Is the List<T>.AddRange() thread safe?

No, its documentation does not say it is thread safe, therefore it is not.

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

As to what can go wrong, think about what AddRange(newItems) does:

• Check if there is enough space in the internal array
• If not:
  • Allocate a new array
  • Copy the current items to the new array
  • Set a field to point at the new array
• Copy the newItems to the correct local in the internal array
• Update the “count” field (this is used to control where the next item is inserted)

Now think what will happen if the above is mixed up with another call to AddRange() or even just a call to read an item.

No it's not, but I'd like to add it's more efficient to do an myList.AddRange(...); within a lock than doing several lock (syncLock) { myList.Add(...) };.

Which sort of trouble would you run into? When one thread is adding an item while another is enumerating the list, List<T> will throw a certain exception because it does some internal versioning, as it wants to prevent us poor developers from hitting nasty side effects.

Also the List<T> internally keeps an array in which it stores its items. Maybe setting an item in an array is pretty atomic, but whenever the capacity of this array is reached, a new one will be created and the items will be copied over from the old one. So when a thread wants to add something while that copying takes place, you can imagine that things would go out of sync.

Depending on your usage, SynchronizedCollection could work.

You'd have no single-shot AddRange though. If you are only using this to seed the collection, you can do this as there is an IEnumerable constructor overload.