通用参数 - 使用具体类型编译，但实现的接口不编译 [英] Generic parameter - using a concrete type compiles, but an implemented interface does not

问题描述

给定这些接口和类...

Given these interfaces and classes...

public interface IPage
{
    string PageTitle { get; set; }
    string PageContent { get; set; }
}


public abstract class Page
    : IPage
{
    public string PageTitle { get; set; }
    public string PageContent { get; set; }
}


public class AboutPage
    : Page
    , IPage
{
}


public interface IPageAdminViewModel<out T>
    where T : IPage
{
    IEnumerable<T> Pages { get; }
}


public abstract class PageAdminViewModel<T>
    : IPageAdminViewModel<T>
    where T: IPage
{
    public IEnumerable<T> Pages { get; set; }
}

为什么使用 IPage 作为接口类型参数不编译....

Why does using IPage as an interface type parameter not compile....

public class AboutPageAdminViewModel
    : PageAdminViewModel<AboutPage>
    , IPageAdminViewModel<IPage> // ERROR HERE - I realise this declaration is not required, just indicating where the error is (not)
{
}

'AboutPageAdminViewModel'未实现接口成员'IPageAdminViewModel< IPage> .Pages'。 'PageAdminViewModel< AboutPage> .Pages'无法实现'IPageAdminViewModel< IPage> .Pages'，因为它没有匹配的返回类型'IEnumerable< IPage>'。

'AboutPageAdminViewModel' does not implement interface member 'IPageAdminViewModel<IPage>.Pages'. 'PageAdminViewModel<AboutPage>.Pages' cannot implement 'IPageAdminViewModel<IPage>.Pages' because it does not have the matching return type of 'IEnumerable<IPage>'.

....当使用具体类 AboutPage 吗？

....when using the concrete class AboutPage does?

public class AboutPageAdminViewModel
    : PageAdminViewModel<AboutPage>
    , IPageAdminViewModel<AboutPage> // NO ERROR - I realise this declaration is not required, just indicating where the error is (not)
{
}

基本上，我不明白为什么返回类型 IEnumerable< IPage> 与<$ c的返回类型不匹配$ c> IEnumerable< AboutPage> 。

Essentially, I don't understand why the return type of IEnumerable<IPage> does not match the return type of IEnumerable<AboutPage>.

我想创建一个方法，它需要 IPageAdminViewModel< IPage> 作为参数。我想知道如何/如果我可以使 AboutPageAdminViewModel 符合该要求。

I would like to create a method that takes IPageAdminViewModel<IPage> as an argument. I am wondering how / if I can make AboutPageAdminViewModel fit that requirement.

推荐答案

捡起Eric的例子：

class Animal {}
class Tiger : Animal {}

interface ICage { 
    Animal GetAnimal(); }

class TigerCage : ICage { 
    //Won’t compile
    public Tiger GetAnimal() => 
        new Tiger(); }

您可以通过实现显式接口成员和强类型方法来解决此问题，其中一种方法委托给另一方以避免代码重复：

You can solve this by implementing both an explicit interface member and a strongly typed method, one of which will delegate to the other to avoid code duplication:

class TigerCage: ICage {
    Animal ICage.GetAnimal() => GetAnimal();
    public Tiger GetAnimal() => new Tiger(); }

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