强制转换通用通用< Base> [英] Cast Generic<Derived> to Generic<Base>

问题描述

我有一个基本的WPF用户控件，该控件处理派生的用户控件的一些常用功能.在任何派生的UserControl的代码背后，我都调用了一个事件

I have a base WPF UserControl that handles some common functionality for derived UserControls. In the code-behind of any derived UserControl I call an event

private void SomeClick(object sender, RoutedEventArgs e) {
    HandleClick(sender);
    MyDataGrid.Items.Refresh();
}

在我的基本UserControl中，

In my base UserControl I do

public class BaseUserControl : UserControl {
   protected void HandleClick(object sender) {
       var vm = (BaseViewModel<Part>)DataContext;
       ...
   }
}

这将引发 InvalidCastException ，因为DataContext的类型为BaseViewModel，但具有派生类型，例如BaseViewModel<Wire>或BaseViewModel<Connector>.

This throws an InvalidCastException since DataContext is of type BaseViewModel but of a derived type like BaseViewModel<Wire> or BaseViewModel<Connector>.

我该如何投放?

推荐答案

您不能将Generic<Derived>强制转换为Generic<Base>.

试想一下，如果可以的话.您有一个List<Wolf>并将其强制转换为List<Animal>.现在，您可以.Add()一个Sheep到List<Animal>.但是等等...现在您的List<Wolf>包含一个Sheep.真是一团糟.

Just imagine if you could. You have a List<Wolf> and cast it to a List<Animal>. Now you could .Add() a Sheep to your List<Animal>. But wait... now your List<Wolf> contains a Sheep. What a mess.

仅当您可以确保投射到的东西为所有可能的形式都是只读的时，这才可行.这是协和逆性为所有关于.不过，它仅适用于接口.

This would only work out if you could make sure that the thing you cast to is read-only in all possible forms. This is was co- and contravariance is all about. It only works for interfaces though.

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