Adding to a list in a Parallel.ForEach loop in a threadsafe manner

Question

I have a bit of code that works like this on a list of obj objects called ListofObjects:

List<SomeObject> NewListofObjects<SomeObject>();

Parallel.ForEach(ListofObjects, obj =>

//Do some operations here on obj to get a newobj

NewListofObjects.Add(newobj);

);

Now I am out of the Parallel.ForEach loop, and I want to do an operation on NewListofObjects. However, I get this error when I try to: "Attempted to read or write protected memory. This is often an indication that other memory is corrupt".

Is this because my NewListofObjects.Add(newobj) method is not threadsafe? If so, how can I make it threadsafe?

Answer 1

Is this because my NewListofObjects.Add(newobj) method is not threadsafe?

Correct. It is not threadsafe.

Any instance members are not guaranteed to be thread safe.

That's from MSDN referring to List<T> (scroll to the section titled "Thread Safety").

If so, how can I make it threadsafe?

Use a concurrent collection, like ConcurrentBag<T>. Note that you lose the ability to keep track of the order that items were inserted.

Answer 2

You can use the locking block like the following code to insert items into your list in a thread-safe manner.

var sync = new object();
var myNewList = new List<SomeObject>();
Parallel.ForEach(myListOfSomethings, a =>
    {
        // Some other code...
        var someObj = new SomeObject();
        // More other code...
        lock(sync)
        {
            myNewList.Add(someObj);
        }
        // Even more code...
    });

Answer 3

The .NET Framework 4 introduces the System.Collections.Concurrent namespace, which includes several collection classes that are both thread-safe and scalable. https://learn.microsoft.com/en-us/dotnet/standard/collections/thread-safe/

BlockingCollection<int>[] sourceArrays = new BlockingCollection<int>[5];
for (int i = 0; i < sourceArrays.Length; i++)
    sourceArrays[i] = new BlockingCollection<int>(500);
Parallel.For(0, sourceArrays.Length * 500, (j) =>
{
    int k = BlockingCollection<int>.TryAddToAny(sourceArrays, j);
    if (k >= 0)
        Console.WriteLine("added {0} to source data", j);
});