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);