is locking necessary for Dictionary lookup?

Question

lock(dictionaryX)
{
   dictionaryX.TryGetValue(key, out value);
}

is locking necessary while doing lookups to a Dictionary ?

THe program is

Answer 1

If you have multiple threads accessing the Dictionary, then you do need to lock on updates and on lookup. The reason you need to lock on lookup is that there could be an update taking place at the same time you're doing the lookup, and the Dictionary could be in an inconsistent state during the update. For example, imagine that your have one thread doing this:

if (myDictionary.TryGetValue(key, out value))
{
}

and a separate thread is doing this:

myDictionary.Remove(key);

What could happen is that the thread doing the TryGetValue determines that the item is in the dictionary, but before it can retrieve the item, the other thread removes it. The result would be that the thread doing the lookup would either throw an exception or TryGetValue would return true but the value would be null or possibly an object that does not match the key.

That's only one thing that can happen. Something similarly disastrous can happen if you're doing a lookup on one thread and another thread does an add of the value that you're trying to look up.