如何将泛型集合转换为泛型集合的祖先？ [英] How to cast a generic collection to an generic collection of ancestors?

问题描述

列表标签：

 集合< Label> labels = new Collection< Label>（）; 
  

现在我想将它转换为 Controls $ b $ pre $ public void ScaleControls（ICollection< Control> controls）{}


我会尝试调用：



  ScaleControls标签）; 
  

但不能编译。

  ScaleControls（（ICollection的<控制>）标签）; 
  

编译，但在运行时崩溃。



 的ICollection<控制> controls =（ICollection< Control>）标签; 
 ScaleControls（c）; 
  

编译，但在运行时崩溃。



有没有办法绕过通用的对象列表？

---

另一种方法是放弃通用列表，并使用typed列表：

  public class ControlList：Collection< Control> 
 {
} 
 
 pubic void InvalidateControls（ControlList控件）
 {
} 
  

但是，这意味着所有使用泛型的代码都必须进行改进。

解决方案

你不能施放它;您必须自己转换它。

  InvalidateControls（新列表< Control>（labels））; // C＃4或更高版本
  

您的问题在于 ICollection< T> 不是协变的。 ICollection不协变的原因是为了防止像这样的方法变得邪恶：
$ b $ pre $ void AddControl（ICollection< Control> controls，Control控制）
{
controls.Add（control）;
}


为什么会是邪恶的？因为如果ICollection是协变的，该方法将允许您将TextBox添加到标签列表中：

  AddControl（new List< Label> （），new TextBox（））; 
  

List< Control> 需要 IEnumerable< Control> 。为什么我们可以将标签传递给？因为 IEnumerable< T> 是协变的：你不能将任何东西放到 IEnumerable< T> 你只能把东西拿出来。因此，您知道可以将任何从 IEnumerable< T> 中检索到的任何东西视为T（当然）或其任何基类型。
$ b

编辑

我刚刚注意到您正在使用.Net 3.5。在这种情况下， IEnumerable< T> 不是协变的，您需要更多代码才能转换集合。像这样：

  ICollection< T> ConvertCollection< T，U>（ICollection< U> collection）其中U：T 
 {
 var result = new List< T>（collection.Count）; 
 foreach（集合中的var项）
 result.Add（item）; 
} 
  

Consider a list of Label:

Collection<Label> labels = new Collection<Label>();

Now i want to cast this to a collection of Controls:

public void ScaleControls(ICollection<Control> controls) {}

i would try calling:

ScaleControls(labels);

but that doesn't compile.

ScaleControls((ICollection<Control>)labels);

compiles, but crashes at runtime.

ICollection<Control> controls = (ICollection<Control>)labels;
ScaleControls(c);

compiles, but crashes at runtime.

Is there a way to pass around generic lists of objects?

---

The alternative is to abandon generic lists, and use typed lists:

public class ControlList : Collection<Control>
{
}

pubic void InvalidateControls(ControlList controls)
{
}

But that means all code using generics must be retrofitted.

解决方案

You can't cast it; you have to convert it yourself.

InvalidateControls(new List<Control>(labels));  //C# 4 or later

Your problem here is that ICollection<T> is not covariant. The reason ICollection is not covariant is to prevent methods like this from being evil:

void AddControl(ICollection<Control> controls, Control control)
{
    controls.Add(control);
}

Why would that be evil? Because if ICollection were covariant, the method would allow you to add TextBoxes to lists of labels:

AddControl(new List<Label>(), new TextBox());

List<Control> has a constructor that takes IEnumerable<Control>. Why can we pass labels in? Because IEnumerable<T> is covariant: You can't put anything into an IEnumerable<T>; you can only take things out. Because of that, you know that you can treat anything retrieved from the IEnumerable<T> as a T (of course) or any of its base types.

EDIT

I just noticed that you're using .Net 3.5. In that case, IEnumerable<T> is not covariant, and you'll need more code to convert the collection. Something like this:

ICollection<T> ConvertCollection<T, U>(ICollection<U> collection) where U : T
{
    var result = new List<T>(collection.Count);
    foreach (var item in collection)
        result.Add(item);
}

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