inherit from two classes in C# [duplicate] - c#

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Multiple Inheritance in C#
I have two classes Class A and Class B. These two classes cannot inherit each other. I am creating new class called Class C. Now, I want to implement the methods in class A and class B by inheriting. I am aware that multiple inheritance is not possible in C# but is there any other way to do this?

Multitiple inheritance is not possible in C#, however it can be simulated using interfaces, see Simulated Multiple Inheritance Pattern for C#.
The basic idea is to define an interface for the members on class B that you wish to access (call it IB), and then have C inherit from A and implement IB by internally storing an instance of B, for example:
class C : A, IB
{
private B _b = new B();
// IB members
public void SomeMethod()
{
_b.SomeMethod();
}
}
There are also a couple of other alternaitve patterns explained on that page.

An common alternative to inheritance is delegation (also called composition): X "has a" Y rather than X "is a" Y.
So if A has functionality for dealing with Foos, and B has functionality for dealing with Bars, and you want both in C, then something like this:
public class A() {
private FooManager fooManager = new FooManager(); // (or inject, if you have IoC)
public void handleFoo(Foo foo) {
fooManager.handleFoo(foo);
}
}
public class B() {
private BarManager barManager = new BarManager(); // (or inject, if you have IoC)
public void handleBar(Bar bar) {
barManager.handleBar(bar);
}
}
public class C() {
private FooManager fooManager = new FooManager(); // (or inject, if you have IoC)
private BarManager barManager = new BarManager(); // (or inject, if you have IoC)
... etc
}

If you want to literally use the method code from A and B you can make your C class contain an instance of each. If you code against interfaces for A and B then your clients don't need to know you're giving them a C rather than an A or a B.
interface IA { void SomeMethodOnA(); }
interface IB { void SomeMethodOnB(); }
class A : IA { void SomeMethodOnA() { /* do something */ } }
class B : IB { void SomeMethodOnB() { /* do something */ } }
class C : IA, IB
{
private IA a = new A();
private IB b = new B();
void SomeMethodOnA() { a.SomeMethodOnA(); }
void SomeMethodOnB() { b.SomeMethodOnB(); }
}

Use composition:
class ClassC
{
public ClassA A { get; set; }
public ClassB B { get; set; }
public C (ClassA a, ClassB b)
{
this.A = a;
this.B = b;
}
}
Then you can call C.A.DoA(). You also can change the properties to an interface or abstract class, like public InterfaceA A or public AbstractClassA A.

Make two interfaces IA and IB:
public interface IA
{
public void methodA(int value);
}
public interface IB
{
public void methodB(int value);
}
Next make A implement IA and B implement IB.
public class A : IA
{
public int fooA { get; set; }
public void methodA(int value) { fooA = value; }
}
public class B : IB
{
public int fooB { get; set; }
public void methodB(int value) { fooB = value; }
}
Then implement your C class as follows:
public class C : IA, IB
{
private A _a;
private B _b;
public C(A _a, B _b)
{
this._a = _a;
this._b = _b;
}
public void methodA(int value) { _a.methodA(value); }
public void methodB(int value) { _b.methodB(value); }
}
Generally this is a poor design overall because you can have both A and B implement a method with the same name and variable types such as foo(int bar) and you will need to decide how to implement it, or if you just call foo(bar) on both _a and _b. As suggested elsewhere you should consider a .A and .B properties instead of combining the two classes.

You can define a base class for A and B where you can hold a common methods/properties/fields of those.
After implement C:Base.
Or in order to simulate multiple inheritance, define a common interface(s) and implement them in C
Hope this helps.

Do you mean you want Class C to be the base class for A & B in that case.
public abstract class C
{
public abstract void Method1();
public abstract void Method2();
}
public class A : C
{
public override void Method1()
{
throw new NotImplementedException();
}
public override void Method2()
{
throw new NotImplementedException();
}
}
public class B : C
{
public override void Method1()
{
throw new NotImplementedException();
}
public override void Method2()
{
throw new NotImplementedException();
}
}

Related

Workaround to template specialization

Abstract Base Class
abstract class Base
{
public abstract void Method<T>(T args) { }
}
Derived Class several of these exist, each with a replacement for T
class Derived : Base
{
public override void Method<int>(int args) { }
}
Now, I know this isn't supported, but I need it because somewhere else in my code
class AnotherBaseClass<T, E>
where E : Base
{
E e; // Will be actually one of the derived classes
T t; // Simple types like string or int
// I want this to work basically
public void Func()
{
e.Method(t);
}
}
this is going on.
What is the easiest solution to this? If I am able to do this, it will save me from writing a lot of code.
If you're fine with making Base a generic class, this is how:
abstract class Base<T>
{
public abstract void Method(T args);
}
class Derived : Base<int>
{
public override void Method(int args) {}
}
class AnotherBaseClass<T, E>
where E : Base<T>
{
E e;
T t;
public void Func()
{
e.Method(t);
}
}
If you need Base to not be generic, you can add this:
abstract class Base
{
public void Method<T>(T args)
{
var genericSelf = this as Base<T>;
genericSelf.Method(args);
}
}
and make Base<T> inherit Base and ensure your concrete classes always derive Base<T> and never Base directly.
I'm not exactly sure what problem you're sovling but under .Net 5 what you have in your question works as is...
abstract class Base
{
public abstract void Method<T>(T args);
}
class Derived : Base
{
public override void Method<T>(T args)
{
Console.Write($"{nameof(Derived)}.Method<{typeof(T)}>({args})");
}
}
class AnotherBaseClass<T, E> where E : Base
{
readonly E e;
readonly T t;
public AnotherBaseClass(T t, E e)
{
this.e = e;
this.t = t;
}
public void Func()
{
e.Method(t);
}
}
Run like so:
static void Main(string[] args)
{
Derived d = new Derived();
var another = new AnotherBaseClass<int,Derived>(5, d);
another.Func();
}
Produces: "Derived.Method<System.Int32>(5)"

Use a subset of methods as member methods in different classes in C#

Is it possible to use a subset of methods as member methods in different classes in C#?
For instance, I have four functions the void A(), the void B(), the void C() and the void D().
And now I want to have three classes. The first class I would like to have the member methods A, B and C. The second I would like to have the B and D. And the third the A, C and D.
How could I achieve this? Is it possible to achieve this only by using interfaces or are there any other approaches?
If all your three classes should have different implementations for these methods, then classic interfaces are the way to go:
public interface IHaveA {
void A();
}
public interface IHaveB {
void B();
}
public interface IHaveC {
void C();
}
public interface IHaveD {
void D();
}
public class Class1 : IHaveA, IHaveB, IHaveC { // Implement A, B and C here }
public class Class2 : IHaveB, IHaveD { // Implement B and D here }
public class Class3 : IHaveA, IHaveC, IHaveD { // Implement A, C and D here }
If all your classes should have the same implementation of A, B, C and D, you could still use interfaces but you would have to duplicate code:
public static class StaticImplementation {
public void A(IHaveA sender) {
// Do stuff here
}
public void B(IHaveB sender) {
// Do stuff here
}
public void C(IHaveC sender) {
// Do stuff here
}
public void D(IHaveD sender) {
// Do stuff here
}
}
public class Class1 : IHaveA, IHaveB, IHaveC {
public void A() { StaticImplementation.A(this) }
public void B() { StaticImplementation.B(this) }
public void C() { StaticImplementation.C(this) }
}
public class Class2 : IHaveB, IHaveD { // Calls to StaticImplementation for B and D here }
public class Class3 : IHaveA, IHaveC, IHaveD { // Calls to StaticImplementation for A, C and D here }
There is no way to enforce that these three classes have the same implementation of these methods using interfaces, because the primary goal interfaces is to ensure that classes implement methods, specifically with different implementations!
This changes in C# 8.0 and .NET Core 3.0 where you can have default implementations for interface methods, and you could ensure that these implementations do not change by sealing them.
The code would become:
public interface IHaveA {
sealed void A() {
// Implementation here
}
}
public interface IHaveB {
sealed void B() {
// Implementation here
}
}
public interface IHaveC {
sealed void C() {
// Implementation here
}
}
public interface IHaveD {
sealed void D() {
// Implementation here
}
}
public class Class1 : IHaveA, IHaveB, IHaveC { // Nothing to do here }
public class Class2 : IHaveB, IHaveD { // Nothing to do here }
public class Class3 : IHaveA, IHaveC, IHaveD { // Nothing to do here }
using System;
public interface MA {}
public static class MAProvider {
public static void A(this MA obj) { Console.WriteLine("MA"); }
}
public interface MB {}
public static class MBProvider {
public static void B(this MB obj) { Console.WriteLine("MB"); }
}
public interface MC {}
public static class MCProvider {
public static void C(this MC obj) { Console.WriteLine("MC"); }
}
public interface MD {}
public static class MDProvider {
public static void D(this MD obj) { Console.WriteLine("MD"); }
}
public class First : MA, MB, MC {}
public class Second : MB, MD {}
public class Third : MA, MC, MD {}
public static class Program {
public static void Main() {
new First().A();
new First().B();
new First().C();
new Second().B();
new Second().D();
new Third().A();
new Third().C();
new Third().D();
}
}
The only issue which is left is the private members. Since we can not access them inside the extension methods.

Return "this" of the subclass from superclass C#

I need to return "this" or the subclass instance from the superclass.
interface IA
{
IA Format();
void Print();
}
interface IB
{
IA Format();
void Print();
void PrintB();
}
abstract class A : IA
{
protected bool isFormated;
public IA Format()
{
isFormated = true;
return this;
}
virtual public void Print()
{
Console.WriteLine("this is A");
}
}
class B : A, IB
{
override public void Print()
{
Console.WriteLine("this is B");
}
public void PrintB()
{
if (isFormated)
{
Console.WriteLine("this is formated B");
}
else
{
Console.WriteLine("this is B");
}
}
}
class Program
{
static void Main(string[] args)
{
var x = new B();
x.Format().PrintB();
}
}
I have two Classes, class A is superclass and class B is the subclass inherited from A.
those two classes implementing Interface A and B.
I need to call 'x.Format().PrintB();' just to format the string.
in other words, I need to return the same object in Format() function and based on the changes in the Format() I need to change the PrintB behavior.
so if I created new Class D and inherits A I want to Implement PrintD with different behavior based on isFormated as well.
I made A as a generic class take type T and I returned this as T
interface IA<T> where T : class
{
T Format { get; }
void Print();
}
abstract class A<T> : IA<T> where T : class
{
protected bool isFormated;
public T Format
{
get
{
isFormated = true;
return this as T;
}
}
virtual public void Print()
{
Console.WriteLine("this is A");
}
}
interface IB
{
void Print();
void PrintB();
}
class B : A<B>, IB
{
override public void Print()
{
Console.WriteLine("this is B");
}
public void PrintB()
{
if (isFormated)
{
Console.WriteLine("this is formated B");
}
else
{
Console.WriteLine("this is B");
}
}
}
class Program
{
static void Main(string[] args)
{
var x = new B();
x.Format.PrintB();
}
}
When you call x.Next(), it returns an instance of something that implements IA. That could be A, B, or any other class that implements IA, but because it returns IA, other classes don't know what the concrete type is. That's usually intentional. If other classes don't know what the implementation of IA is then you can replace one with another.
However, since Next returns an IA, the only way you can call PrintB is if IA has a PrintB method, like this:
public interface IA
{
void PrintB();
}
If A implements IA then it must have a PrintB method. But because it's an abstract class the method can be abstract. That means A doesn't really implement the method. A non-abstract class that inherits from A would be required to implement it.
That would look like this:
abstract class A : IA
{
public IA Next()
{
return this;
}
virtual public void Print()
{
Console.WriteLine("this is A");
}
public abstract void PrintB();
}
PrintB doesn't have to be abstract. A could implement it. It could be virtual and other classes could override it. Or it could be neither abstract nor virtual, and other classes couldn't override it.
Now, when you declare B : A, the compiler will require B to implement PrintB.
Then you can call Next().PrintB() because Next() returns IA and IA has a PrintB method.

C# class constructors creating infinite loop

So lets say I have two classes. Class A and Class B like this:
Class A
{
B classB;
public A
{
classB = new B();
}
public void funcIHaveToUseInClassB()
{
}
}
Class B
{
A classA;
public B
{
classA = new A();
}
public void funcIHaveToUseInClassA()
{
}
}
As you can see both classes contain functions that need to be used in the other class. Class A has a function that class B has to use and the other way around. No I can't just put the functions in the other class because they heavily rely on the class they are currently in. So how would I go about doing this? With my method I create an infinite loop and get a stack overflow exception. I hope someone can help me out, thanks in advance.
EDIT:
People are asking me why I need these 2 classes to rely on each other so here it is: Class A manages everything that has to do with a WebBrowser control and class B Manages everything that has to do with a certain page in my program. Class A is being used by multiple pages, which is the reason it needs to be a seperate class. Class A sometimes needs to push info to class B. Class B sometimes needs info from the WebBrowser control class A is managing and that is why it calls a function.
Make classB and classA into public properties and initialize them from another class instead of constructor.
class A
{
public B classB { get; set; }
public void funcIHaveToUseInClassB()
{
}
public void anotherF()
{
classB.funcIHaveToUseInClassA();
}
}
class B
{
public A classA { get; set; }
public void funcIHaveToUseInClassA()
{
}
public void anotherF()
{
classA.funcIHaveToUseInClassB();
}
}
static void main()
{
// entry point
var a = new A();
var b = new B();
a.classB = b;
b.classA = a;
// do what ever you want with a and b
}
You need to pass an instance of one of your classes to the constructor of the other class.
Try this:
Class A
{
B classB;
public A()
{
classB = new B(this);
}
public void funcIHaveToUseInClassB()
{
}
}
Class B
{
A classA;
public B(A arg)
{
classA = arg;
}
public void funcIHaveToUseInClassA()
{
}
}
Update
Or just pass in the instance as a parameter to the methods like in Matt Jacobsen's answer.
Create a private and public accessor, and instantiate the property only when the private object is null, like so:
class A
{
private B _b;
public B b {
get {
if (_b == null) _b = new B();
return _b;
}
}
// Constructor can now be empty
public A()
{
}
}
Pass your reference from B/A in to A/B each time you need to use it. You don't need the constructors.
Class A
{
public void funcIHaveToUseFromClassB(B classB)
{
}
}
Class B
{
public void funcIHaveToUseFromClassA(A classA)
{
}
}

How to refactor the following code without killing OOPS in C#

I have 3 interfaces with 2 methods each doing the same job.
Interface A
{
Void M1()
Void M2()
}
Interface B
{
Void M1()
Void M2()
}
Interface C
{
Void M1()
Void M2()
}
Now, There are 3 classes implementing each of these interfaces.
Public Class A1:A
{
Public void M1()
{
}
Public void M2()
{
}
}
Public Class B1:B
{
Public void M1()
{
}
Public void M2()
{
}
}
Public Class C1:C
{
Public void M1()
{
}
Public void M2()
{
}
}
Functionality of M1 and M2 is exactly same in 3 classes. Interfaces are a part of library, I cannot change the interface and also cannot declare a new interface.
I want to refactor this code so that this duplication can be removed. I thought of creating a common class containing this functionality and then calling common class from each of these classes.
Please suggest.
It sounds like you should declare your own interface, and then create an adapter - or possibly multiple adapters. For example:
public interface IUnified
{
void M1();
void M2();
}
public class UnifiedAdapter : IUnified
{
private Action m1;
private Action m2;
public UnifiedAdapter(A a)
{
m1 = () => a.M1();
m2 = () => a.M2();
}
public UnifiedAdapter(B b)
{
m1 = () => b.M1();
m2 = () => b.M2();
}
public UnifiedAdapter(C c)
{
m1 = () => c.M1();
m2 = () => c.M2();
}
public M1()
{
m1();
}
public M2()
{
m2();
}
}
(This uses delegates to avoid having to create multiple adapter classes. The best approach depends on your exact situation.)
public abstract class XX : X
{
public void M1()
{
}
public void M2()
{
}
}
public interface X : A, B, C
{
}
Given the odd restrictions (and if you can I really suggest trying to get the interfaces changed) I think this is the best you can do:
public class Base : A, B, C {
public void M1(){}
public void M2(){}
}
Now inherit from Base in A1, B1 and C1.
However, if an A cannot, or should not, also be a B then this pattern won't work.
Therefore you would indeed have to go for the next best thing - a common base with the common functionality:
public class Base {
protected void M1Impl() { /* put your common implementation in here */ }
protected void M2Impl() { /* put your common implementation in here */ }
}
As the comments say - put the duplicated M1 and M2 code in the M1Impl and M2Impl methods here.
Now you can reuse this base for A, B and C implementations:
//common base for any implementation of A
//repeat for B and C
public class A1Base : Base, A
{
public void M1() { M1Impl(); }
public void M2() { M2Impl(); }
}
public class A1 : A1Base { }
I've worked on the basis here that you might have many implementations of A or B or whatever, and therefore you want a common starting point for each of those. If that's not the case, then you can do away with A1Base and simply call it A1.
public class BaseClass : A, B, C
{
public void M1()
{
}
public void M2()
{
}
}
Then just inherit from BaseClass:
public class A1 : BaseClass
{
}
public class B1 : BaseClass
{
}
public class C1 : BaseClass
{
}
A1 still will implement interface A. B1 will implement interface B. Same with C1. So, all your existing code will remain working:
A a = new A1();
a.M1();
If you only want to avoid to duplicate the implementation of those methods, then your initial approach is correct.
public class HelperClass
{
public static void M1()
{
// implementation code
}
public static void M2()
{
// implementation code
}
}
public class A1:A
{
public void M1()
{
HelperClass.M1();
}
public void M2()
{
HelperClass.M2();
}
}
public class B1:B
{
public void M1()
{
HelperClass.M1();
}
public void M2()
{
HelperClass.M2();
}
}
public class C1:C
{
public void M1()
{
HelperClass.M1();
}
public void M2()
{
HelperClass.M2();
}
}
Even if interfaces A, B, and C have the same methods, they may have different semantics and may make sense to have them as separate interfaces. That is, making a class implement A, may mean something different than implementing B, even if their methods have the same signatures.
Adding additional interfaces or common base classes is overkill and adds unneeded coupling. As I said before, if you only need to avoid duplicating the methods' implementation, a helper class is the easiest and cleanest solution.

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