Question

I am using a Parallel.Foreach for populating an external ConcurrentBag. I tried also to use a common List and everything works fine.

I have been lucky or I missed the special scope of ConcurrentBag?

Answer 1

You have been lucky; Parallel.ForEach to populate a List is not thread-safe, you will eventually run into problems.

According to MSDN, List<T> is not thread safe:

Any instance members are not guaranteed to be thread safe.

A List<T> can support multiple readers concurrently, as long as the
collection is not modified. Enumerating through a collection is
intrinsically not a thread-safe procedure. In the rare case where an
enumeration contends with one or more write accesses, the only way to
ensure thread safety is to lock the collection during the entire
enumeration. To allow the collection to be accessed by multiple
threads for reading and writing, you must implement your own
synchronization.

ConcurrentBag is what you should use for this, which is thread-safe for multiple readers and writers.