implementation with derived interface

In the following sample class "SomeClass" does not implement "ISomeInterface". Why can't I implement this by passing a more derived interface which does implement the base requirement. Whatever instance would be passed it would still implement the base, am I missing something?
namespace Test
{
public interface IBaseInterface
{
void DoBaseStuff();
}
public interface IChildInterface : IBaseInterface
{
void DoChildStuff();
}
public interface ISomeInterface
{
void DoSomething(IBaseInterface baseInterface);
}
public class SomeClass : ISomeInterface
{
public void DoSomething(IChildInterface baseInterface)
{
}
}
}

This restriction exists because the ISomeInterface expects that any IBaseInterface will satisfy the contract. That is, if you have the following:
public interface IBase {}
public interface IChildA : IBase {}
public interface IChildB : IBase {}
And an interface that expects IBase:
public interface IFoo { void Bar(IBase val); }
Then restricting this in a derived class as you would like:
public class Foo : IFoo { public void Bar(IChildA val) {} }
Would create the following problem:
IChildB something = new ChildB();
IFoo something = new Foo();
something.Bar(something); // This is an invalid call
As such, you're not implementing the contract you said you would.
In this situation, you have two simple options:
Adjust IFoo to be generic, and accept a T that is a derivation of IBase:
public interface IFoo<T> where T : IBase { void Bar(T val); }
public class Foo : IFoo<IChildA> { public void Bar(IChildA val) {} }
Of course, this means that Foo can no longer accept any IBase (including IChildB).
Adjust Foo to implement IFoo, with an additional utility method for void Bar(IChildA val):
public class Foo : IFoo
{
public void Bar(IBase val) {}
public void Bar(IChildA val) {}
}
This has an interesting side-effect: whenever you call ((IFoo)foo).Bar it will expect IBase, and when you call foo.Bar it will expect IChildA or IBase. This means it satisfies the contract, while also having your derived-interface-specific method. If you want to "hide" the Bar(IBase) method more, you could implement IFoo explicitly:
void IFoo.Bar(IBase val) { }
This creates even more inconsistent behavior in your code, as now ((IFoo)foo).Bar is completely different from foo.Bar, but I leave the decision up to you.
This means, with the second version in this section, that foo.Bar(new ChildB()); is now invalid, as IChildB is not an IChildA.
Why can't I implement this by passing a more derived interface which does implement the base requirement. Whatever instance would be passed it would still implement the base, am I missing something?
This is not allowed because of the reasoning I mentioned above, IFoo.Bar expects any IBase, whereas you want to further constrain the type to IChildA, which is not a super-interface of IBase, and even if it were it would not be allowed because it violates the interface implementation, though you could more easily define a second method at that point that does what you want.
Keep in mind that when you implement an interface, you subscribe to a contract, and C# will not let you violate that contract.

This violates the Liskov substitution principle.
ISomeInterface guarantees that the method can be called with any IBaseInterface instance. Your implementation cannot limit that to only accept IChildInterface interfaces.

From MSDN:
When a class or struct implements an interface, the class or struct must provide an implementation for all of the members that the interface defines
This method in the derived
void DoSomething(IChildInterface baseInterface)
Does not have the same signature as the one in the interface:
void DoSomething(IBaseInterface baseInterface)
IChildInterface and IBaseInterface are not the same types. Therefore your derived class does not implement all methods of the interface and you get the compilation error.
For a possible the logic behind having this as a restriction instead of the compiler understanding the inheritance see Liskov's substitution principle as in SLakes answer

You should change some interface to use some type which implements IBaseInterface,
then change the method signatures to use whichever child your SomeClass wants.
public interface ISomeInterface<TSomeChild> where TSomeChild : IBaseInterface
{
void DoSomething(TSomeChild baseInterface);
}
public class SomeClass : ISomeInterface<IChildInterface>
{
public void DoSomething(IChildInterface baseInterface)
{
}
}

If you could do that, then you could do this:
IAnimal cat = new Cat();
IAnimalTrainer dogTrainer = new DogTrainer();
dogTrainer.Train(cat);
An IAnimalTrainer can train any IAnimal. But a DogTrainer can only train Dogs. Thus it's illegal for DogTrainer to implement the IAnimalTrainer interface.

Base interface in c#

I need some sort of way to mark base interfaces and identify if a class implemented the base interface or its derived interface. c# doesn't allow having 'abstract interface'. Is there any way to do this in c#?
public interface IBaseFoo
{
void BaseMethod();
}
public interface IFoo : IBaseFoo
{
void FooMethod();
}
public class Base
{
}
public class A : Base, IFoo
{
}
public class B : Base, IBaseFoo
{
}
Now in the following method I need to check if the typeCls is implemented the IFoo or IBaseFoo without explicitly specifying types. I need sort of a way to mark the base interface and identify it in the method. (ie: if c# allowed having abstract interface, I could have check if IsAbstract property of interfaces of typeClas)
public bool IsBaseFooImplemented<T>(T typeCls) where T : Base
{
// Here I need to check if the typeCls is implemented the IFoo or IBaseFoo
}

Because IFoo : IBaseFoo, every class implementing IFoo also implements IBaseFoo. But not the other way around, so you can simply check whether typeCls is IFoo.
Do note that changing behavior based on implemented interfaces generally is a design smell that bypasses the use for interfaces in the first place.

//somewhere define
static List<IBaseFoo> list = new List<IBaseFoo>();
public class A : Base, IFoo
{
public A()
{
YourClass.list.add(this);
}
}
public class B : Base, IBaseFoo
{
public B()
{
YourClass.list.add(this);
}
}
//then you can check if a class is IFoo or not.
public bool IsBaseFooImplemented<T>(T typeCls) where T : Base
{
foreach(var c in list )
{
if(typeof(c) == typeCls) return true;
}
return false;
}
I have not tested the code but it should work.

static method cannot implement interface method, why?

interface IXXX
{
void Foo();
}
class XXX : IXXX
{
public static void Foo()
{
Console.WriteLine("From XXX");
}
}
class Program
{
static void Main(string[] args)
{
XXX.Foo();
}
}
Compiler error: XXX.Foo() cannot implement an interface member because it is static.
Why can't a static method implement an interface method?

See this thread from JoelOnSoftware describing the reasons behind this.
Basically the interface is the contract between the consumer and the provider, and a static method belongs to the class, and not each instance of the class as such.
An earlier question on SO also deal with the exact same question:
Why Doesn't C# Allow Static Methods to Implement an Interface?

An interface defines the behaviour that an object must respond to.
As Foo is a static method, the object doesn't respond to it. In other words, you couldn't write...
XXX myXXX = new XXX();
myXXX.Foo();
In other words, myXXX doesn't fully satisfy the requirements of the interface.

IF we look at interfaces as a promise that an object can perform the methods listed in the interface, then ths idea of static implementation becomes problematic. If the implemetion is static, then you can't write new ImplementingObject().ImplementedMthod. The object can't perform the method, the class can.

You use interface to avoid using concrete class during instantiation. You can't access static method through instantiated class, so implementing interface methods with static methods is not allowed.

Well, I believe it should allowed in case of generic type parameter. It probably simplified contractual singleton class. Here is an example:
public interface IEntity {
// some constrains...
DataRow ObjToRow(object obj);
object RowToObj(DataRow dr);
}
//T would be any class inherites from IEntity with default contructor signature.
public interface IMyContract {
T read<T>() where T : IEntity;
void write<T>(T object) where T : IEntity;
}
//everything in the class is static
public static class SqlProvider : IMyContract {
public static T read<T>() where T: IEntity {
DataRow dr = [reading from database]
return T.RowToObj(dr);
}
//compile error here....
public static void write<T>(T obj) where T : IEntity {
DataRow dr = T.ObjToRow(obj);
[ ... commit data row dr to database ... ]
}
}
public static class MyAppleEntity : IEntity {
[... implement IEntity contract normally ... ]
}
public static class MyOrangeEntity : IEntity {
[... implement IEntity contract normally ... ]
}
public class MyTest {
void reading() {
MyAppleEntity apple = SqlProvider.Read<MyAppleEntity>();
MyOrangeEntity orange = SqlProvider.Read<MyOrangeEntity>();
SqlProvider.write<MyAppleEntity>(apple);
SqlProvider.write<MyOrangeEntity>(orange);
}
}
The only time a type reference implicitly is in the SqlProvider.read() and write() and T is well identity at point of invoke. Without static implementation of interface I'm forced to write like this.
public class MyAppleEntity : IEntity {
[... implement IEntity contract normally ... ]
}
.....
public T read<T>() where T: IEntity, new() {
DataRow dr = [reading from database]
return new T().RowToObj(dr);
}
Very little different but not quite as elegant.

Because interface member are public and overridable, and that static method cannot by design be overrided or abstract, Interfaces are here to define an accessible contract that must be implemented by their concrete implementation (with as many steps of abstract implementations & inherited interfaces between) and as far as I know there is no way to create an abstract static method.

Explicitly implementing an interface with an abstract method

Here is my interface:
public interface MyInterface {
bool Foo();
}
Here is my abstract class:
public abstract class MyAbstractClass : MyInterface {
abstract bool MyInterface.Foo();
}
This is the compiler error:
"The modifier 'abstract' is not valid for this item.
How should I go on about explicitly implementing an abstract with an abstract method?

You can't, basically. Not directly, anyway. You can't override a method which is explicitly implementing an interface, and you have to override an abstract method. The closest you could come would be:
bool MyInterface.Foo() {
return FooImpl();
}
protected abstract bool FooImpl();
That still implements the interface explicitly and forces derived classes to actually provide the implementation. Are those the aspects you're trying to achieve?

You have to use an implicit implementation of the interface member instead of an explicit implementation:
public abstract class MyAbstractClass : MyInterface
{
public abstract bool Foo();
}

In fact there is another option than using an abstract helper method which still keeps the implementation private:
public abstract class MyAbstractClass : MyInterface
{
bool MyInterface.Foo() // must be overridden
{ throw NotImplementedException(); // never called
}
}
public class MyDerivedClass : MyAbstractClass, MyInterface
{
bool MyInterface.Foo() // overrides MyInterface.Foo
{ // Place your implementation here
}
}
This pattern will also work if the interface has many methods and only some of them are redefined in the derived class. And, of course, you can also use this to override private interface implementations in general.
The major disadvantage is that Foo cannot be declared abstract in MyAbstractClass, so the compiler cannot ensure that the method is actually overridden. (It's a pity that abstract classes may not have incomplete interface implementations in C#.)
The advantage is that you save one calli instruction that is likely to cause CPU pipeline stalls. However, the impact is quite small, since the method cannot be inlined anyway because of the interface call. So I would recommend it only for performance critical cases.

I'm not sure why you need to. Why not let the concrete implementation of the abstract class implement the member from the interface? It's the same thing really.

An abstract method has no implementation, so it can't be used to explicitly implement an interface method.

I am able to do this
public interface SampleInterface
{
void member1();
void member2();
void member3();
}
public abstract class Client2 : SampleInterface.SampleInterface
{
public void member1()
{
throw new NotImplementedException();
}
public abstract void member2();
public void member3()
{
throw new NotImplementedException();
}
}
public class Client3 : Client2
{
public Client3()
{
}
public override void member2()
{
throw new NotImplementedException();
}
}

In addition to Jon's explanation: this is a way to bypass the problem instead of solving it directly and will work only in certain circumstances, but maybe someone will benefit from this idea.
If you plan to pass all (or at lest most) interface method calls to the derived class, you can do it in the following way:
public interface MyInterface
{
bool Foo();
}
public abstract class MyAbstractClass
{
public abstract MyInterface AsMyInterface();
}
public class MyDerivedClass : MyInterface
{
public override MyInterface AsMyInterface()
{
return this;
}
public bool Foo()
{
return false;
}
}
...
MyAbstractClass c = new MyDerivedClass();
MyInterface i = c.AsMyInterface();
bool b = i.Foo();

Abstract all the interface methods that are implemented in a abstract class, even if you do not use them.
This particular case requires that you are implementing a hierarchy of 2 or more abstract classes, with a interface.
I was trying to implement a hierarchy in C# as well. I needed a Interface but I wanted a Abstract class, because most of the properties are the same for the interface. To do this I had to create a separate Abstract class, with the implementation, and then my Concrete classes, or in my case another abstract class, would inherit the Interface and the Abstract Class.
I do not think that this first one is a good example for many reasons, but I had to have it because the compiler would not allow FooBar to implement Foo and then have another abstract class to inherit FooBar. So I had a abstract class with a abstract method bar(), and the interface with the bar() method.
public interface Foo {
bool bar();
//other stuffs
}
public abstract class FooBar {
public abstract bool bar();
//Other stuffs
}
public abstract class FooBarAbstraction: FooBar, Foo {
//other stuffs
//Don't supply the interface and abstract here
}
public class FooBarConcrete: FooBarAbstraction {
public override bool bar() {
return true;
}
//other stuffs
}
This was my first attempt, then I got curious and started to think about it. I came across this solution. The better solution.
public interface Foo {
bool bar();
bool buzz();
//other stuffs
}
public abstract class FooBar : Foo{
public abstract bool bar();
public abstract bool buzz();
//Other stuffs
}
public abstract class FooBarAbstraction: FooBar {
//other stuffs
//Don't supply the interface and abstract here
// override everything else
public override bool buzz() {
return false;
}
}
public class FooBarConcrete: FooBarAbstraction {
public override bool bar() {
return true;
}
//other stuffs
}

Need help understanding abstract classes that implement an interface

Consider the following example. I have an interface MyInterface, and then two abstract classes MyAbstractClass1 and MyAbstractClass2. MyAbstractClass1 implements MyInterface, but MyAbstractClass2 does not.
Now I have three concrete classes.
MyConcreteClass1 is derived from MyAbstractClass1 but does not implement MyInterface.
MyConcreteClass2 is derived from MyAbstractClass2, but does implement MyInterface.
MyConcreteClass3 is derived from MyAbstractClass1, and does implement MyInterface.
Does ConcreteClass1 also implicitly implement MyInterface because it derives from MyAbstractClass1? Assuming MyAbstractClass1 implements the methods of MyInteface implicitly then ConcreteClass1 should not have to be cast to a MyInterface to access the MyInteface methods right?
MyAbstractClass1 can implicitly implement a method of MyInterface as an abstract method, but can't explicitly implement a method of MyInterface as an abstract method. Why is this?
Is MyConcreteClass3 excessive because it's implementing an interface that is already implemented by its base class? Would there be a reason you would want to do that even if you knew all classes that derive from MyAbstractClass1 should also implement MyInterface.
Here's a class diagram
alt text http://files.getdropbox.com/u/113068/abstractclassesandinterfaces.png
Here's the code:
//interface
public interface MyInterface
{
void MyMethodA();
void MyMethodB();
void MyMethodC();
}
//abstract classes
public abstract class MyAbstractClass1 : MyInterface
{
public void MyMethodA()
{
}
void MyInterface.MyMethodB()
{
}
//Error: "the modifier abstract is not valid for this item"
//abstract void MyInterface.MyMethodC();
//This works
public abstract void MyMethodC();
public abstract void MyMethodZ();
}
public abstract class MyAbstractClass2
{
public void MyMethodX()
{
}
public abstract void MyMethodY();
}
//Concrete classes
//ConcreteClass 1: Only Abstract class implements the interface
public class ConcreteClass1 : MyAbstractClass1
{
public override void MyMethodC()
{
}
public override void MyMethodZ()
{
}
}
//ConcreteClass 1: Only Concrete class implements the interface
public class ConcreteClass2 : MyAbstractClass2, MyInterface
{
public override void MyMethodY()
{
}
public void MyMethodA()
{
}
public void MyMethodB()
{
}
public void MyMethodC()
{
}
}
//ConcreteClass 1: Both concrete and abstract class implement the interface
public class ConcreteClass3 : MyAbstractClass1, MyInterface
{
public override void MyMethodC()
{
}
public override void MyMethodZ()
{
}
}

Does ConcreteClass1 also implicitly implement MyInterface because it derives from MyAbstractClass1?
Yes.
ConcreteClass1 should not have to be cast to a MyInterface to access the MyInteface methods right?
Correct. (myConcreteClass1 is MyInterface) will evaluate true.
MyAbstractClass1 can implicitly implement a method of MyInterface as an abstract method, but can't explicitly implement a method of MyInterface as an abstract method.
Explicit implementation is to distinguish between overlapping member signatures. The explicit implementation is private to the class you are implementing it on, so it is not accessible to derived classes (and thus cannot be abstract). You also cannot force classes which derive from MyAbstractClass1 to explicitly implement MyInterface, so there is no way to ensure the abstract member will ever be implemented.
Is MyConcreteClass3 excessive because it's implementing an interface that is already implemented by its base class? Would there be a reason you would want to do that even if you knew all classes that derive from MyAbstractClass1 should also implement MyInterface.
Not necessarily, If you need to explicitly implement a member of the interface to distinguish it from an overlapping member on MyConcreteClass3. Otherwise it is unnecessary.

In this case, all three classes implement the interface (directly or indirectly). This is because MyAbstractClass1 implements MyInterface, and since MyConcreteClass1 derives from MyAbstractClass1, it also follows that you can treat MyConcreteClass1 as a MyInterface. MyConcreteClass2 can be treated with something that derives from MyAbstractClass1, as long as you treat it as a MyInterface. the derivation from MyInterface in ConcreteClass3 is a bit redundant since MyAbstractClass1 already implements MyInterface.
With all of that information, i'd say that yes, it is redundant to implement MyInterface on MyConcreteClass3 since it derives from MyAbstractClass1 which already implements MyInterface.
I think the reason that you cant have an abstract implementation of an interface method is that it provides no code itself and you cannot guarantee that it will be overriden in subcalsses. Use Virtual instead.

It's not redundant. Consider this class setup to simplify ...
public interface I
{
void A();
}
public abstract class B : I
{
public void A( )
{
Console.WriteLine("Base");
}
}
public class D : B
{
public void A()
{
Console.WriteLine("Hide");
}
}
public class U
{
public void M(I i)
{
Console.WriteLine("M!");
}
}
Executing this ...
var d = new D();
var i = (I)d;
var u = new U();
i.A();
d.A();
u.M(d);
u.M(i);
You will get ...
Base
Hide
M!
M!
If you add the interface from the derived class ...
public class D : B, I
{
public void A()
{
Console.WriteLine("Hide");
}
}
You will get ...
Hide
Hide
M!
M!
So, it effects which implementation of the interface method you get when you get the Iness of your derived class.