允许访问但阻止外部类实例化嵌套类 [英] Allow access to but prevent instantiation of a nested class by external classes

问题描述

我想定义一个嵌套类，容器类和外部类可以访问该嵌套类，但是我想控制嵌套类的实例化，以便只有容器类的实例才能创建嵌套类的新实例。 。

I'm looking to define a nested class that is accessible to the container class and external classes, but I want to control instantiation of the nested class, such that only instances of the container class can create new instances of the nested class.

后续代码应有望证明这一点：

The proceeding code should hopefully demonstrate this:

public class Container
{
    public class Nested
    {
        public Nested() { }
    }

    public Nested CreateNested()
    {
        return new Nested();  // Allow
    }
}

class External
{
    static void Main(string[] args)
    {
        Container containerObj = new Container();
        Container.Nested nestedObj;

        nestedObj = new Container.Nested();       // Prevent
        nestedObj = containerObj.CreateNested();  // Allow

    }
}

嵌套必须是公开的，以便外部可以访问。我尝试将嵌套的构造函数设置为受保护的，但是这会阻止 Container 创建实例，例如容器不是嵌套的基类。我可以将 Nested 的构造函数设置为 internal ，但是我想防止所有外部用户访问该构造函数类，包括同一程序集中的类。有办法做到吗？

Nested must be public in order for it to be accessible by External. I tried making the constructor for Nested protected, however that prevents Container from creating instances, as Container isn't a base class of Nested. I could set the constructor for Nested to internal, but I'm looking to prevent access to the constructor by all external classes, including those in the same assembly. Is there a way to do this?

如果无法通过访问修饰符实现此功能，我想知道是否可以在 Nested（）中引发异常。 。但是，我不知道如何测试调用 new Nested（）的上下文。

If this cannot be achieved through access modifiers, I wonder if I could throw an exception within Nested(). However, I don't know how to test for the context within which new Nested() is called.

推荐答案

如何通过接口进行抽象？

How about abstraction via an interface?

public class Container
{
    public interface INested
    {
        /* members here */
    }
    private class Nested : INested
    {
        public Nested() { }
    }

    public INested CreateNested()
    {
        return new Nested();  // Allow
    }
}

class External
{
    static void Main(string[] args)
    {
        Container containerObj = new Container();
        Container.INested nestedObj;

        nestedObj = new Container.Nested();       // Prevent
        nestedObj = containerObj.CreateNested();  // Allow

    }
}

您也可以与抽象基类相同的东西：

You can also do the same thing with an abstract base-class:

public class Container
{
    public abstract class Nested { }
    private class NestedImpl : Nested { }
    public Nested CreateNested()
    {
        return new NestedImpl();  // Allow
    }
}

class External
{
    static void Main(string[] args)
    {
        Container containerObj = new Container();
        Container.Nested nestedObj;

        nestedObj = new Container.Nested();       // Prevent
        nestedObj = containerObj.CreateNested();  // Allow

    }
}

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