Parallel.ForEach with adding to list

You can use a concurrent collection.

The System.Collections.Concurrent namespace provides several thread-safe collection classes that should be used in place of the corresponding types in the System.Collections and System.Collections.Generic namespaces whenever multiple threads are accessing the collection concurrently.

You could for example use ConcurrentBag since you have no guarantee which order the items will be added.

Represents a thread-safe, unordered collection of objects.