List thread safety | 易学教程

问题

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.

回答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.

回答2:

Use:

var processed = new ConcurrentBag<Guid>();

See parallel foreach loop - odd behavior.

回答3:

To quote from Jon Skeet before he gets here:

As part of Parellel 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>, and ConcurrentDictionary<TKey, TValue> among others.

回答4:

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.

回答5:

Using ConcurrentBag of type Something

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

回答6:

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.

来源：https://stackoverflow.com/questions/5020486/listt-thread-safety

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