How does using interfaces overcome the problem of

问题:

I understand that C# does not support multiple inheritance, and that the solution is to use interfaces instead. But what I don't understand is why interfaces doesn't create the diamond problem the same way as multiple inheritance would. How does using interfaces avoid the pitfalls of multiple inheritance?

回答1:

One class may implement any number of interfaces, even if those interfaces extend other interfaces as well. Multiple inheritance is not possible only with classes.

// This is not allowed
class A { void A() {} }
class B { void B() {} }
class C : A, B {}

// This is allowed
interface IA { void A(); }
interface IB { void B(); }

class A : IA, IB
{
    public void A() {}
    public void B() {}
}

The diamond problem exists with classes because there is a possibility of clashing implementations (if A and B have the same method and C extends both, which method does it take?). Interfaces, on the other hand, simply require an implementing type to have the methods that they declare.

If the exact same method is defined in two interfaces, and a class implements both interfaces, that doesn't matter. All the class needs to do is provide an implementation for the method so that code can be written to call that method. Meaning, this works:

interface IA { void Method(int x); }
interface IB { void Method(int x); }

class A : IA, IB
{
    public void Method(int x)
    {
        Console.WriteLine(x);
    }
}

Note that a class may still inherit from one other class, plus any number of interfaces:

class A : B, IA, IB {}

回答2:

The "diamond problem" is not present when just using interfaces because there is no ambiguous implementation possible. Read the Wikipedia article, which contains a full explanation.

回答3:

Multiple interfaces will not create the diamond problem because each class must provide their own unique implementation, which means there is no sharing of resources.

C# does not allow multiple inheritance because they care about you, and made the language as shoot-yourself-in-the-foot-proof as possible.

回答4:

You can inherite only one class and unlimited number of interfaces at one time

回答5:

I think it is not fair to think about this in scope of C# only.

CLR itself does not support multiple inheritance. May be because they wanted to support other languages that do not support it currently or cannot support it in future.

回答6:

If you ask about inheritance: class can inherit only one class, but can implement any number of interfaces. See this article about inheritance and polymorphism in C#.

回答7:

This is a possible way to try to achieve something that you usually can achieve with multiple inheritance.

interface IA
{
    void DoAnything(string name);
}

class A : IA
{
    public void DoSomething()
    {
        // some code
    }
    public void DoAnything(string name)
    {
        // Some code
    }
}

class B : IA
{
    public void DoNothing()
    {
        // Some code
    }

    public void DoAnything(string name)
    {
        // Some code
    }
}

class StorageCache : IA
{
    private A ObjA;
    private B ObjB;

    public void DoAnything(string name)
    {
        ObjA.DoAnything(name);
        ObjB.DoAnything(name);
    }
    public void DoNothing()
    {
        ObjB.DoNothing();
    }
    public void DoSomething()
    {
        ObjA.DoSomething();
    }
}