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
}
Related
In an Abstract class there are two abstract methods Method1() and Method2(),
but I like to inherit only one Method1() in derived Class, how to handle the situation?
public abstract class BaseClass
{
public abstract void Method1();
public abstract void Method2();
}
Really you can't... If you have to (and I would really question the reasons) some options are:
If you do not have any control over the abstract classes involved, and must use this specific abstract class, then, only way is to make the implementation in derived class throw a NotImplementedException.
public MyDerivedClass: BaseClass
{
public override void Method1()
{
// implementation of Method1
}
public override void Method2()
{ throw new NotImplementedException(); }
}
... or create another abstract base class called, say OnlyDOMethod1
public abstract class OnlyDoMethod1
{ public abstract void Method1(); }
then, modify Baseclass so it inherits from OnlyDoMethod1
public abstract class BaseClass: OnlyDoMethod1
{ public abstract void Method2(); }
and use OnlyDoMethod1 anywhere you only want Method1
public MyDerivedClass: OnlyDoMethod1
{
public override void Method1()
{
// implementation of Method1
}
}
It sounds like what you're looking for is interfaces. Something like this:
public interface ICanDoMethod1
{
void Method1();
}
public interface ICanDoMethod2
{
void Method2();
}
Then in your classes you can selectively implement them:
public class JustMethod1 : ICanDoMethod1
{
// implement Method1 here
}
public class Both : ICanDoMethod1, ICanDoMethod2
{
// implement both here
}
// etc.
Essentially, any given class either can or can not be polymorphically interpreted as any given type. If you want to be only part of a type, then what you really have is two types. C# is single-inheritance, so to implement multiple types you would use interfaces.
Conversely, you could also chain your inheritance. Something like this:
public abstract class Base1
{
public abstract void Method1();
}
public abstract class BaseBoth : Base1
{
public abstract void Method2();
}
public class JustOne : Base1
{
// only implement Method1 here
}
public class Both : BaseBoth
{
// implement both here
}
That'll work if the options stack, that is if you don't want to be able to pick and choose and either want "1" or "1 and 2" (but not just "2").
As a last resort, you can "selectively implement" methods by explicitly not implementing the others. It would looks something like:
public class JustOne : BaseClass
{
public override void Method1()
{
// implement
}
public override void Method2()
{
throw new NotImplementedException();
}
}
But this would be something of an anti-pattern, where your objects would advertise functionality that they intentionally do not support. This would mean that the type BaseClass should be considered very unstable/unreliable, because there's no way for anything consuming that type to know how it should actually behave.
Ultimately, it sounds like you've painted yourself into a corner with your types and you need to back up a little and re-think them. Liskov Substitution shouldn't be taken so lightly.
This is basic example of violation of one of SOLID principles Interface segregation principle
A client should never be forced to implement an interface that it
doesn’t use or clients shouldn’t be forced to depend on methods they
do not use
If you have abstraction where you need only some of method you need to split them in separated abstractions.
.NET do not support multi-inheritance from classes, nut have nice workaround for this problem -> interfaces.
If you care about your code, then you have only one option - split abstract class into two separated classes which have only one method.
If you work only with abstraction then interfaces is better approach, because you can implement multiply interfaces in one class.
public interface IMethodOne
{
void Method1();
}
public interface IMethodTwo
{
void Method2();
}
Then you can implement that both interfaces in the class which needs both methods. And use only one interface in the class with one method needs.
public abstract class BaseClass : IMethodOne, IMethodTwo
{
public abstract void Method1();
public abstract void Method2();
}
And class with one method
public abstract class BaseClassOneMethod : IMethodOne
{
public abstract void Method1();
}
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.
I'm trying to call an explicitly implemented interface method implemented on the base class but just can't seem to get it to work. I agree the idea is ugly as hell, but I've tried every combination I can think of, to no avail. In this case, I can change the base class, but thought I'd ask the question to satisfy my general curiosity.
Any ideas?
// example interface
interface MyInterface
{
bool DoSomething();
}
// BaseClass explicitly implements the interface
public class BaseClass : MyInterface
{
bool MyInterface.DoSomething()
{
}
}
// Derived class
public class DerivedClass : BaseClass
{
// Also explicitly implements interface
bool MyInterface.DoSomething()
{
// I wish to call the base class' implementation
// of DoSomething here
((MyInterface)(base as BaseClass)).DoSomething(); // does not work - "base not valid in context"
}
}
You cannot (it is not part of the interface available to subclasses). In that scenario, use something like:
// base class
bool MyInterface.DoSomething()
{
return DoSomething();
}
protected bool DoSomething() {...}
Then any subclass can call the protected DoSomething(), or (better):
protected virtual bool DoSomething() {...}
Now it can just override rather than re-implement the interface:
public class DerivedClass : BaseClass
{
protected override bool DoSomething()
{
// changed version, perhaps calling base.DoSomething();
}
}
I am working on a small project and I came across that problem.
The project output is a library containing an interface. I would like to implement that interface and seal the functions in it like this if possible:
public interface ITest
{
void SomeMethod();
}
class A : ITest
{
public sealed override SomeMethod()
{
}
}
The idea is to have the interface available to everyone and have some specialized class that implements it. The exception is that I want to make sure that if someone create a specialized class of type A, he/she won't be able to change the method's behavior.
The problem is you can't put the "override" keyword in there since the method isn't declared as "virtual" in the interface. And you can't declare it as "virtual" in the interface since it's not allowed. And you can't remove the "override" keyword since it's needed by "sealed".
Any workaround or brainstorming idea would be welcome, but if someone can come up with a solution that includes an interface, I'd be really happy to learn it!
Thanks!
EDIT: Forget this question! Like Ani said, I forgot that by default method in C# are sealed. Seems like it's always good to go back to the basics once in a while...
I may have completely misunderstood the question, but if your intention is to seal the method in A, you can just do:
class A : ITest
{
public void SomeMethod() { ... }
}
Unlike Java, methods in C# are sealed by default. Subclasses of A won't be able to override the method since it hasn't been marked virtual.
On the other hand, if your intention is to mark the method 'almost sealed' in the interface, so that it forces upon an implementing class to immediately seal it, that isn't possible. It isn't (and shouldn't be) the business of the interface to dictate such details of implementation - an interface is meant to represent a specification.
Use an abstract base class with internal visibility. This base class is not visible outside of the library but allows you to seal the method and the class still implements the interface.
public interface ITest
{
void SomeMethod();
}
internal abstract class SuperA : ITest
{
public abstract void SomeMethod();
}
class A : SuperA
{
public sealed override void SomeMethod()
{
}
}
Your understanding of sealed keyword is incorrect. As a method modifier, sealed is used to prevent a virtual method(defined in the base class) to be override in the next generation of derived classes. For example:
class Base
{
public virtual void M() { }
}
class Derived : Base
{
public sealed override void M() { }
}
class A : Derived
{
public override void M() { } //compile error, M is sealed in Derived
}
Developers can always use new modifier to define a method with the same name in the derived class, that hides the one defined in the base class.
if someone create a specialized class
of type A, he/she won't be able to
change the method's behavior.
If "specialized class" means a class derived from A, the answer is: he can always hide the method in A, but he can't change the method's behavior.
Why not use an abstract class like below.
Haven't tested it but this should work?
public abstract class Test
{
public virtual void SomeMethod() {}
//OR
public abstract void SomeMethod();//MSDN says:
//an abstract method is implicitly virtual
}
class A : Test
{
public sealed override SomeMethod()
{
}
}
Methods in C# are sealed by default.. Here is a sample
class Program
{
static void Main(string[] args)
{
A obj = new A();
obj.SomeMethod();
b ss = new b();
ss.SomeMethod();
Console.ReadLine();
}
}
public interface ITest { void SomeMethod(); }
class A : ITest { public void SomeMethod() {
Console.WriteLine("SomeMethod Called from Class A object");
} }
class b : A
{
//public override void SomeMethod()
//{
// Console.WriteLine("Called from Class B Object");
//}
}
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.