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");
//}
}
Related
I want to build a class that would have a property, in which there is an instance of a class, which implements an abstract class. Here's and example.
public class MyClass {
public MyDerivedClassA derived;
public void mainClassUtility () {
derived.foo();
}
}
public abstract class MyAbstractBaseClass {
public abstract void foo();
}
public class MyDerivedClassA : MyAbstractBaseClass {
public override void foo(){
return;
}
}
public class MyDerivedClassB : MyAbstractBaseClass
{
public override void foo()
{
return;
}
}
Basically, I want to make sure the object I'm using is derived from an abstract class and implements all the methods I will need to use. There will be many implementations of the abstract class and depending on the current state of the program, MyClass might be using different implementations of the ABC. I want to write the program in a way, that no matter what implementation of the ABC is currently being used, there is a way to call it's methods by MyClass. What would be the best solution to this problem?
Unless I'm misunderstanding the question, you're pretty much there. Have MyClass expect a property of the abstract base class and you should be all set.
using System;
public class Program
{
public static void Main()
{
var myClassOne = new MyClass(new MyDerivedClassA());
var myClassTwo = new MyClass(new MyDerivedClassB());
myClassOne.mainClassUtility();
myClassTwo.mainClassUtility();
}
public class MyClass
{
public MyAbstractBaseClass Derived;
public MyClass(MyAbstractBaseClass derived)
{
Derived = derived;
}
public void mainClassUtility ()
{
Derived.foo();
}
}
public abstract class MyAbstractBaseClass
{
public abstract void foo();
}
public class MyDerivedClassA : MyAbstractBaseClass
{
public override void foo()
{
Console.WriteLine("I am MyDerivedClassA");
return;
}
}
public class MyDerivedClassB : MyAbstractBaseClass
{
public override void foo()
{
Console.WriteLine("I am MyDerivedClassB");
return;
}
}
}
How to require an implementation of an abstract class in C#?
You can not instantiate a abstract class - and thus can not use it for most cases. Except as variable/argument/generic type argument. You need to make a concrete (non-abstract) class that inherits from it. You can only use the abstract class as a variable/argument type. To guarantee that only stuff that inherits from it can be used there.
Basically, I want to make sure the object I'm using is derived from an abstract class and implements all the methods I will need to use.
Then use the abstract class as type argument. It means only instaces of the abstract class (of wich there can be no instance) or instances of classes that inherit from it (that somebody else writes) can be used at that place.
Note that Abstract classes and Interfaces overlap in nearly all uses. There is a miriad small differences, but I do not think they mater. The only big difference I can see, is one of exclusivity:
a class can implement as many Interfaces as it wants.
You can only inherit from one abstract class. that means it is for a primary, exclusive purpose. That way you prevent some dumb ideas, like someone trying to make a Windows Form that is also a DBConnection.
I am wondering if the following code can be written in C#:
AbstractClass a = new ConcreteClass1();
a.Operations.Method1();
AbstractClass b = new ConcreteClass2();
b.Operations.Method2();
where Method1() is exclusive to the ConcreteClass1 instance and Method2() is exclusive to the ConcreteClass2() instance. As a result, a.Operations.Method2() and b.Operations.Method1() would be invalid.
This is not possible by design - a and b have the same type, and the compiler will treat them as such. The only way to make it work is by using runtime exceptions.
The concept behind using abstract classes or interfaces conflicts with what you are attempting to do; it sounds like ConcreteClass1 and ConcreteClass2 do not server the same purpose, should they still use the same abstract base class?
I don't know what exactly you are trying to do - so I'll provide a few options:
Use interfaces to show that specific classes implement specific operations:
interface IOperation1
{
void Operation1();
}
interface IOperation2
{
void Operation2();
}
Then reference the interfaces based on what you are trying to achieve.
If Method1 and Method2 are supposed to be invoked at the same time, consider a design where AbstractClass declares the method to be invoked, and the concrete classes do different operations based on that:
abstract class AbstractClass
{
...
abstract void DoSomeOperation();
}
class ConcreteClass1
{
override void DoSomeOperation()
{
this.Operations.Method1();
}
}
class ConcreteClass2
{
override void DoSomeOperation()
{
this.Operations.Method2();
}
}
It's okay for ConcreteClass1 and ConcreteClass2 to have some different methods, but share some functionality that they both inherit from AbstractClass. (If you cast them as their base type then you can only call common methods that they inherit from the base type.)
It sounds like the difference between whether a concrete class uses Method1 or Method2 is an internal detail that should be handled inside the class. The class should know what method it needs to call. In fact, does a consumer of that class even need to know that it depends on Operations? Probably not. Consumers should just call a method on the class, and then whether that class uses Operations.Method1, Operations.Method2, or even depends on Operations at all is an internal implementation detail.
Maybe what you want is something like this:
public abstract class AbstractClass
{
public abstract void DoSomething();
}
public class Operations
{
public void Method1()
{
//Does something
}
public void Method2()
{
//Apparently does something comopletely different
}
}
public class ConcreteClass1 : AbstractClass
{
private Operations _operations;
public override void DoSomething()
{
_operations.Method1();
}
}
public class ConcreteClass2 : AbstractClass
{
private Operations _operations;
public override void DoSomething()
{
_operations.Method2();
}
}
Operations should only be in the base class if it's required that every derived class will use it. (That doesn't happen too much. If all of the derived classes have the same behavior, why isn't it in the base class?) If that's the case then you can hide it in the base class, like this:
public abstract class AbstractClass
{
private Operations _operations;
protected Operations Operations { get { return _operations; } }
public abstract void DoSomething();
}
That way it's exposed to the derived classes but hidden from everything else.
I created two abstract classes and tried to create a class that inherits from both. But I get an error message.
abstract class AbstractClassOne
{
public abstract void ShowMessage();
public abstract void DisplayName();
}
abstract class AbstractClassTwo
{
public abstract void ShowMessage();
public abstract void DisplayPlace();
}
class DerivedClass : AbstractClassOne, AbstractClassTwo // here under AbstractClassTwo it shows the error "cannot have multiple base classes:"
{
}
So a class can only derive from one abstract class?
If can derive from more than one abstract class, then what happens if both classes define the same method, as is the case above (abstract class one and two both have a method showmessage(), so which one will be in the derived class)?
Multiple inheritance is not allowed by C# but it is allowed by C++.
To answer your question regarding the ShowMessage() method that is a known problem in c++ with multiple inheritance called "The Diamond Problem". see: http://en.wikipedia.org/wiki/Multiple_inheritance
So basically you will have to excitability state to which method you are refereeing when calling it e.g. ParentA::ShowMessage()
if you want to have a type that is polymorphic to 2 other types than you should create two separate interfaces and implement them. and if you want to reuse the same methods than you will have to use compositions.
Interfaces example:
public interface ISomeInterface
{
public void ShowMessage();
public void DisplayName();
}
public class ClassOne : ISomeInterface
{
public void ShowMessage()
{
//implementation
}
public void DisplayName()
{
//implementation
}
}
public class ClassTwo : ISomeInterface
{
public void ShowMessage()
{
//implementation
}
public void DisplayPlace()
{
//implementation
}
}
Interface with reusable Show Message Method using composition:
public class ClassTwo : ISomeInterface
{
private ISomeInterface _MyPrivateReusableComponent = new ClassOne();
public void ShowMessage()
{
_MyPrivateReusableComponent.ShowMessage()
}
public void DisplayPlace()
{
_MyPrivateReusableComponent.DisplayName()
//implementation
}
}
In C# it's not allowed to inherit from more than one class. To do what you want here, you need to use interfaces.
abstract class AbstractClassOne
{
public abstract void ShowMessage();
public abstract void DisplayName();
}
Interface IClassTwo
{
void ShowMessage();
void DisplayPlace();
}
class DerivedClass : AbstractClassOne, IClassTwo
{
}
You can't inherit from more than one class (abstract or otherwise), but in your case the abstract classes are pretty much interfaces, so you can turn them into interfaces and inherit from them (you can inherit from any number of interfaces).
No, abstract class whether having all abstract methods or only some, makes no difference as far as inheritance in concerned. you can inherit only one class (in C#) and as many interfaces as you want.
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.
I'm learning C# coming from C++ and have run into a wall.
I have an abstract class AbstractWidget, an interface IDoesCoolThings, and a class which derives from AbstractWidget called RealWidget:
public interface IDoesCoolThings
{
void DoCool();
}
public abstract class AbstractWidget : IDoesCoolThings
{
void IDoesCoolThings.DoCool()
{
Console.Write("I did something cool.");
}
}
public class RealWidget : AbstractWidget
{
}
When I instantiate a RealWidget object and call DoCool() on it, the compiler gives me an error saying
'RealWidget' does not contain a
definition for 'DoCool'
I can cast RealWidget object to an IDoesCoolThings and then the call will work, but that seems unnecessary and I also lose polymorphism (AbstractWidget.DoCool() will always be called even if i define RealWidget.DoCool()).
I imagine the solution is simple, but I've tried a variety of things and for the life of me can't figure this one out.
You're running into the issue because you used explicit interface implementation (EII). When a member is explicitly implemented, it can't be accessed through a class instance -- only through an instance of the interface. In your example, that's why you can't call DoCool() unless you cast your instance to IDoesCoolThings.
The solution is to make DoCool() public and remove the explicit interface implementation:
public abstract class AbstractWidget : IDoesCoolThings
{
public void DoCool() // DoCool() is part of the abstract class implementation.
{
Console.Write("I did something cool.");
}
}
// ...
var rw = new RealWidget();
rw.DoCool(); // Works!
In general, you use EII in two cases:
You have a class that must implement two interfaces, each of which contains a member that has an identical name/signature to another member in the other interface.
You want to force clients not to depend on the implementation details of your class, but rather on the interface that's being implemented by your class. (This is considered a good practice by some.)
The way you implement the interface is explicit implement void IDoesCoolThings.DoCool(), if you choose implicit implement interface.
public abstract class AbstractWidget : IDoesCoolThings
{
public void DoCool()
{
Console.Write("I did something cool.");
}
}
Then it will work.
Read this :
C# Interfaces. Implicit implementation versus Explicit implementation
Change your declaration to:
public abstract class AbstractWidget : IDoesCoolThings
{
public void DoCool()
{
Console.Write("I did something cool.");
}
}
You should do it this way:
public interface IDoesCoolThings
{
void DoCool();
}
public abstract class AbstractWidget
{
public void DoCool()
{
Console.WriteLine("I did something cool.");
}
}
public class Widget : AbstractWidget, IDoesCoolThings
{
}
Usage:
var widget = new Widget();
widget.DoCool();