C# 无法将“System.Collections.Generic.List`1[Item]”类型的对象转换为“ItemList”
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Unable to cast object of type 'System.Collections.Generic.List`1[Item]' to type 'ItemList'
提问by PositiveGuy
For some reason, my boss likes to create custom types to represent a generic list (even in most cases where his custom type has no members! I think he's just lazy and doesn't like to type the List or something but to me this is lame and is causing me many headaches per the issue below.
出于某种原因，我的老板喜欢创建自定义类型来表示通用列表（即使在大多数情况下，他的自定义类型没有成员！我认为他只是懒惰，不喜欢键入 List 之类的东西，但对我来说这是跛脚，每个下面的问题让我很头疼。
Point in case:
以防万一：
public class ItemnList : List<Item>
{
public Personalization FindById(int id)
{
...blahblah blah, this is really an extension method that should be elsewhere
}
}
Consequently, when I'm using a standard List (mabye I hate his custom class and like to use plain .NET types like they should be used), OR maybe I'm using a LINQ expression like below, I always run into casting problems even though the custom type is inheriting from that List
因此，当我使用标准列表时（也许我讨厌他的自定义类，喜欢使用应该使用的普通 .NET 类型），或者也许我正在使用如下所示的 LINQ 表达式，我总是遇到强制转换问题即使自定义类型是从该 List 继承的
private ItemList someMethod(ItemList itemList)
{
...
itemList = (ItemList)items.Where(x => x.ItemType != ItemType.Car && x.ItemType != ItemType.Truck).ToList();
return itemList;
....
}
采纳答案by Lee
As Grzenio points out, you can't use ToList() and cast, however you could create your own extension method to create an instance of the derived type from a sequence:
正如Grzenio 指出的那样，您不能使用 ToList() 和强制转换，但是您可以创建自己的扩展方法来从序列创建派生类型的实例：
public static TDerived CreateFromEnumerable<TDerived, T>(this IEnumerable<T> seq) where TDerived : List<T>, new()
{
TDerived outList = new TDerived();
outList.AddRange(seq);
return outList;
}
So for your example you would do:
因此，对于您的示例，您将执行以下操作：
ItemList outList = itemList
.Where(x => x.ItemType != ItemType.Car && x.ItemType != ItemType.Truck)
.CreateFromEnumerable<ItemList, Item>();
回答by Grzenio
Unfortunately ToList() will return a normal list, and not ItemnList, so you can't cast it. I don't really see a reasonable workaround, it would probably be better to encapsulate the List in ItemnList, instead deriving from it.
不幸的是，ToList() 将返回一个普通列表，而不是 ItemnList，因此您无法投射它。我真的没有看到合理的解决方法，最好将 List 封装在 ItemnList 中，而不是从中派生。
