Question

Im just confused why adding to a list would not be thread safe like below

object aLock = new object();
List<string> aList = new List<string>

lock(aLock)
   aList.Add("abc");

Not sure why a lock would be required where all you are doing is adding to it.
Why would such a scenario be not thread safe?

Answer 1

The example code is useless- as @Jon mentions, all threads would be locking on their own object, which means they wouldn’t be blocking each other at all. Might as well leave out the lock statement entirely.

First of all, you need to lock on an object that’s common to all of the threads (like the list itself). For instance:

lock (aList)
    aList.Add("abc");

As for “why”, the internal implementation of the List may (does) perform actions that are not safe to perform in parallel on multiple threads. This is documented in the List class MSDN docs:

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

A List 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.