Foreach loop and variables with locking

Question

Basically what I want to do is this psuedo code

List<DatabaseRecord> records;

List<ChangedItem> changedItems;

Parallel.ForEach<DatabaseRecord>(records, (item, loopState) =>
{
    if (item.HasChanged)
    {
        lock (changedItems)
        {
            changedItems.Add(new ChangedItem(item));
        }
    }
});

But what I'm worried about is locking on the changedItems. While it works, I have heard that it has to serialize the locked object over and over. Is there a better way of doing this?

Answer 1

Why don't you use PLinq instead? No locking needed:

changedItems = records.AsParallel()
                      .Where(x => x.HasChanged)
                      .Select(x => new ChangedItem(x))
                      .ToList();

Since you are projecting into a new list of ChangedItem and do not have any side effects this would be the way to go in my opinion.