c# Trying to reverse a list
C#ListReverseC# Problem Overview
I have the following code:
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();
}
But I get the following error:
> Cannot implicitly convert type 'void' to 'System.Collections.Generic.List<Lite.CategoryNavItem>'
Any ideas why this might be?
C# Solutions
Solution 1 - C#
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();
Solution 2 - C#
One workaround would be Return NavItems.AsEnumerable().Reverse();
Solution 3 - C#
.Reverse()
on a list reverses the items within the list, it does not return a new reversed list.
Solution 4 - C#
Reverse()
does not returns reversed list itself, it modifies original list. So rewrite it as following:
return NavItems.Reverse();
TO
NavItems.Reverse();
return NavItems;
Solution 5 - C#
Reverse()
does not return a List as expected of your function.
NavItems.Reverse();
return NavItems;
Solution 6 - C#
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;
Solution 7 - C#
.Reverse
reverses the "in-place"..., try
NavItems.Reverse();
return NavItems;
Solution 8 - C#
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 theAmbiguous invocation
error
You can try Enumerable.Reverse(someList)
instead.
Don't forget the:
using System.Linq;