ConcurrentBag of strings and using .Contains in Parallel.ForEach

ConcurrentBag of strings and using .Contains in Parallel.ForEach

 

The collection is threadsafe, as in "using it concurrently won't corrupt its state". It doesn't make your "contains then add" operation atomic. It's your code that isn't threadsafe

ConcurrentBag is threadsafe, but your code isn't:

if (!SystemNames.Contains(name))
{
    SystemNames.Add(name);
}

Contains will execute in a thread-safe way, then Add will also execute in a thread-safe way, but you have no guarantee that an item haven't been added in-between.

For your needs, I recommend using a ConcurrentDictionary instead. Just ignore the value as you won't need it.

var SystemNames = new ConcurrentDictionary<string, bool>();

Then use the TryAdd method to do the "if not contains then add" in a single atomic operation:

SystemNames.TryAdd(name, true);