Thread safe collections in .NET: ConcurrentBag

Concurrent collections in .NET work very much like their single-thread counterparts with the difference that they are thread safe. These collections can be used in scenarios where you need to share a collection between Tasks. They are typed and use a lightweight synchronisation mechanism to ensure that they are safe and fast to use in parallel programming.

Concurrent bags

Concurrent bags are similar to concurrent stacks and concurrent queues but there’s a key difference. Bags are unordered collections. This means that the order of the items is not the same as how they were inserted. So concurrent bags are ideal if you would like to share a List of T generic collection among several tasks.

Important methods:

  • Add(T element): adds an item of type T to the collection
  • TryPeek(out T): tries to retrieve the next element from the collection without removing it. The value is set to the out parameter if the method succeeds. Otherwise it returns false.
  • TryTake(out T): tries to get the first element. It removes the item from the collection and sets the out parameter to the retrieved element. Otherwise the method returns false

The ‘try’ bit in the method names imply that your code needs to prepare for the event where the element could not be retrieved. If multiple threads retrieve elements from the same list you cannot be sure what’s in there when a specific thread tries to read from it.

Example

The example is almost identical to what we saw for the collections discussed previously in the posts on TPL.

Declare and fill a concurrent bag:

ConcurrentBag<int> concurrentBag = new ConcurrentBag<int>();

for (int i = 0; i < 5000; i++)
{
	concurrentBag.Add(i);
}

Next we’ll try to take every item from the bag. The bag will be accessed by several tasks at the same time. The counter variable – which is also shared and synchronised- will be used to check if all items have been retrieved.

int counter = 0;

Task[] bagTasks = new Task[20];
for (int i = 0; i < bagTasks.Length; i++)
{
	bagTasks[i] = Task.Factory.StartNew(() =>
	{
		while (concurrentBag.Count > 0)
		{
			int bagElement;
			bool success = concurrentBag.TryTake(out bagElement);
			if (success)
			{
				Interlocked.Increment(ref counter);
			}
		}
	});
}

The while loop will ensure that we’ll try to take the items as long as there’s something left in the collection.

Wait for the tasks and print the number of items processed – the counter should have the same value as the number of items in the bag:

Task.WaitAll(bagTasks);
Console.WriteLine("Counter: {0}", counter);

View the list of posts on the Task Parallel Library here.

About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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