C＃中的单一继承 - 对象类？

Question

I have been asking myself this question for a long time now. Thought of posting it. C# doesn't support Multiple Inheritance(this is the fact). All classes created in C# derive out of 'Object' class(again a fact).

So if C# does not support Multiple inheritance, then how are we able to extend a class even though it already extends Object class?

Illustating with an example:

1. class A : object - Class A created.
2. class B : object - Class B created.
3. class A : B - this again is supported. What happens to the earlier association to object.

We are able to use object class methods in A after step 3. So is the turned to multi level inheritance. If that is the case, then

1. class A : B
2. class C : B
3. class A : C - I must be able to access class B's methods in A. Which is not the case?

Can anyone please explain?

Answer 1

You're confusing mutliple inheritance with an inheritance tree. You can inherit from something other than Object. It's just that Object is sitting way up there at the top of your tree. And someone can inherit your class, but because Object is still up there at the top that class will also inherit from object. Your "Multi-level" inheritance is not multiple inheritance.

Multiple inheritance is when you inherit from two different trees, and .Net actually does support this after a fashion via interfaces.

Answer 2

Joel's answer is correct. There is a difference between multiple inheritance and an inhertance tree (or derivation chain). In your example, you actually show an inhertance tree: One object inherits (derives) from another object higher in the tree. Multiple inheritance allows one object to inherit from multiple base classes.

Take, for example, the following tree:

public class BaseClass { }

public class SpecialBaseClass : BaseClass {}

public class SpecialtyDerivedClass : SpecialBaseClass {}

This is perfectly valid and says that SpecialtyDerivedClass inherits from SpecialBaseClass (SpecialtyDerivedClass' parent) which, in turn, derives from BaseClass (SpecialtyDerivedClass' grandparent).

Under the idea of multiple inheritance, the example would look like this:

public class BaseClass { }

public class SpecialBaseClass {}

public class SpecialtyDerivedClass : BaseClass, SpecialBaseClass {}

This is not allowed in .NET, but it says that SpecialityDerivedClass inherits from both BaseClass and SpecialBaseClass (which are both parents).

.NET does allow a form of multiple inheritance by allowing you to inherit from more than one interface. Changing the example above slightly:

public class BaseClass { }

public interface ISpecialBase {}

public interface ISpecialDerived {}

public class SpecialtyDerivedClass : BaseClass, ISpecialBase, ISpecialDerived {}

This says that SpecialtyDerivedClass inherits from BaseClass (it's parent) and also ISpecialBase and ISpecialDerived (also parent's but more like step-parents as interfaces can't specify functionality).

Answer 3

All classes ultimately derive from Object.

public class A

is implicitly equivalent to

public class A : System.Object

When you derive from another class

public class A : B

where

public class B : System.Object

B becomes the parent class, and Object becomes the grandparent class.

And so on.

So it is the parent, grandparent, great-grandparent (etc) class of all other classes.

Answer 4

一种看待它的方法是：C＃有一个继承树 ，而C ++（或其他多重继承语言）有一个继承点 。

Answer 5

Given below.

public class A : B
{

}

public class B : C
{
    public int BProperty { get; set; }
}

public class C
{
    public int CProperty { get; set; }
}

public class Test
{
    public void TestStuff()
    {
        A a = new A();

        // These are valid.
        a.CProperty = 1;
        a.BProperty = 2;
    }

}

This is valid. Object is a base for C in this case.

Answer 6

In the example, the reason that B can extend A is because A extends Object. A class can only specify one parent class, but that class must either be object or have object as one of its ancestors.

Answer 7

A class inherits from object if you do not specify a base class . Thus:

class C {}

is the same as

class C : Object {}

However, if you specify a base class, it will inherit from that class instead of Object . Thus,

class B : C {}

B directly inherits from C instead of Object. Another example,

class A : B {}

In this case, A inherits from B instead of Object. To summarize, in this hierarchy:

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

Class A derives from B, which derives from C. So Class A is indirectly derived from C because B is derived from C. C also derived from Object which in not explicitly specified but it is there by default. So A is indirectly derived from Object too.

Answer 8

A class in C# can only have one parent, but it can have multiple ancestors. You can implement multiple interfaces, but that only means that your class agrees to implement the signatures defined by those interfaces. You don't actually inherit any functionality from those interfaces.