How do I get the index of an object from a LINQ query in addition to the result set

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I would like to know how to get the index of an object from a LINQ query in addition to the result set itself. Once I have the index of the result set, I would like to remove the item from the same list I am querying.

Base Class: Animal
Derived Class: Fish : Animal

//Fish has a Scales property

What I have tried:

Currently my code looks like this and works perfectly but I want to get the actual item/index so I can remove it from NoahsArk when i'm done with it.

List<animal> NoahsArk = new List<animal>();

//just assume separate code populates this list (NoahsArk) with a bunch of random
//animals and that there are a bunch of fish in the list

var FishQuery = NoahsArk.Where(f => f is Fish).OrderByDescending (f => (f as Fish).Scales)

Fish f = FishQuery.Cast<fish>().First();

1) Now that I have the fish I want (a fish with the highest amount of scales), I want to remove that specific fish from NoahsArk by index (or some other method if that's easier).

2) Is there a way to condense the cast line into the LINQ query creation line to make it more condense? IF not that's totally fine and not the main issue. My main issue with the first question.

Posted 23-May-21 15:00pm

Nostromo_Khan

Updated 27-May-21 9:50am

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Gerry Schmitz · Answer 1 · 2021-05-23T17:13:00

Solution 1

Use .Remove(); it removes the instance; you don't need the index in this case.

List<T>.Remove(T) Method (System.Collections.Generic) | Microsoft Docs[^]

Posted 23-May-21 17:13pm

Gerry Schmitz

OriginalGriff · Answer 2 · 2021-05-23T19:12:00

First off, you code won't compile:
1) C# is case sensitive, so Fish and fish are different classes.
2) You can't use f as a lambda parameter, because you are using f as a Fish in the outer method - you can't have two variables in the same scope with the same name (and yes, yes - I know the lambda parameter scope is restricted to the expression: the compiler doesn't and will complain!)
3) Don't forget your semicolons! :laugh:

Having said that, if you use Cast earlier you get a clearer result:

C#

Fish fish = NoahsArk.Where(f => f is Fish).Cast<Fish>().OrderByDescending(f => f.Scales).FirstOrDefault();

Note that I used FirstOrDefault - which returns a null if there were no Fishes on the boat!
I would probably enter is as:

C#

Fish fish = NoahsArk.Where(f => f is Fish)
                    .Cast<Fish>()
                    .OrderByDescending(f => f.Scales)
                    .FirstOrDefault();

As it's easier to read than a single line, but that's personal preference.

You definitely don't want to use an index to remove items as any index you could get would be indexing the input list as filtered by the Where which effectively creates a new sequence: If the input has ten items, and the last three are the ones you want, the index post Where would be 0 to 2 which doesn't reference any of them!
As Gerry said - use Remove rather than RemoveAt, it finds the instance and removes it which is much safer!

Richard Deeming · Answer 3 · 2021-05-23T23:16:00

You can combine the .Where(f => f is Fish) and the cast by using .OfType<Fish>():

C#

Fish fish = NoahsArk.OfType<Fish>().OrderByDescending(f => f.Scales).FirstOrDefault();

Also, don't forget that FirstOrDefault will return null if there are no Fish in the ark. You'll need to check for null before trying to remove or examine the fish.

If you really want the index, you'll need to use an overload of Select:

C#

var fishWithIndex = NoahsArk
    .Select((animal, index) => new { fish = animal as Fish, index })
    .Where(pair => pair.fish != null)
    .OrderByDescending(pair => pair.fish.scales)
    .FirstOrDefault();

if (fishWithIndex != null)
{
    NoahsArk.RemoveAt(fishWithIndex.index);
    ShowFish(fishWithIndex.fish);
}

How do I get the index of an object from a LINQ query in addition to the result set

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