Multiple Inheritance and Objects C#

Question

Sorry didn't get any funky title for the question.

I had been taught that .Net(C#) doesn't support Multiple Inheritance. But looking at the foo example below I wonder is this really truth ??

class fooList
{
    public int Index()
    {
        return 0;
    }
}    
class foo : fooList
{
    public foo()
    { }
}
class testFoo
{
    void test()
    {
        foo obj = new foo();

        // From object
        obj.Equals(obj);
        obj.GetHashCode();
        obj.GetType();
        obj.ToString();

        // From fooList
        obj.Index();
    }
}

As we can see that I have a class fooList and a class foo which inherits fooList according to the sayings(C# doesn't support Multiple Inheritance) the object of class foo should have only one method which is Index() from the fooList class, But it has more methods from the object class. It clearly indicates that by default all the classes inherit object class. So questions raises

Is it really completely true that C# doesn't support Multiple Inheritance ?
I guess it supports it at CLR level, Than why it doesn't support it in coding ?

This is not a case of multiple inheritence. Every class inherits from object if not otherwise specified. If you inherit from another class directly, you indirectly inherit from object since that class inherits from object. Object is the root to all objects. — Andreas
– Andreas, Commented May 27, 2013 at 6:37
class fooList is a shorthand for class fooList : object. That should clear your confusion. — user1096188
– user1096188, Commented May 27, 2013 at 6:42

Anthony Pegram · Accepted Answer · 2013-05-27 06:44:01Z

14

C# does not support multiple inheritance of classes at a single level. C# does support multiple levels of inheritance in a hierarchy. What you are seeing is that at the top of your inheritance hierachy is System.Object, which is the top level base class for classes and structs in .NET.

What you cannot have is:

class A { }
class B { }
class C : A, B { }

What you can have:

class A { }
class B : A { }
class C : B { }

Make sense?

For a bit more completeness, C# does allow you to simulate multiple inheritance via the usage of interfaces. So let's walk it back.

class A { }
interface B { }
class C : A, B { } // legal

In this example, the declaration of C is legal, and you would need to implement the various members that interface B defines as part of C.

edited May 27, 2013 at 6:44

answered May 27, 2013 at 6:37

Anthony Pegram

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Comments

Thorsten Dittmar · Accepted Answer · 2013-05-27 06:39:07Z

Multiple Inheritance is not what you're seeing here. Multiple Inheritance means that a class derives from two base classes "in parallel", like maybe this:

public class Test : Base1, Base2 {}

That is not possible.

What you're seeing here (and this works in any object oriented language) is that of course a class A inherits methods and properties from all the entire hierarchy.

For example:

public class Test : Base {}

inherits all methods from Base and Object, if Base is directly derived from Object.

So if you have this hierarchy:

public class Base : Object {}
public class A : Base {}

Base inherits all methods and properties from Object, and A inherits all methods and properties from Base and thus also from Object.

Otherwise it would not be possible to build class hierarchies.

Ishu · Accepted Answer · 2013-05-27 06:41:28Z

1

You are showing example of multi label inheritance and not multiple inheritance.

Multiple inheritance means a single class can inherit more than one class.

class A: B,C

Which is not true in case of c#

answered May 27, 2013 at 6:41

Ishu

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Mehmet Ataş · Accepted Answer · 2013-05-27 06:43:48Z

You are missunderstanding multiple inheritance. A class can inherit a class which already inherits another class. But a class cannot inherit two or more different classes at the same time.

// Default inheritance from Object.
// Class A inherits System.Object
class A     

// Simple inheritance. This is not multiple inheritance.   
// Class B inherits class A, which inherits Object
class B : A     

// Simple inheritance. This is not multiple inheritance.
// Class C inherits class B, which inherits class A, which inherits Object
class C : B  

// Default inheritance from Object.
// Class D inherits System.Object
class D     

// Multiple inheritance
// Class E inherits both class A and class D
class E : A, D

// Multiple inheritance
// Class F inherits both class A and class B
class F : A, B

// Multiple inheritance
// Class G inherits both class C and class D
class G : C, D

Vikram · Accepted Answer · 2013-05-27 07:16:00Z

Apart from Index() method other methods are from the Object class which is the base class for all the reference type as well as value types. It is very true that C# doesn't allow multiple inheritance but it does support multilevel inheritance, again C# supports multiple inheritance if we are using Interfaces.

C# doesn't support multiple inheritance to remove the ambiguity which can arise if two base classes have a same methods defined in them and it would be confusing by looking at the code that which version of the method would be called. Check out this link.

This uncertainity can be removed by having two interface as shown in the example below;

public  interface IA
{
  string SomeMethod();
}

public interface IB
{
  string SomeMethod();
}

class MyConcreteClass:IA, IB
{
  string IA.SomeMethod()
  {
     return "For IA";       
  }

  string IB.SomeMethod()
  {
     return "For IB";       
  }
}

IA concA = new MyConcreteClass();
IB concB = new MyConcreteClass();

 Console.WriteLine(concA.SomeMethod()); // print "For IA"
 Console.WriteLine(concB.SomeMethod()); // print "For IB"

As you can see we can have different behaviors for the functions of both the interface by explicitly defining them in the derived class which would prevent from any ambiguity.

One more point to note here is that the function definitions in the derived class cannot be public as it cant be accessed by the instance of the derived type.

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