.NET parallel processing of ArrayList

Question

I am attempting at embedding multi-threading for the first time ever and running into some unexpected issues, hope you can help.

Here's the code fragment that gives me troubles:

ArrayList recordsCollection = new ArrayList();
ArrayList batchCollection = null;
int idx = 0;

while(true)
{
  // Some code to generate and assign new batchCollection here
  recordsCollection.Add(batchCollection);

  ThreadPool.QueueUserWorkItem(delegate
  {
    ProcessCollection(recordsCollection.GetRange(idx, 1));
  });
  Interlocked.Increment(ref idx);
}

private void ProcessCollection(ArrayList collection)
{
   // Do some work on collection here
}

Once the Process Collection method is invoked and I am attempting to iterate through collection I am getting "The range in the underlying list is invalid".

Thanks in advance!

Update: Guys, thank you to each and every one of you. Through applying your suggestions I was able to greatly simplify and get it to work.

Answer 1

You have a couple of problems.

• Like Mark pointed out you are capturing a loop variable which will really confuse things here.
• You are modifying the collection while at the same time reading it without the use of synchronization mechanisms.

I am assuming you have omitted the code for acquiring a batchCollection and then periodically removing them from recordsCollection for brevity, otherwise there would be issues there as well.

Here is how you can fix it.

ArrayList recordsCollection = new ArrayList();  
ArrayList batchCollection = null;  
int idx = 0;  

while(true)  
{  
  lock (recordsCollection) 
  {
    recordsCollection.Add(batchCollection);  
  }

  int capturedIndex = idx; // Used for proper capturing.

  ThreadPool.QueueUserWorkItem(delegate  
  {
    ArrayList range;
    lock (recordsCollection)
    {
      range = recordsCollection.GetRange(capturedIndex, 1);
    }
    ProcessCollection(range);  
  });  

  idx++;
}  

Or my refactored version which, as best I can tell anyway, does the exact same thing...

List<List<Record>> recordsCollection = new ArrayList();  
List<Record> batchCollection = null;  

while(true)  
{  
  recordsCollection.Add(batchCollection);

  List<List<Record>> range = new List<List<Record>>();
  range.Add(batchCollection);

  ThreadPool.QueueUserWorkItem(delegate  
  {
    ProcessCollection(range);  
  });      
}