使用继承可选参数基本方法的模糊性 [英] Ambiguity with inheriting an optional-parameter base method

问题描述

我有一个基类有一个可选的默认参数，子类自动提供一个值：

I have a base class with one optional default parameter, which a child class automatically provides a value for:

public class Merchant
{
    public string WriteResults(List<string> moreFields = null)
    {

        List<string> ListOfObjects = new List<string>() {Name, Address};
        if (moreFields != null)
        {
            ListOfObjects.AddRange(moreFields);
        }
            return ListOfObjects.ToString() //not real output
}


public class SpecificMerchant : Merchant 
{   
    new public string WriteResults()
        {
            return ((Merchant)this).WriteResults(new List<string>() {
                    Address, Phone //class-specific parameters
            });
        }
}

我使用 / code>关键字当调用 SpecificMerchant.WriteResults 时，因为父和基本可以不带参数，但编译器说这是不必要的：

I used the new keyword when calling SpecificMerchant.WriteResults because both the parent and the base can take no parameters, but the compiler says this is unnecessary:

成员'SpecificMerchant.WriteResults（）'不隐藏
继承的成员。不需要新的关键字。

The member 'SpecificMerchant.WriteResults()' does not hide an inherited member. The new keyword is not required.

为什么？

推荐答案

因为可选参数是编译时构造，而不是运行时

Because optional parameters are a compile time construct, not a runtime construct.

你的基类总是有一个参数的方法。如果你调用没有参数的方法，编译器只是在编译时替换null 。

Your base class is always going to have a method with one parameter. The compiler just "substitutes" null at compile time if you call that method without an argument.

做你正在实施的上面。即使你删除了 new 关键字，这将让它编译，你添加了很多困惑。如果需要，我可以使基类实现为virtual，或者向基类中添加两个方法，而不是使用可选的参数。

That being said, I would avoid trying to do what you're implementing above. Even if you remove the new keyword, which will let it compile, you're adding a lot of confusion. I would, personally, make the base class implementation virtual, if required, or add two methods to the base class and override one instead of using optional arguments.

资源，我建议阅读 James Michael Hare关于可选参数的帖子 - 他讨论了这个陷阱，当你将可选参数与继承混合时。

For a good resource, I'd recommend reading James Michael Hare's post on Optional Parameters - He discusses the pitfalls, like this one, when you mix optional arguments with inheritance.

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