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
– Jon Skeet, Commented Sep 20, 2013 at 13:14
Remember Capacity and Count are different and may not be the same. — OneFineDay
– OneFineDay, Commented Sep 20, 2013 at 13:15

Khan · Accepted Answer · 2013-09-20 13:20:16Z

109

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, 2013 at 13:20

answered Sep 20, 2013 at 13:14

Khan

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5 Comments

Mateusz Over a year ago

But for list there is no Remove(Predicate), it needs to be RemoveAll()

user9993 Over a year ago

x.UniqueID does not have an extension method, apparently.

Khan Over a year ago

Do you mean ChunkList? What type is ChunkList?

user9993 Over a year ago

private List<Chunk> ChunkList = new List<Chunk>(); Chunk being a class with some properties in it.

Khan Over a year ago

The above should work then. Are you sure you have the syntax correct?

bland · Accepted Answer · 2013-09-20 13:24:56Z

20

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, 2013 at 13:24

answered Sep 20, 2013 at 13:15

bland

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4 Comments

user9993 Over a year ago

This does not work, because I get "The best overload method for... has some invalid arguments" when I hover over ChunkList.Remove(i)

bland Over a year ago

@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.

Khan Over a year ago

Try ChunkList.RemoveAt(i)

user9993 Over a year ago

private List<Chunk> ChunkList = new List<Chunk>(); Chunk being a class with some properties in it. ChunkList.RemoveAt seems to work thanks

Moo-Juice · Accepted Answer · 2013-09-20 13:16:20Z

11

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

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

answered Sep 20, 2013 at 13:16

Moo-Juice

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Comments

vasily.sib · Accepted Answer · 2018-12-28 11:17:06Z

9

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, 2018 at 11:17

vasily.sib

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answered Dec 28, 2018 at 10:25

vivek

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2 Comments

vivek Over a year ago

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

karthik suryadevara Over a year ago

do we need to implement IEquatable for this to work? or does it just compare reference?

DGibbs · Accepted Answer · 2013-09-20 13:17:47Z

3

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);
     }
 }

answered Sep 20, 2013 at 13:17

DGibbs

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Comments

Sergey Kalinichenko · Accepted Answer · 2013-09-20 13:15:53Z

2

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.

answered Sep 20, 2013 at 13:15

Sergey Kalinichenko

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Comments

Raz Harush · Accepted Answer · 2013-09-20 13:17:31Z

2

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);

answered Sep 20, 2013 at 13:17

Raz Harush

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Comments

Matthew Watson · Accepted Answer · 2013-09-20 13:33:50Z

2

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);

answered Sep 20, 2013 at 13:33

Matthew Watson

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Comments

user164226 · Accepted Answer · 2013-09-20 13:15:40Z

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, 2013 at 13:15

user164226

Comments

knguyen · Accepted Answer · 2013-09-20 14:20:34Z

1

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++;
      }
   }
}

answered Sep 20, 2013 at 14:20

knguyen

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Comments

knocte · Accepted Answer · 2016-01-20 03:11:49Z

0

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.)

answered Jan 20, 2016 at 3:11

knocte

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