I understand this is not possible in C#, because Generics are not Templates and they are implented differently(processing on runtime rather than during compile):
public class Foo<T> : T
{
}
The question though remains. Is there an equivalent or perhaps an alternative way to achieve this?
In my case I have three different parent classes I want to inherit from, let's call them A, B, C:
public class A {}
public class B {}
public class C {}
Then I have the Foo class, and then MANY more inheriting from Foo but each of them needing only one of the A, B, C:
public class X : Foo<A> {}
public class Y : Foo<B> {}
public class Z : Foo<C> {}
So class X needs all the functionality in Foo and all the functionality in A, Y from Foo and B and so on...
How do I do this in C#?
I suppose you cannot modify the A, B and C classes (otherwise you could just inherit all of them from Foo), therefore this is my idea:
I would define "Foo" as an interface (that I would call "IFoo"):
public interface IFoo {}
and implement all its methods as Extension methods:
public static class IFooExtension
{
public static void Method1(this IFoo f) {
// do whatever here;
}
}
then I'd declare the further classe as follow:
public class X : A, IFoo {}
public class Y : B, IFoo {}
public class Z : C, IFoo {}
The only issue (if it is an issue) is that you can implement whatever method you want, but no properties.
If class X: Foo<A> { } needs all the functionality of Foo and A, then simply expose A as property and you will have it:
public class Foo<T> where T : new()
{
public T Bar { get; } = new T();
public void FooMethod() => Console.WriteLine("Foo method");
}
public class A
{
public void AMethod() => Console.WriteLine("A method");
}
public class X : Foo<A>
{
}
class Program
{
static void Main(string[] args)
{
var x = new X();
x.FooMethod();
x.Bar.AMethod(); // access via property
}
}
Related
Is there a design pattern that allows me to define an object as extending another object depending on the functionality that I need? For example if I have the following classes:
using System;
namespace C_
{
public class Program
{
static void Main(string[] args)
{
Console.WriteLine("------------------------------------------------");
C c = new C();
Console.WriteLine("------------------------------------------------");
D d = new D();
Console.WriteLine("------------------------------------------------");
}
}
public abstract class A
{
public A() {}
public void DoSomething()
{
Console.WriteLine("Doing something");
}
public virtual void CallSomething()
{
DoSomething();
}
}
public abstract class B : A
{
public B() {}
public override void CallSomething()
{
Console.WriteLine("Do something extra");
DoSomething();
}
}
public class C : A
{
public C()
{
CallSomething();
}
}
public class D : B
{
public D()
{
CallSomething();
}
}
}
Essentially class B is an extension of class A. The minimum functionality I want is defined in class A while in some cases I may need more which is defined in class B. Is it possible to have another class extend either A or B depending on what functionality is required at runtime? I could create separate classes each extending A or B but that would lead to a lot of repeated code
EDIT: Added a more concrete example. Classes C and D have the same functionality except that D requires a little more done before it can proceed hence extending B instead of A. Is there a design pattern where I wouldn't need class D and can pick and choose the functionality of C based on what I need?
I have a couple of classes which realize one method:
class ClassA : BaseClass
{
void Copy(ClassA a) {}
}
class ClassB : BaseClass
{
void Copy(ClassB b) {}
}
I want to describe these methods in the interface. Is it possible?
you can make use of Generic interface , for example as below
interface ICopy<T>
{
void Copy<T>(T t)
}
Class A: ICopy<A>,BaseClass//(if you need baseclass)
{
public void Copy(A a)
{}
}
Class B: ICopy<B>,BaseClass//(if you need baseclass)
{
public void Copy(B b)
{}
}
you can also try ICloneable inbuilt interface , if you just want to make clone of you class,
This is just suggestion
class Rock : ICloneable
{
int _weight;
bool _round;
bool _mossy;
public Rock(int weight, bool round, bool mossy)
{
this._weight = weight;
this._round = round;
this._mossy = mossy;
}
public object Clone()
{
return new Rock(this._weight, this._round, this._mossy);
}
}
Use a generic interface. Using where you can constrain the type parameter T to BaseClass and its derived types.
interface Interface<T> where T : BaseClass
{
void Copy<T>(T t);
}
class ClassA : BaseClass, Interface<ClassA>
{
public void Copy(ClassA b) {}
}
class ClassB : BaseClass, Interface<ClassB>
{
public void Copy(ClassB b) {}
}
I have the following classes:
public class A {
public C mObj:
}
public class B : A {
}
public class C {
public int val1;
}
public class D : C {
public int val2;
}
I initialize all the class B instances in a way that inst.mObj = new D();. When I have an instance of class B I would ideally like to access all the members of class D by using mObj, but due to inheritance I can not do it without casting into D first.
I would like class B to have a member object of class D, but I automatically inherit a member from class C. Is there a way to achieve something like that? If not then how is it usually done when a similar structure is required?
It is difficult to determine your exact requirements, but you could try to use generics with type constraints:
public class A<T>
where T : C
{
public T mObj:
}
public class B : A<D>
{
}
public class C
{
public int val1;
}
public class D : C
{
public int val2;
}
In this case the mObj in B will be of type D, so no conversion will be required.
How would I override DerivedZ() in the child, without having to specify a U in the base class? The latter solution appears a bit excessive.
public abstract class Z {}
public class DerivedZ : Z
{
public DerivedZ (B someB, int num)
{
// initialize here
}
}
// will not compile
// error: 'B.GetZ(B, int)': no suitable method found to override
// error: 'B' does not implement inherited abstract member 'A<DerivedZ>.GetZ(A<DerivedZ>, int)'
public abstract class A<T> where T : Z
{
public abstract T GetZ (A<T> inputA, int optional=1);
}
public class B : A<DerivedZ>
{
public override DerivedZ GetZ (B someB, int optional=1)
{
return new DerivedZ (someB, optional)
}
}
this works though...
public abstract class A<T,U> where T : Z where U : A<T,U>
{
public abstract T GetZ (U inputA, int optional=1);
}
public class B : A<DerivedZ,B>
{
public override DerivedZ GetZ (B someB, int optional=1)
{
return new DerivedZ (someB, optional);
}
}
You can't use the first form, because it's not properly overriding the method. If you could do that, imagine this code:
public class C : A<DerivedZ> {}
A<DerivedZ> x = new B();
x.GetZ(new C());
That should work fine, after all - A<T>.GetZ is just declared to accept an A<T>, and C is a A<DerivedZ>.
The approach you've shown is fine.
I agree that sometimes it would be useful to be able to say "something of the same type" but it's not part of the C# type system.
I have a theoretical question concerning how to deal with the following scenario in a language which does not allow multiple inheritance.
Imagine I have a base class Foo and from it I am wishing to create three sub-classes:
Class Bar inherits Foo and implements functionality "A"
Class Baz inherits Foo and implements functionality "B"
Class Qux inherits Foo and implements functionalities "A" and "B"
Imagine that the code to implement functionalities "A" and "B" is always the same. Is there a way to write the code for "A" and "B" only once, and then have the appropriate classes apply (or "inherit") it?
Well the only way I can see you achieving this in C#/Java is by composition. Consider this:
class Foo {
}
interface A {
public void a();
}
interface B {
public void b();
}
class ImplA implements A {
#Override
public void a() {
System.out.println("a");
}
}
class ImplB implements B {
#Override
public void b() {
System.out.println("b");
}
}
class Bar extends Foo {
A a = new ImplA();
public void a() {
a.a();
}
}
class Baz extends Foo {
B b = new ImplB();
public void b() {
b.b();
}
}
class Qux extends Foo {
A a = new ImplA();
B b = new ImplB();
public void b() {
b.b();
}
public void a() {
a.a();
}
}
Now Qux has both the functionality of Foo via normal inheritance but also the implementations of A and B by composition.
The more general term for this is a Mixin. Some languages provide support out of the box, such as Scala and D. There are various ways to achieve the same results in other languages though.
One way you can create a pseudo-mixin in C# is to use empty interfaces and provide the methods with extension methods.
interface A { }
static class AMixin {
public static void aFunc(this A inst) {
... //implementation to work for all A.
}
}
interface B { }
static class BMixin {
public static void bFunc(this B inst) {
...
}
}
class Qux : Foo, A, B {
...
}
This is achievable in languages providing traits (here: scala):
class Foo {
def fooM() {}
}
trait A {
def aFunc() {}
}
trait B {
def bFunc() {}
}
class Bar extends Foo with A {}
class Baz extends Foo with B {}
class Qux extends Foo with A with B {}
Because Scala runs on top of Java (having neither multiple inheritance nor traits) it is translated into something like this (simplified) - which might be a hint how to implement it in Java/C# manually:
class Foo {
}
interface A {
void aFunc();
}
interface B {
void bFunc();
}
class Bar extends Foo implements A {
public void aFunc() {
$A.aFunc();
}
}
class Baz extends Foo implements B {
public void bFunc() {
$B.bFunc();
}
}
class Qux extends Foo implements A, B {
public void aFunc() {
$A.aFunc();
}
public void bFunc() {
$B.bFunc();
}
}
class $A {
public static void aFunc() {}
}
class $B {
public static void bFunc() {}
}
There are several ways to do something like this. More specifically, if we abandon the inheritance aspect for a moment, there are ways to introduce the same unit of functionality to different classes, while writing the unit only once.
Okay, I love AOP Frameworks, and they exist for many languages (C# and Java having several). AOP Frameworks basically allow you add self-contained functionality into different classes throughout your inheritance structure.
For C#, you have PostSharp and for Java you have AspectJ, among many others.
Many AOP frameworks allow 'hijacking' or 'overriding' method calls without using inheritance.