List<T> thread safety

Question

I am using the below code

var processed = new List<Guid>();
Parallel.ForEach(items, item => 
{
    processed.Add(SomeProcessingFunc(item));
});

Is the above code thread safe? Is there a chance of processed list getting corrupted? Or should i use a lock before adding?

var processed = new List<Guid>();
Parallel.ForEach(items, item => 
{
    lock(items.SyncRoot)
        processed.Add(SomeProcessingFunc(item));
});

thanks.

Answer 1

No! It is not safe at all, because processed.Add is not. You can do following:

items.AsParallel().Select(item => SomeProcessingFunc(item)).ToList();

Keep in mind that Parallel.ForEach was created mostly for imperative operations for each element of sequence. What you do is map: project each value of sequence. That is what Select was created for. AsParallel scales it across threads in most efficient manner.

This code works correctly:

var processed = new List<Guid>();
Parallel.ForEach(items, item => 
{
    lock(items.SyncRoot)
        processed.Add(SomeProcessingFunc(item));
});

but makes no sense in terms of multithreading. locking at each iteration forces totally sequential execution, bunch of threads will be waiting for single thread.

Answer 2

From Jon Skeet's Book C# in Depth:

As part of Parallel Extensions in .Net 4, there are several new collections in a new System.Collections.Concurrent namespace. These are designed to be safe in the face of concurrent operations from multiple threads, with relatively little locking.

These include:

• IProducerConsumerCollection<T>
• BlockingCollection<T>
• ConcurrentBag<T>
• ConcurrentQueue<T>
• ConcurrentStack<T>
• ConcurrentDictionary<TKey, TValue>
• and others

Answer 3

Use:

var processed = new ConcurrentBag<Guid>();

See parallel foreach loop - odd behavior.

Answer 4

reading is thread safe, but adding is not. You need a reader/writer lock setup as adding may cause the internal array to resize which would mess up a concurrent read.

If you can guarantee the array won't resize on add, you may be safe to add while reading, but don't quote me on that.

But really, a list is just an interface to an array.

Answer 5

As alternative to the answer of Andrey:

items.AsParallel().Select(item => SomeProcessingFunc(item)).ToList();

You could also write

items.AsParallel().ForAll(item => SomeProcessingFunc(item));

This makes the query that is behind it even more efficient because no merge is required, MSDN. Make sure the SomeProcessingFunc function is thread-safe. And I think, but didn't test it, that you still need a lock if the list can be modified in an other thread (adding or removing) elements.

Answer 6

Using ConcurrentBag of type Something

var bag = new ConcurrentBag<List<Something>>;
var items = GetAllItemsINeed();
Parallel.For(items,i =>                          
   {
      bag.Add(i.DoSomethingInEachI());
   });