First look at this code:
class Program
{
static void Main(string[] args)
{
var x =(Base) new Derived();
((IMethod)x).DoWork();
Console.ReadKey();
}
}
interface IMethod
{
void DoWork();
}
abstract class Base : IMethod
{
void IMethod.DoWork()
{
Console.WriteLine("Base.DoWork");
}
}
class Derived : Base, IMethod
{
public void DoWork()
{
//here I where I want to call base.DoWork();
Console.WriteLine("Derived.DoWork");
}
}
Output:
Derived.DoWork
Desired:
Base.DoWork
Derived.DoWork
I'm dealing with an API that exposes an interface that when implemented, the DoWork method will be called at some part of the procession.
Now in the above example, the class Base is a part of the API, that internally (in the API) already explicitly implements that interface and does some important executions in the DoWork method.
I need to override the implementation of the IMethod in my derived class as well, so I get notified when needed, the problem is I can't 'override' the method and call the base method, neither can I cast base to IMethod.
Any solution?
Note: reflection won't work since it's a Silveright project, and private method invoking is prohibited.
Are you able to just compose the classes, rather than using inheritance? Then you can implement DoWork() however you like, and still call DoWork() on the Base object as well. Since Base is abstract, you'll need to derive a dummy type to get everything to work.
class Derived : IMethod
{
private class SneakyBase : Base
{
// abstract implementations here
}
private IMethod baseObject = new SneakyBase();
void DoWork()
{
baseObject.DoWork();
// Custom DoWork code here
}
}
It's obviously a bit of pain to do things this way, but the API designers made an odd choice with the explicit interface implementation, and you're now paying for it.
Are you looking for:
public class Derived : Base
{
public override void DoWork()
{
base.DoWork();
}
}
I've found DanBryant's comment to be the answer, although as he mentions is a bit risky since we can't assure the implementer will call the base method, but is a decent way tho.
I made a protected virtual method that is called from the private interface implementer, then, in the derived class, instead of worrying about the interface, I just care about overriding the base class and calling the base implementation from it, that works perfect, example:
abstract class Base : IMethod
{
void IMethod.DoWork()
{
DoWork();
}
protected virtual void DoWork()
{
Console.WriteLine("Base.DoWork");
}
}
class Derived : Base
{
protected override void DoWork()
{
base.DoWork();
//here I where I want to call base.DoWork();
Console.WriteLine("Derived.DoWork");
}
}
Related
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.
What do I need to do to first run the method in the base class then run the same method in the derived class? Is this a good idea?
I want run common actions in base class and extend it in the derived class in the same method. Is this how it is usually done?
public abstract class MyBase
{
void DoStuff()
{
//some common implementation
}
}
public class MyDerived : MyBase
{
void DoStuff()
{
// DoStuff in the base first
// Then DoStuff in here
}
}
are you talking about something like that?
class base
{
protected virtual void method()
{
// do some stuff in base class, something common for all derived classes
}
}
class derived : base
{
public override void method()
{
base.method(); // call method from base
// do here some more work related to this instance of object
}
}
that's not a bad idea, I do use it a lot when I have some common functionality for all derived classes.
If you want to guarantee that the base class logic is run (and not rely on the derived class being polite), you can do this:
public void Method()
{
//Stuff that should always happen in base class
OnMethod();
}
protected virtual void OnMethod()
{
//Default base class implementation that derived class can either override or extend
}
Use base.TheMethod() to run a method in the base class from a derived class.
If you want to run a method of a derived class from a base class, then you have to cast the base class to the derived class. This means that your class needs to be aware of who is deriving it, which breaks encapsulation and should be avoided.
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");
//}
}
I have a base class and a class inheriting base. The base class has several virtual functions that the inherited class may override. However, the virtual functions in the base class has code that MUST to run before the inherited class overrides get called. Is there some way that I can call the base classes virtual functions first then the inherited class overrides. Without making a call to base.function().
I know I can simply make two functions, one that gets called, the other virtual. But is there a way I can keep the same names as well? I know I may need to change some things around.
class myBase
{
public virtual myFunction()
{ /* must-run code, Called first */ }
}
class myInherited : myBase
{
public override myFunction()
{ /* don't use base.myFunction();,
called from base.myFunction(); */ }
}
Similar question here.
C# doesn't have support for automatically enforcing this, but
you can enforce it by using the template method pattern. For example, imagine you had this code:
abstract class Animal
{
public virtual void Speak()
{
Console.WriteLine("I'm an animal.");
}
}
class Dog : Animal
{
public override void Speak()
{
base.Speak();
Console.WriteLine("I'm a dog.");
}
}
The trouble here is that any class inheriting from Animal needs to call base.Speak(); to ensure the base behavior is executed. You can automatically enforce this by taking the following (slightly different) approach:
abstract class Animal
{
public void Speak()
{
Console.WriteLine("I'm an animal.");
DoSpeak();
}
protected abstract void DoSpeak();
}
class Dog : Animal
{
protected override void DoSpeak()
{
Console.WriteLine("I'm a dog.");
}
}
In this case, clients still only see the polymorphic Speak method, but the Animal.Speak behavior is guaranteed to execute. The problem is that if you have further inheritance (e.g. class Dachshund : Dog), you have to create yet another abstract method if you want Dog.Speak to be guaranteed to execute.
A common solution that can be found in the .NET Framework is to split a method in a public method XXX and a protected, virtual method OnXXX that is called by the public method. For your example, it would look like this:
class MyBase
{
public void MyMethod()
{
// do something
OnMyMethod();
// do something
}
protected virtual void OnMyMethod()
{
}
}
and
class MyInherited : MyBase
{
protected override void OnMyMethod()
{
// do something
}
}
public abstract class BaseTemp
{
public void printBase() {
Console.WriteLine("base");
print();
}
public abstract void print();
}
public class TempA: BaseTemp
{
public override void print()
{
Console.WriteLine("TempA");
}
}
public class TempB: BaseTemp
{
public override void print()
{
Console.WriteLine("TempB");
}
}
There is no way to do what you're seeking other than the 2 ways you already named.
Either you make 2 functions in the base class, one that gets called and the other virtual.
Or you call base.functionName in the sub-class.
Not exactly. But I've done something similar using abstract methods.
Abstract methods must be overriden by derived classes. Abstract procs are virtual so you can be sure that when the base class calls them the derived class's version is called. Then have your base class's "Must Run Code" call the abstract proc after running. voila, your base class's code always runs first (make sure the base class proc is no longer virtual) followed by your derived class's code.
class myBase
{
public /* virtual */ myFunction() // remove virtual as we always want base class's function called here
{ /* must-run code, Called first */
// call derived object's code
myDerivedMustcallFunction();
}
public abstract myDerivedMustCallFunction() { /* abstract functions are blank */ }
}
class myInherited : myBase
{
public override myDerivedMustCallFunction()
{ /* code to be run in derived class here */ }
}
What do you think of this?
class myBase
{
public void myFunctionWrapper()
{
// do stuff that must happen first
// then call overridden function
this.myFunction();
}
public virtual void myFunction(){
// default implementation that can be overriden
}
}
class myInherited : myBase
{
public override void myFunction()
{
}
}
I have an interface so class writers are forced to implement certain methods. I also want to allow some default implemented methods, so I create a abstract class. The problem is that all classes inherit from the base class so I have some helper functions in there.
I tried to write : IClass in with the abstract base, but I got an error that the base didn't implement the interface. Well of course because I want this abstract and to have the users implement those methods. As a return object if I use base I can't call the interface class methods. If I use the interface I can't access base methods.
How do I make it so I can have these helper classes and force users to implement certain methods?
Make sure methods in the base class have the same name as the interface, and they are public. Also, make them virtual so that subclasses can override them without hiding them.
interface IInterface {
void Do();
void Go();
}
abstract class ClassBase : IInterface {
public virtual void Do() {
// Default behaviour
}
public abstract void Go(); // No default behaviour
}
class ConcreteClass : ClassBase {
public override void Do() {
// Specialised behaviour
}
public override void Go() {
// ...
}
}
Move the interface methods into the abstract class and declare them abstract as well. By this, deriving classes are forced to implement them. If you want default behaviour, use abstract classes, if you want to only have the signature fixed, use an interface. Both concepts don't mix.
Having faced with the same problem recently, I've came up with a somewhat more elegant (to my mind) solution. It looks like:
public interface IInterface
{
void CommonMethod();
void SpecificMethod();
}
public abstract class CommonImpl
{
public void CommonMethod() // note: it isn't even virtual here!
{
Console.WriteLine("CommonImpl.CommonMethod()");
}
}
public class Concrete : CommonImpl, IInterface
{
void SpecificMethod()
{
Console.WriteLine("Concrete.SpecificMethod()");
}
}
Now, according to C# spec (13.4.4. Interface mapping), in the process of mapping IInterface on Concrete class, compiler will look up for CommonMethod in CommonImpl too, and it doesn't even have to be virtual in the base class!
The other significant advantage, compared to Mau's solution, is that you don't have to list every interface member in the abstract base class.