What's wrong with this code - #6

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After a long hiatus, here's the next entry in my not-really-regular-enough-to-honestly-be-called-periodic-(though-I-try-to-keep-up-a-somewhat-reasonable-schedule-I-don't-seem-to-keep-it-since-I'm-lazy-are-you-still-reading-this)-series about C# code.

Anyway, here's some code that I've seen a fair bit, and written myself:

 class Notifier

{

   ArrayList items = new ArrayList();

   public void Add(object o) {

      lock (this) {

         items.Add(o);

      }

   }

}

 

This code will work fine, but it has a latent issue. What is the issue, and how should it be addressed?