转换失败：IList到实现 ICollection的自定义类
这是我的自定义收集声明。
public interface IMenuCollection<T> : ICollection<T>
public class HTMLMenuCollection: IMenuCollection<HTMLMenu>
我正在尝试从另一个集合 IList 转换为它。
IList<HTMLMenu> htmlMenuList = new List<HTMLMenu>();
...
HTMLMenuCollection tempColl = (HTMLMenuCollection)htmlMenuList;
我不知道为什么这行不通。如果我将 IList 转换为 ICollection 它工作得很好，但这样我会得到一个无效的转换异常。我做错了什么？
This is my custom collection declaration.
public interface IMenuCollection<T> : ICollection<T>
public class HTMLMenuCollection: IMenuCollection<HTMLMenu>
I am trying to cast to it from another collection IList<T>.
IList<HTMLMenu> htmlMenuList = new List<HTMLMenu>();
...
HTMLMenuCollection tempColl = (HTMLMenuCollection)htmlMenuList;
I have no idea why this won't work. If I cast IList<T> to ICollection<T> it works just fine, but with this I get an invalid cast exception. What am I doing wrong?
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另一种解决方法是使用通过实现
ICollection提供给HtmlMenuCollection的AddRange(IEnumerable) 方法>。Another work-around could be to use the
AddRange(IEnumerable<HtmlMenu>)method that is provided toHtmlMenuCollectionthrough the implementation ofICollection<HtmlMenu>..net 中的转换是在运行时完成的，因此您必须查看实例的类型而不是变量的声明类型。
htmlMenuList实例的类型为List，无法转换为HTMLMenuCollection，因为List不是，也不从HtmlMenuCollection继承。Casting in .net is done at run-time, so you must look at the type of the instance instead of the declared type of the variable. The type of
htmlMenuListinstance isList<HTMLMenu>, which cannot be casted toHTMLMenuCollectionbecauseList<HtmlMenu>is not, and does not inherited fromHtmlMenuCollection.HTMLMenuCollection未实现IList。ICollection强制转换有效，因为它们都继承自ICollection。此外，您从
List向上转换为HTMLMenuCollection，这将不起作用，因为HTMLMenuCollection不是从List< 继承的/code>，并且无论如何上转换都无法工作，因为编译器会认为HTMLMenuCollection比List拥有更多数据（即使没有）。将列表传递到构造函数中可能更有用，并且将
ICollection方法的内部实现传递到您在内部存储它的任何容器字段。HTMLMenuCollectiondoes not implementIList. TheICollectioncast works because they both inherit fromICollection.Also, you're upcasting from a
Listto anHTMLMenuCollection, which won't work becauseHTMLMenuCollectionis not inheriting fromList, and regardless the upconvert can't work since the compiler will considerHTMLMenuCollectionto have more data (even if it doesn't) than aList.It might be more useful to pass the list into a constructor, and have the internal implementations of the
ICollectionmethods just pass to whatever container field you're storing it in internally.它不会进行强制转换，因为
List根本没有实现HTMLMenuCollection，即使它具有相同的成员集也是如此。It doesn't cast because
List<HTMLMenu>simply does not implementHTMLMenuCollection, even if it has the same set of members.将继承视为一种“是”关系。
那么你可以认为它是因为
HTMLMenuCollection是一个List但不是每个List都是一个HTMLMenuCollection（因为List不是从HTMLMenuCollection继承的）。您可以从 List 转换为 IList 再转换为 ICollection 的原因如下
：
Think of inheritance as a 'is a' relationship.
Then you can think of it as being because
HTMLMenuCollectionis aList<HTMLMenu>but not everyList<HTMLMenu>is aHTMLMenuCollection(sinceList<HTMLMenu>doesn't inherit fromHTMLMenuCollection).The reason that you can cast from a List to IList to ICollection is the following:
and