模拟多重继承 C# [重复]

Question

public class A2
{
  protected virtual void f() {Console.WriteLine("from A2");}
  protected virtual void g() {Console.WriteLine("From A2");}
  protected virtual void h1() {Console.WriteLine("from A2");}
}

public class B2
{
  protected virtual void f() {Console.WriteLine("B2");}
  protected virtual void g() {Console.WriteLine("B2");}
  protected virtual void h2() {Console.WriteLine("B2");}
}

给出上面的代码，我试图通过创建一个新类来继承这两个类的所有功能，如下所示：

public class C2 
{
    private A2 a2 = new A2();
    private B2 b2 = new B2();

    public void f()
    {
        a2.f();
    }

    public void h1()
    {
        a2.h1();
    }

    public void g()
    {
        b2.g();
    } 

    public void h2()
    {
        b2.h2();
    } 


    static public void Main() 
    {
        C2 n = new C2();
        n.f();
        n.g();
        n.h1();
        n.h2();

    }

}

但是，当我尝试编译程序时，我遇到了一个错误，即我无法访问受保护的方法。 如何在不修改 A2 和 B2 类的情况下继承所有功能？

Answer 1

派生在公共方法中公开行为的类。从派生类型组成复合类。

public class A2_Derived : A2
{
    public void FMethod() => f();
    public void GMethod() => g();
    public void H1Method() => h1();
}

public class B2_Derived : B2
{
    public void FMethod() => f();
    public void GMethod() => g();
    public void H2Method() => h2();
}

public class C2 
{
    private A2_Derived a2 = new A2_Derived();
    private B2_Derived b2 = new B2_Derived();

    // implement the methods you need...
}

Answer 2

这是 C# 抽象类和虚拟类的缺点之一 - 没有多重继承。抽象本质上是对父级的依赖，但为什么要有一个依赖呢？

至于你的问题的解决方案，你需要在合并之前将它们公开：

public class A2
{
  protected virtual void f() {Console.WriteLine("from A2");}
  protected virtual void g() {Console.WriteLine("From A2");}
  protected virtual void h1() {Console.WriteLine("from A2");}
}

//make a wrapper which will publish protected methods
//ADVICE: always use interfaces, they MUCH more flexible as you will see in your own example, use abstract/static modifier with caution.
public interface IA2
{
    void f_public();
    void g_public();
    void h1_public();
}
public class PublicA2 : A2, IA2
{  
   public void f_public(){f();}
   public void g_public(){g();}
   public void h1_public(){h1();}
}
//do the same for B2

然后你可以将它们组合起来，甚至用对应的接口标记它们：

public class C2 : IA2, IB2
{
    private readonly IA2 _a2;
    private readonly IB2 _b2;

    public C2(IA2 a2, IB2 b2)
    {
        _a2 = a2;
        _b2 = b2;
    }

    public void f_public()
    {
        _a2.f_public();
    }

    public void h1_public()
    {
        _a2.h1_public();
    }

    public void g_public()
    {
        _b2.g_public();
    } 

    public void h2_public()
    {
        _b2.h2_public();
    } 
}