解决“最派生的"问题.虚拟替代中的方法 [英] Resolving the "most derived" method in a virtual override

问题描述

我有一个简单的基类和派生类:

I have a simple base class and derived class:

class Base
{
    public virtual void Write(int value)
    {
        Console.WriteLine("base int: {0}", value);
    }

    public virtual void Write(string value)
    {
        Console.WriteLine("base string: {0}", value);
    }
}

class Derived : Base
{
    public override void Write(int value)
    {
        Console.WriteLine("derived int: {0}", value);
    }

    public virtual void Write(IEnumerable enumerable)
    {
        Console.WriteLine("derived IEnumerable: {0}", enumerable);
    }

    public virtual void Write(object o)
    {
        Console.WriteLine("derived obj: {0}", o);
    }
}

如果我运行此命令:

    static void Main(string[] args)
    {
        Derived d = new Derived();
        Console.WriteLine("derived:");
        d.Write(42);
        d.Write("hello");

        Console.WriteLine("base:");
        Base b = d;
        b.Write(42);
        b.Write("hello");
    }

我得到:

derived:
derived obj: 42
derived IEnumerable: hello
base:
derived int: 42
base string: hello

但是我期望"b.Write(42)"和"d.Write(42)"是相同的.字符串大小写相同.

But I am expecting "b.Write(42)" and "d.Write(42)" to be identical. The same for the string case.

我不明白什么?在无法修改基本"的约束下，如何使行为达到预期?

What am I not understanding? How can I get the behavior to be what I am expecting given the constraint that I cannot modify "Base"?

更新:请参见 Eric的发布.

推荐答案

之所以会发生这种情况，是因为C#首先考虑以类型声明的方法，包括替代方法.请参阅: C#规范的7.3节.

This happens because C# considers methods declared in a type before anything else, including override methods. See: Section 7.3 of the C# spec.

此博客帖子很好地说明了这一点，并说明原因.

This blog post explains it pretty well, and also explains the reasons.

以下两个规则证明了这种高度非直觉的行为是正确的:

This highly unintuitive behavior is justified by the following two rules:

1. 是否重写方法是实现的详细信息 应该被允许改变 不破坏客户代码.
2. 更改为不会破坏继承类的基类 不破坏继承的客户 课.

1. Whether or not a method is overridden is an implementation detail that should be allowed to change without breaking client code.
2. Changes to a base class that don't break an inherited class should not break clients of the inherited class.

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