C # 试图推翻一个列表

我有以下密码:

public class CategoryNavItem
{
public int ID { get; set; }
public string Name { get; set; }
public string Icon { get; set; }


public CategoryNavItem(int CatID, string CatName, string CatIcon)
{
ID = CatID;
Name = CatName;
Icon = CatIcon;
}
}


public static List<Lite.CategoryNavItem> getMenuNav(int CatID)
{
List<Lite.CategoryNavItem> NavItems = new List<Lite.CategoryNavItem>();


-- Snipped code --


return NavItems.Reverse();
}

但我得到了以下错误:

无法将类型‘ void’隐式转换为 'System.Collections.Generic.List<Lite.CategoryNavItem>'

知道为什么会这样吗？

109992 次浏览
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.Reverse() on a list reverses the items within the list, it does not return a new reversed list.

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最佳答案

Try:

NavItems.Reverse();
return NavItems;

List<T>.Reverse() is an in-place reverse; it doesn't return a new list.

This does contrast to LINQ, where Reverse() returns the reversed sequence, but when there is a suitable non-extension method it is always selected in preference to an extension method. Plus, in the LINQ case it would have to be:

return someSequence.Reverse().ToList();
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Reverse() does not return a List as expected of your function.

NavItems.Reverse();
return NavItems;
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Reverse() does not returns reversed list itself, it modifies original list. So rewrite it as following:

return NavItems.Reverse();

TO

NavItems.Reverse();
return NavItems;
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.Reverse reverses the "in-place"..., try

NavItems.Reverse();
return NavItems;
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One workaround would be Return NavItems.AsEnumerable().Reverse();

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If you have a list like in your example:

List<Lite.CategoryNavItem> NavItems

You can use the generic Reverse<> extensions method to return a new list without modifiying the original one. Just use the extension method like this:

List<Lite.CategoryNavItem> reversed = NavItems.Reverse<Lite.CategoryNavItem>();

Notes: You need to specify the <> generic tags to explicit use the extension method. Don't forget the 

using System.Linq;
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I had a situation where none of the suggested options suited me. So, if you:

  • don't want to use someList.Reverse() because it returns nothing (void)
  • don't want to use someList.Reverse() because it modifies source list
  • use someList.AsEnumerable().Reverse() and get the Ambiguous invocation error

You can try Enumerable.Reverse(someList) instead.

Don't forget the:

using System.Linq;

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