How to remove a single, specific object from a ConcurrentBag<>?

Question

With the new ConcurrentBag<T> in .NET 4, how do you remove a certain, specific object from it when only TryTake() and TryPeek() are available?

I'm thinking of using TryTake() and then just adding the resulting object back into the list if I don't want to remove it, but I feel like I might be missing something. Is this the correct way?

This gets me every time. Finally realized that 'bag' means you don't get to look inside. Like in a game of Scrabble. — Simon_Weaver, Jun 21 '23 at 21:35
I haven't found a case where ConcurrentBag is appropriate yet, and it's touted constantly as a replacement for List. Lots of misinformation and obscurity around the issue -- and lots of good information, too, but almost all of it misses the mark on using a List concurrently. — omJohn8372, Aug 14 '23 at 14:55

score 120 · Accepted Answer · answered Jun 12 '10 at 19:26

120

The short answer: you can't do it in an easy way.

The ConcurrentBag keeps a thread local queue for each thread and it only looks at other threads' queues once its own queue becomes empty. If you remove an item and put it back then the next item you remove may be the same item again. There is no guarantee that repeatedly removing items and putting them back will allow you to iterate over the all the items.

Two alternatives for you:

Remove all items and remember them, until you find the one you want to remove, then put the others back afterwards. Note that if two threads try to do this simultaneously you will have problems.
Use a more suitable data structure such as ConcurrentDictionary.

answered Jun 12 '10 at 19:26

Mark Byers

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SynchronizedCollection might also be a suitable substitute. – ILIA BROUDNO Oct 07 '16 at 19:59
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@ILIABROUDNO -- you should put that as answer! That is SO MUCH better than a kludgey ConcurrentDictionary when you don't need a Dictionary – Denis Jul 14 '17 at 17:25
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FYI, SynchronizedCollection is not available in .NET Core. As of the date of this comment, the System.Collections.Concurrent types are the way current way to go for .NET Core based implementations. – Matthew Snyder Jun 13 '19 at 14:58
2

I'm not sure which version of .NET Core was being used, but I'm working on a project based on the .NET Core 2.1 SDK and SynchronizedCollection is available in the Collections.Generic namespace now. – Lucas Leblanc Sep 25 '19 at 18:28
Be aware that SynchronizedCollection says it implements the collection as a `List`. Thus removal will cost O(n). – Oliver Bock Apr 27 '22 at 00:14

score 18 · Answer 2 · answered Jun 12 '10 at 19:33

18

You can't. Its a bag, it isn't ordered. When you put it back, you'll just get stuck in an endless loop.

You want a Set. You can emulate one with ConcurrentDictionary. Or a HashSet that you protect yourself with a lock.

answered Jun 12 '10 at 19:33

Hans Passant

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Please expand. What would you use as the key in the underlying ConcurrentDictionary? – Denise Skidmore Jan 10 '14 at 20:19
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Well, I assume the key would be the type of the object you're trying to store, and then the value would be a collection of some sorts. That would "emulate" a `HashSet` as he describes. – Mathias Lykkegaard Lorenzen May 25 '14 at 21:22

score 14 · Answer 3 · edited May 23 '17 at 12:18

14

The ConcurrentBag is great to handle a List where you can add items and enumerate from many thread, then eventually throw it away as its name is suggesting :)

As Mark Byers told, you can re-build a new ConcurrentBag that does not contains the item you wish to remove, but you have to protect this against multiple threads hits using a lock. This is a one-liner:

myBag = new ConcurrentBag<Entry>(myBag.Except(new[] { removedEntry }));

This works, and match the spirit in which the ConcurrentBag has been designed for.

edited May 23 '17 at 12:18

Community

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answered Mar 17 '16 at 14:21

Larry

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I feel this answer is misleading. To be clear, this does NOT provide any thread safety in the desired Remove operation. And putting a lock around it kinda defeats the purpose of using a concurrent collection. – ILIA BROUDNO Oct 07 '16 at 19:53
1

I agree. Well, to clarify a bit, the ConcurrentBag is designed to be filled, enumerated and thrown away with its whole content when it's done. Any attempts -including mine- to remove an item will result in a dirty hack. At least I tried to provide a answer, although the best is to use a better concurrent collection class, like the ConcurrentDictionary. – Larry Oct 07 '16 at 20:01

score 5 · Answer 4 · answered Sep 02 '16 at 12:31

Mark is correct in that the ConcurrentDictionary is will work in the way you are wanting. If you wish to still use a ConcurrentBag the following, not efficient mind you, will get you there.

var stringToMatch = "test";
var temp = new List<string>();
var x = new ConcurrentBag<string>();
for (int i = 0; i < 10; i++)
{
    x.Add(string.Format("adding{0}", i));
}
string y;
while (!x.IsEmpty)
{
    x.TryTake(out y);
    if(string.Equals(y, stringToMatch, StringComparison.CurrentCultureIgnoreCase))
    {
         break;
    }
    temp.Add(y);
}
foreach (var item in temp)
{
     x.Add(item);
}

score 4 · Answer 5 · answered Jun 12 '10 at 19:26

4

As you mention, TryTake() is the only option. This is also the example on MSDN. Reflector shows no other hidden internal methods of interest either.

answered Jun 12 '10 at 19:26

Mikael Svenson

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score -1 · Answer 6 · edited Feb 19 '18 at 16:42

-1

public static void Remove<T>(this ConcurrentBag<T> bag, T item)
{
    while (bag.Count > 0)
    {
        T result;
        bag.TryTake(out result);

        if (result.Equals(item))
        {
            break; 
        }

        bag.Add(result);
    }

}

edited Feb 19 '18 at 16:42

robinCTS

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answered Feb 19 '18 at 08:16

huda

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`ConcurrentBag` is an unordered collection, but your code expects that `bag.TryTake` and `bag.Add` work in a FIFO manner. Your code assumes that `bag` includes `item`, it loops until it finds `item` in `bag`. Code only answers are discouraged, you should explain your solution. – GDavid Feb 27 '20 at 14:55
In your example, there are situations where the collection will not have an item that is required by other threads. – nim Dec 22 '21 at 13:44

score -4 · Answer 7 · answered Feb 20 '18 at 11:53

This is my extension class which I am using in my projects. It can a remove single item from ConcurrentBag and can also remove list of items from bag

public static class ConcurrentBag
{
    static Object locker = new object();

    public static void Clear<T>(this ConcurrentBag<T> bag)
    {
        bag = new ConcurrentBag<T>();
    }


    public static void Remove<T>(this ConcurrentBag<T> bag, List<T> itemlist)
    {
        try
        {
            lock (locker)
            {
                List<T> removelist = bag.ToList();

                Parallel.ForEach(itemlist, currentitem => {
                    removelist.Remove(currentitem);
                });

                bag = new ConcurrentBag<T>();


                Parallel.ForEach(removelist, currentitem =>
                {
                    bag.Add(currentitem);
                });
            }

        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.Message);
        }
    }

    public static void Remove<T>(this ConcurrentBag<T> bag, T removeitem)
    {
        try
        {
            lock (locker)
            {
                List<T> removelist = bag.ToList();
                removelist.Remove(removeitem);                

                bag = new ConcurrentBag<T>();

                Parallel.ForEach(removelist, currentitem =>
                {
                    bag.Add(currentitem);
                });
            }

        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.Message);
        }
    }
}

It's hard to believe it can work because you are creating new ConcurrentBag in local variable, but I am not sure. any test? — shtse8, Aug 06 '18 at 22:27
No, it doesn't work. How should it work, you are creating a only a new reference. The old one is still pointing to the old object.... It would work if you work with "ref" — Bin4ry, Sep 30 '18 at 19:12

score -6 · Answer 8 · edited Jul 24 '15 at 11:42

-6

public static ConcurrentBag<String> RemoveItemFromConcurrentBag(ConcurrentBag<String> Array, String Item)
{
    var Temp=new ConcurrentBag<String>();
    Parallel.ForEach(Array, Line => 
    {
       if (Line != Item) Temp.Add(Line);
    });
    return Temp;
}

edited Jul 24 '15 at 11:42

GrandMasterFlush

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answered Jul 24 '15 at 11:39

Evgeniy Nikulov

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score -15 · Answer 9 · edited Jul 04 '13 at 13:47

-15

how about:

bag.Where(x => x == item).Take(1);

It works, I'm not sure how efficiently...

edited Jul 04 '13 at 13:47

tkanzakic

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answered Jul 04 '13 at 13:28

Simon Gullive

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This does not remove anything from the bag. The item you are retrieving remains inside the bag. – Keith Dec 19 '13 at 19:49
5

should be "bag = new ConcurrentBag(bag.Where(x => x != item))" – atikot Nov 19 '14 at 09:26
6

@atikot, that line made me laugh – parek Jun 23 '15 at 09:14

How to remove a single, specific object from a ConcurrentBag<>?

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