ConcurrentBag - Add Multiple Items?

问题:

Is there a way to add multiple items to ConcurrentBag all at once, instead of one at a time? I don't see an AddRange() method on ConcurrentBag, but there is a Concat(). However, that's not working for me:

ConcurrentBag<T> objectList = new ConcurrentBag<T>();

timeChunks.ForEach(timeChunk =>
{
    List<T> newList = Foo.SomeMethod<T>(x => x.SomeReadTime > timeChunk.StartTime);
    objectList.Concat<T>(newList);
});

This code used to be in a Parallel.ForEach(), but I changed it to the above so I could troubleshoot it. The variable newList indeed has objects, but after the objectList.Concat<> line, objectList always has 0 objects in it. Does Concat<> not work that way? Do I need to add items to ConcurrentBag one at a time, with the Add() method?

回答1:

Yes :)

Concat is perhaps one of the Enumerable extensions. It doesn't add anything to the ConcurrentBag, it just returns some funky object containing the original bag and whatever you tried to add there.

Beware that the result of Concat is not a ConcurrentBag anymore, so you would not want to use it. It's a part of general LINQ framework, making possible to combine immutable sequences. This framework, of course, doesn't try to extend the concurrent properties of the operands to the result, so the resulting object will not be so well suited for multithreaded access.

(Basically, Concat applies to ConcurrentBag because it exposes IEnumerable<T> interface.)

回答2:

(I know this is an old post, thought I'd add a little something).

Like others have said: yes, you need to add them one by one. In my case, I added a small extension method to make things a bit cleaner, but under the hood it does the same thing:

    public static void AddRange<T>(this ConcurrentBag<T> @this, IEnumerable<T> toAdd)
    {
        foreach (var element in toAdd)
        {
            @this.Add(element);
        }
    }

And then:

    ConcurrentBag<int> ccBag = new ConcurrentBag<int>();
    var listOfThings = new List<int>() { 1, 2, 4, 5, 6, 7, 8, 9 };
    ccBag.AddRange(listOfThings);

I also looked at using AsParallel to add within the extension method, but after running some tests on adding a list of strings of various sizes, it was consistantly slower to use AsParallel (as shown here) as opposed to the traditional for loop.

    public static void AddRange<T>(this ConcurrentBag<T> @this, IEnumerable<T> toAdd)
    {
        toAdd.AsParallel().ForAll(t => @this.Add(t));
    }

回答3:

Concat is an extension method provided by LINQ. It is an immutable operation that returns another IEnumerable that can enumerate the source collection followed immediately by the specified collection. It does not, in any way, change the source collection.

You will need to add your items to the ConcurrentBag one at a time.

回答4:

I faced a similar issue, trying to process smaller chunks of data in parallel, because one large chunk was timing out the web service I was using to access my data on the sending side but I did not want things to run slower by processing each chunk serially. Processing the data record by record was even slower - since the service I was calling could handle bulk requests, it would be better to submit as many as possible without timing out.

Like Vlad said, concatting a concurrent bag to a list of an object type doesn't return a concurrent bag, so concat won't work! (It took me a while to realize I couldn't do that.)

Try this instead - create a List<T>, and then create a ConcurrentBag<List<T>>. On each parallel iteration, it will be adding a new list to the concurrent bag. When the parallel loop is done, loop through the ConcurrentBag and concat (or union if you want to eliminate possible duplicates) to the first List<T> that you created to "flatten" everything into one list.