This introduces a race condition: imagine that two threads execute line 39 at the same time (and both find that `EncoderLookup == null`). Both threads could then execute `CreateEncoderLookup()`. The first thread will take the lock and execute the code in that method. Once it exits, the second thread will take the lock; the first statement it executes is `EncoderLookup = new Dictionary<char, string>();`, which will replace the initialised lookup with an empty dictionary. The first thread may then execute the rest of this method using an empty dictionary, which will generate incorrect output.