C＃铸造继承的通用接口 [英] C# casting an inherited Generic interface

问题描述

我有一些麻烦我的头周围投界面我来了。
这是一个MVP设计C＃Windows窗体。我有我对我的表单类实现一个IVIEW类。还有，我得出成各种具体演示了IPresenter。每个主持人将管理的iView根据不同的角色，例如打开对话框输入一个新的数据集与AddPresenter，而不是用这将写入数据到形式的EditPresenter编辑现有的数据。每一种从IPresenter继承。我想使用的代码这样：

I'm having some trouble getting my head around casting an interface I've come up with. It's an MVP design for C# Windows Forms. I have an IView class which I implement on my form classes. There's also an IPresenter which I derive into various specific Presenters. Each Presenter will manage the IView differently depending on the role, for example opening the dialog to enter a new set of data with an AddPresenter as opposed to editing existing data with an EditPresenter which would preload data onto the form. Each of these inherit from IPresenter. I want to use the code as such:

AddPresenter<ConcreteView> pres = new AddPresenter<ConcreteView>();

我基本上有这个工作，但这些主持人和被加载后，他们管理已被捆绑的意见纳入插件运行时，这意味着我需要的插件接口采取了模式参数，其作用Manager类。此模式参数是使用一个工厂方法来创建或者添加或编辑演示，而是因为以显示对话框中调用时以后那我需要通过像这样的IPresenter界面拨打电话：

I basically have this working but these presenters and the views they manage are bundled into plugins which are loaded post runtime which means I need a Manager class which acts as the plugin interface take a "mode" parameter. This mode parameter is use for a factory method to create either the Add or Edit Presenter, but because the call to show the dialog is made later on then I need to make the call via the IPresenter interface like so:

private IPresenter<IView> pres;
public ShowTheForm()
{
    pres.ShowDialog();
}

现在我有问题，当涉及到我的套管的一个具体实例AddPresenter说，到PRES成员。
这里是什么我有一个砍伐简化版本：

Now I'm having issues when it comes to casing my concrete instantiation of an AddPresenter say, to the 'pres' member. Here's a cut down simplified version of what I have:

    interface IView
    {
        void ViewBlah();
    }

    interface IPresenter<V> where V : IView
    {
        void PresBlah();

    }

    class CView : IView
    {
        public void ViewBlah()
        {

        }
    }

    class CPresenter<T> : IPresenter<T> where T : IView
    {
        public void PresBlah()
        {
        }
    }

    private void button3_Click(object sender, EventArgs e)
    {
        CPresenter<CView> cpres = new CPresenter<CView>();
        IPresenter<IView> ipres = (IPresenter<IView>)cpres;

    }

这是错误：

Unable to cast object of type 'CPresenter`1[MvpApp1.MainForm+CView]' to type 'IPresenter`1[MvpApp1.MainForm+IView]'.

无论是演示，从我可以告诉通用类型规范是接口的子类，所以我可以不懂为什么它不会投。

Both the Presenter and the Generic type specification from what I can tell ARE subclasses of the interfaces so I can't understand why it won't cast.

有什么想法？

史蒂夫

推荐答案

问题是泛型类型参数。如果你的接口参数协再投会工作。

The problem is the generic type parameter. If you make the interface parameter covariant then the cast will work.

这是通过添加退出关键字来完成，像这样：

This is accomplished by adding the out keyword, like so:

interface IPresenter<out V> where V : IView
{
    void PresBlah();

}

您可以了解更多关于如何适用于以下MSDN文章：协变和逆变泛型中。本节通用接口有协类型参数专门适用于您的问题。

You can learn more about how this works with the following MSDN article: Covariance and Contravariance in Generics. The section Generic Interfaces with Covariant Type Parameters specifically applies to your question.

的更新：请务必检查@phoog和我之间的意见。如果你的实际代码接受 V 作为输入，您将无法使其协变的。在引用的文章和@ phoog的回答解释了这种情况下，进一步的细节。的

Update: Make sure you check the comments between @phoog and me. If your actual code accepts a V as an input, you will be unable to make it covariant. The referenced article and @phoog's answer explains this case in further detail.

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