I have one base class:
class BaseClass{ }
I want to implement interface, that uses base class:
public interface IClass
{
BaseClass GetValue();
}
and then create child classes, that implemented this interface, but in method returned thier own type of class:
class Child: BaseClass, IClass
{
Child GetValue();
}
How can I do that correctly?
Use generics:
public interface IClass<T> where T : BaseClass
{
T GetValue();
}
class Child: BaseClass, IClass<Child>
{
Child GetValue();
}
That's not so easy. The way you would expect it to work doesn't because any class implementing IClass must exactly implement the method from the interface (If you've got an instance of IClass, how would you know that the static type returned by GetValue() is Child? There are workarounds using Generics, but they may be not so nice to use, i.e.
public T GetValue<T>();
where you can exactly specify the type you want to have returned. This, however, may get really ugly if used in combination with virtual overloads. Another alternative is creating a new implementation:
class Child: BaseClass, IClass
{
BaseClass IClass.GetValue();
new Child GetValue();
}
Like this, you get a BaseClass if the static type of the object in the call is IClass and a Child if it is statically a Child.
Remember that
IClass a = new Child();
Child b = a.GetValue(); // Error: Cannot convert BaseClass to Child
will never work, since the static return type of IClass.GetValue() is BaseClass.
Related
The question I asked here brought me thus far on my project:
abstract class Base<T> where T:ContentBase
{
//base functionality
}
class Foo : Base<FooContent>
{
//overrides
}
class Bar : Base<BarContent>
{
//overrides
}
Is there a way to make an array or List that can hold both Foo and Bar objects or would I have to look at making a record or something?
Thanks in advance.
You could add a non-generic abstract class Base that Base<T> inherits from, and then make a List<Base>. So extending your code might look something like this:
abstract class Base
{
//base functionality (non-generic)
}
abstract class Base<T> : Base where T : ContentBase
{
//base functionality acting on T
}
class Foo : Base<FooContent>
{
//overrides
}
class Bar : Base<BarContent>
{
//overrides
}
I'm assuming from the example code that FooContent and BarContent derive from ContentBase. I'm also assuming that the code follows the SOLID principles and especially LSP.
A generic collection could be defined in terms of ContentBase.
var collection = new List<ContentBase>();
The collection can hold instances of FooContent, BarContent, and ContentBase.
But apparently that is a need to hold the ContentBase and derived types within another type. (It is not clear from the question why this is needed.)
public class ContentAdaptor
{
public ContentAdaptor(ContentBase content)
{
_content = content;
}
...
private ContentBase _content;
}
The ContentAdaptor type holds a reference to an instance of ContentBase. The instance could be of a derived type.
A collection of ContentAdaptor:
var collection = new List<ContentAdaptor>();
I have a suspicion that FooContent and BarContent may violate LSP and are not substitutable for each other. Is that the case? If you have types that are fundamentally different but you want to process the objects in a polymorphic way, consider an interface.
public interface IProcess
{
bool Process();
}
Here ContentBase implements the IProcess interface. But note that a SomeOtherContent type that doesn't derive from ContentBase can also implement the IProcess interface.
public class ContentBase : IProcess
{
// Implement the IProcess interface.
}
A collection of IProcess:
var collection = new List<IProcess>();
The Process method can be called on each object in the collection. Each type provides its own implementation of the Process method.
This question already has answers here:
Can a child class implement the same interface as its parent?
(5 answers)
Closed 3 years ago.
For example,I have a base class like this:
class Base: ICloneable
{
public object Clone()
{
//something
}
}
And is there any difference between these Derived classes?
class Derived: Base, ICloneable
{
//something
}
class Derived: Base
{
//something
}
Well, you may need to do it; here is a possible scenario:
When cloning, we usually want to return not universal object by cloned object type:
class Base {
// We can't declare this method as virtual: Derived class will return Derived
public Base Clone() {
...
}
}
...
Base test = new Base();
Base duplicate = test.Clone();
If we want to implement IClonable we have to do it explictly (in order to solve Clone() names conflict):
class Base : ICloneable {
public Base Clone() {
...
}
// This method is efficiently private, and that's why can't be virtual
object ICloneable.Clone() => Clone();
}
Now we want a Derived class which can be cloned as well:
class Derived : Base {
// Note that now we have new Clone method which returns Derived instance (not Base one)
public new Derived Clone() {
...
}
}
...
Derived test = new Derived();
Derived duplicate = test.Clone();
If we keep it like this then we'll have wrong behaviour:
Derived test = new Derived();
// This will be wrong:
// Base.ICloneable.Clone() will be called which executes "Base Clone()" method
// instead of expected "new Derived Clone()"
object clone = (test as IClonable).Clone();
So we have to reimplement IClonable interface:
class Derived : Base, ICloneable {
// Please, note that now we have new Clone method which returns Derived instance
public new Derived Clone() {
...
}
// This ICloneable implementation will call Derived Clone()
object ICloneable.Clone() => Clone();
}
...
// Derived ICloneable.Clone() will be called
// which executes "new Derived Clone()" method
object clone = (test as IClonable).Clone();
Actually, you can do this, if your derived class provides its own (explicit) implementation of the interface:
class B : A, Icloneable
{
object IClonable.Clone()
{
// do something completely new
}
}
As this is an explicit implementation, you have to cast your B-instance to the interface in order to call the new implementation. Otherwise the implementation from A is used (see my fiddle: https://dotnetfiddle.net/tuUnCm).
No, you do not have to implement the interface in the derived class. The derived class can also be cast as the interface type when the base class implements the interface. Like this:
public interface ISomething
{
void Say();
}
public abstract class BaseClass : ISomething
{
public void Say(string something)
{
Console.WriteLine("Say: " + something);
}
}
public class DerivedClass : BaseClass
{
}
var x = new DerivedClass() as ISomething;
x.Say("Derived");
If understanding the problem correctly, you don't need to inherit for base and ICloneable as base already inherits from it.
This is fine
class base:ICloneable
{
public object Clone()
{
//something
}
}
This is fine too, as derived1 will inherit ICloneable, as its part of your base class
class derived1:base
{
//something
}
However this is saying it will, inherit from base (which already inherits from ICloneable) AND ICloneable. Its redundant and defeats the purpose of having an interface.
class derived1:base,Icloneable
{
//something
}
Hope that helps!
No need, when your base class is implementing interface or other classes all the properties and functionality introduced in base class will inherit to derived class.
I am having trouble overriding a method that returns a generic class that is strongly typed.
This is just setting up for the example below
public class Something : ISomething {}
public interface ISomething {}
My issue is with the Clone method below, only the second one works.
public class ClassA : AbstractClass<Something>
{
public override ClassA Clone() // <--- this doesn't work
{
return this; // this is just dummy code
}
public override AbstractClass<Something> Clone() // <-- this works
{
return this; // this is just dummy code
}
}
public abstract class AbstractClass<T> where T : ISomething
{
public abstract AbstractClass<T> Clone();
}
I would prefer to have the first Clone method, as the name of the class doesn't change. But only the second method compiles. Is there any way to avoid this?
The first override changes the actual signature:
Your method on the base class establishes that it will return an AbstractClass<T>.
Since your subclass says that T is going to be something, this now implies that the method returns an AbstractClass<Something>. While ClassA is an AbstractClass<Something> that doesn't mean that all AbstractClass<Something> are a ClassA.
Consider the following example:
AbstractClass<Something> myVar = new ClassA(); // works, because ClassA is an AbstractClass<Something>
// Now we hold a reference to an AbstractClass<Something>
myVar.Clone(); // => This should return an AbstractClass<Something> not ClassA
Extending the example: consider a ClassB:
public class ClassB : AbstractClass<Something> {}
Both ClassA and ClassB are AbstractClass<Something>. If we retake the first example:
AbstractClass<Something> myVar;
myVar = new ClassA();
myVar = new ClassB();
// Here you don't know whether it's a ClassA or ClassB, you only know it's an `AbstractClass<Something>`
myVar.Clone();
You could add another generic parameter for the return type:
public abstract class AbstractClass<TC, T> where TC: AbstractClass<TC, T> where T : ISomething
{
public abstract TC Clone();
}
then change ClassA to:
public class ClassA : AbstractClass<ClassA, Something>
{
public override ClassA Clone()
{
return this;
}
}
It is impossible to change the return type while overriding a method.
You could remove the override from the first implementation, and this will define a new method. But, because you have that abstract Clone method in the base class, you are forced to implement it as well, and you'll end up with 2 methods with the same name, and the same parameters, so the code will not compile.
Do you really need that Clone method in the abstract class ?
I think you should remove the Clone method from the abstract class, and add the Clone method in the derived class (perhaps also implementing ICloneable interface).
I currently have a small object hierarchy that looks like this:
public class BaseClass {
// this class is empty and exists only so the others can extend it and share the
// same base type
}
public class ChildA : BaseClass {
public Subject<AssociatedClassA> Results;
}
public class ChildB : BaseClass {
public Subject<AssociatedClassB> Results;
}
In my design I would like to enforce that every class that extends from BaseClass should contain a Subject<SomeType> called Results. I'm wondering if there is a way that I can move Results into the base class or an interface such that I can supply the generic type for the Subject when constructing the base class. For example, it would be awesome if I could do something like this:
ChildA<AssociatedClassA> instance = new ChildA<AssociatedClassA>();
Or even better since there should really only be one template parameter that matches with ChildA if when I constructed it that could be taken care of for me:
ChildA instance = new ChildA();
// Results is automatically set to Subject<AssociatedClassA>
I'm stuck trying to implement this now as if I try to move Results into the base class the Subject requires a template parameter which I can't necessarily supply. There could potentially be more than 2 derived classes and I don't like the idea that someone extending this system has to know to add Results manually to each child class.
Following the suggestions of the 2 answers below this solves my desire to move Results into the base class, however I've run into another issue in that I was hoping to be able to use BaseClass as a generic parameter to methods such that any of the derived classes could be used. For example:
public void ProcessBaseClass(BaseClass base) {
// base could be ChildA or ChildB here
}
This no longer works since BaseClass now requires a type argument. Is there any way that I can have the best of both worlds here or am I stuck due to my design choices?
If appropriate, you can make the parent generic:
public class BaseClass<T> {
public Subject<T> Results;
}
public class ChildA : BaseClass<AssociatedClassA> {
}
public class ChildB : BaseClass<AssociatedClassB> {
}
You can make the base class itself generic:
public class BaseClass<T> {
public T Results { get; protected set; }
}
I have class A:
public class ClassA<T>
Class B derives from A:
public class ClassB : ClassA<ClassB>
Class C derives from class B:
public class ClassC : ClassB
Now I have a generic method with constraints
public static T Method<T>() where T : ClassA<T>
OK, now I want to call:
ClassC c = Method<ClassC>();
but I get the compile error saying:
Type argument 'ClassC' does not inherit from or implement the constraint type 'ClassA<ClassC>.
Yet, the compiler will allow:
ClassB b = Method<ClassB>();
My understanding is that this fails because ClassC inherits ClassA<ClassB> instead of ClassA<ClassC>
My real question is, is it possible to create a class deriving from ClassB that can be used in some way with the generic method?
This may seem like generics are overused and I would agree. I am trying to create business layer objects deriving from the subsonic data objects in a separate project.
Note: I have put the < T > with extra spaces otherwise they get stripped from the question.
Well, you could change Method to:
public static T Method<T,U>() where T : ClassA<U> where U : T
Does that help at all? It's not much use if you can't change Method of course...
No. You must change or wrap this method.
Here is the reason.
ClassC inherits from ClassB which inherits from ClassA(ClassB)
ClassC does not inherit from ClassA(ClassC)
No child of ClassB will inherit from ClassA(child class), because they instead inherit from ClassB and ClassB does not inherit from ClassA(child class).
Generic types are invariant.
In most cases it is possible to solve this scenario by having a base non-generic abstract class:
public abstract class BasicClassA
{
}
public class ClassA<T> : BasicClassA
{
}
public class ClassB : ClassA<ClassB>
{
}
public class ClassC : ClassB
{
}
public static T Method<T>() where T : BasicClassA
{
return null;
}
Your mileage may vary, though.