方法签名中具有基本类型的派生类型集合的扩展方法 [英] Extension Method for a Collection of Derived Types with Base Type in Method Signature

问题描述

我想为使用基类作为类型要求的对象集合编写扩展方法.我了解这不一定是做事的最佳方法，但我很好奇，因为我有兴趣学习语言的细微差别.这个例子说明了我想做什么.

I want to write an extension method for a collection of objects that uses base class as a type requirement. I understand this is not necessarily the best way to do things, but I am curious because I'm interested in learning the nuances of the language. This example explains what I would like to do.

public class Human { public bool IsHappy { get; set; } }
public class Man : Human { public bool IsSurly { get; set; } }
public class Woman : Human { public bool IsAgreeable { get; set; } }

public static class ExtMethods
{
    public static void HappinessStatus(this IEnumerable<Human> items)
    {
        foreach (Human item in items)
        {
            Console.WriteLine(item.IsHappy.ToString());
        }
    }
}

// then in some method, I wish to be able to do the following

List<Woman> females = RetreiveListElements(); // returns a list of Women
females.HappinessStatus(); // prints the happiness bool from each item in a collection

我可以获取扩展方法的唯一方法是创建一个Humans集合.只要我仅引用基本类型的成员，就可以在派生类型上调用这种扩展方法吗?

The only way I can get the extension method to expose is to create a collection of Humans. Is it possible to call this type of extension method on derived types as long as I only reference members of the base type?

推荐答案

您的代码实际上将与C#4编译器一样编译，因为该版本支持

Your code will actually compile as is with the C# 4 compiler, as that version supports contravariant type parameters.

要使其与C#3配合使用，您可以为 IEnumerable< T> 创建通用扩展方法，并具有对通用类型起作用的 where T:Human 约束，而不是专门用于 IEnumerable< Human> :

To get it working with C# 3, you can create a generic extension method for IEnumerable<T> with a where T : Human constraint that acts on the generic type, instead of specifically only for IEnumerable<Human>:

public static void HappinessStatus<T>(this IEnumerable<T> items) where T : Human
{
    foreach (T item in items)
    {
        Console.WriteLine(item.IsHappy.ToString());
    }
}

然后，您可以按照自己的描述在 List< Woman> 集合上调用扩展方法.

Then you can call the extension method on your List<Woman> collection as you describe.

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