C# Remove object from list of objects

Question

I have a list of objects and I am trying to remove a specific object in the list by first checking a property in the object.

Originally I used a foreach but then realised you can't use this while modifying a collection, so I decided to use a normal for but then I'm not sure how to write code that does what I originally wrote.

How do I go about writing code to do what I originally had?

Thanks

Here's my code:

    public void DeleteChunk(int ChunkID)
    {
        //foreach (Chunk i in ChunkList)
        //{
        //    if (i.UniqueID == ChunkID)
        //    {
        //        ChunkList.Remove(i);
        //    }
        //}

        //This won't work because here i is just an integer so i.UniqueID won't exist.
        for (int i = 0; i < ChunkList.Capacity; i++)
        {
            if (i.UniqueID == ChunkID)
            {
                ChunkList.Remove(i);
            }
        }

    }

"This won't work" is *not* a good description of what goes wrong. — Jon Skeet, Sep 20 '13 at 13:14
Remember `Capacity` and `Count` are different and may not be the same. — OneFineDay, Sep 20 '13 at 13:15

Khan · Answer 1 · 2013-09-20T13:20:16.507

105

You can simplify this with linq:

var item = ChunkList.SingleOrDefault(x => x.UniqueId == ChunkID);
if (item != null)
    ChunkList.Remove(item);

You can also do the following, which will also work if there is more than one match:

ChunkList.RemoveAll(x => x.UniqueId == ChunkID);

edited Sep 20 '13 at 13:20

answered Sep 20 '13 at 13:14

Khan

17,904
5
47
59

But for list there is no Remove(Predicate), it needs to be RemoveAll() – Mateusz Sep 20 '13 at 13:19
x.UniqueID does not have an extension method, apparently. – user9993 Sep 20 '13 at 13:20
Do you mean `ChunkList`? What type is `ChunkList`? – Khan Sep 20 '13 at 13:23
private List ChunkList = new List(); Chunk being a class with some properties in it. – user9993 Sep 20 '13 at 13:24
The above should work then. Are you sure you have the syntax correct? – Khan Sep 20 '13 at 13:27

bland · Answer 2 · 2013-09-20T13:24:56.173

19

You're removing and then incrementing, which means you'll be one ahead of yourself. Instead, remove in reverse so you never mess up your next item.

for (int i = ChunkList.Count-1; i >=0; i--)
{
    if (ChunkList[i].UniqueID == ChunkID)
    {
        ChunkList.RemoveAt(i);
    }
}

edited Sep 20 '13 at 13:24

answered Sep 20 '13 at 13:15

bland

1,968
1
15
22

This does not work, because I get "The best overload method for... has some invalid arguments" when I hover over ChunkList.Remove(i) – user9993 Sep 20 '13 at 13:22
@0x4C4A41 You need to provide the type of collection that ChunkList is in order to ensure we provide correct help. Otherwise, we can only guide you so far. – bland Sep 20 '13 at 13:24
1

Try `ChunkList.RemoveAt(i)` – Khan Sep 20 '13 at 13:24
private List ChunkList = new List(); Chunk being a class with some properties in it. ChunkList.RemoveAt seems to work thanks – user9993 Sep 20 '13 at 13:26

score 11 · Answer 3 · answered Sep 20 '13 at 13:16

11

If ChunkList is List<Chunk>, you can use the RemoveAll method:

ChunkList.RemoveAll(chunk => chunk.UniqueID == ChunkID);

answered Sep 20 '13 at 13:16

Moo-Juice

38,257
10
78
128

score 7 · Answer 4 · edited Dec 28 '18 at 11:17

7

First you have to find out the object in the list. Then you can remove from the list.

       var item = myList.Find(x=>x.ItemName == obj.ItemName);
       myList.Remove(item);

edited Dec 28 '18 at 11:17

vasily.sib

3,871
2
23
26

answered Dec 28 '18 at 10:25

vivek

248
3
6

1

First you have to find out the object in the list. Then you can remove from the list. – vivek Dec 28 '18 at 10:28
do we need to implement IEquatable for this to work? or does it just compare reference? – karthik suryadevara Feb 01 '23 at 07:05

score 3 · Answer 5 · answered Sep 20 '13 at 13:17

Originally I used a foreach but then realised you can't use this while modifying a collection

You can create a copy of the collection and iterate over that using ToList() to create to copy:

 foreach(Chunk chunk in ChunkList.ToList())
 {
     if (chunk.UniqueID == ChunkID)
     {
         ChunkList.Remove(chunk);
     }
 }

score 2 · Answer 6 · answered Sep 20 '13 at 13:15

There are two problems with this code:

Capacity represents the number of items the list can contain before resizing is required, not the actual count; you need to use Count instead, and
When you remove from the list, you should go backwards, otherwise you could skip the second item when two identical items are next to each other.

score 2 · Answer 7 · answered Sep 20 '13 at 13:17

You're checking i's UniqueID while i is actually an integer. You should do something like that, if you want to stay with a for loop.

for (int i = 0; i < ChunkList.Capacity; i++)
{
    if (ChunkList[i].UniqueID == ChunkID)
    {
        ChunkList.Remove(i);
    }
}

You can, and should, however, use linq:

ChunkList.Remove(x => x.UniqueID == ChunkID);

score 2 · Answer 8 · answered Sep 20 '13 at 13:33

Firstly, you are using Capacity instead of Count.

Secondly, if you only need to delete one item, then you can happily use a loop. You just need to ensure that you break out of the loop after deleting an item, like so:

int target = 4;

for (int i = 0; i < list.Count; ++i)
{
    if (list[i].UniqueID == target)
    {
        list.RemoveAt(i);
        break;
    }
}

If you want to remove all items from the list that match an ID, it becomes even easier because you can use List<T>.RemoveAll(Predicate<T> match)

int target = 4;

list.RemoveAll(element => element.UniqueID == target);

score 1 · Answer 9 · answered Sep 20 '13 at 13:15

1

One technique is to create a copy of the collection you want to modify, change the copy as needed, then replace the original collection with the copy at the end.

answered Sep 20 '13 at 13:15

score 1 · Answer 10 · answered Sep 20 '13 at 14:20

You can use a while loop to delete item/items matching ChunkID. Here is my suggestion:

public void DeleteChunk(int ChunkID)
{
   int i = 0;
   while (i < ChunkList.Count) 
   {
      Chunk currentChunk = ChunkList[i];
      if (currentChunk.UniqueID == ChunkID) {
         ChunkList.RemoveAt(i);
      }
      else {
        i++;
      }
   }
}

score 0 · Answer 11 · answered Jan 20 '16 at 03:11

Simplest solution without using LINQ:

    Chunk toRemove = null;
    foreach (Chunk i in ChunkList)
    {
        if (i.UniqueID == ChunkID)
        {
            toRemove = i;
            break;
        }
    }
    if (toRemove != null) {
        ChunkList.Remove(toRemove);
    }

(If Chunk is a struct, then you can use Nullable<Chunk> to achieve this.)

C# Remove object from list of objects

11 Answers11

Linked

Related