The lapsed listener problem is one of the primary causes of leaks in .NET applications. It occurs when a subscriber (or 'listener') signs up for a publisher's event, but fails to unsubscribe. The failure to unsubscribe means that the publisher maintains a reference to the subscriber as long as the publisher is alive. For some publishers, this may be the duration of the application.This situation causes two problems. The obvious problem is the leakage of the subscriber object. The other problem is the performance degredation due to the publisher sending redundant notifications to 'zombie' subscribers.
There are at least a couple of solutions to the problem. The simplest is to make sure the subscriber is unsubscribed from the publisher, typically by adding an Unsubscribe() method to the subscriber. Another solution, documented here by Shawn Van Ness, is to change the publisher to use weak references in its subscriber list.
Serialization is the process of converting an object into a stream of bytes. Deserialization is the opposite process, i.e. creating an object from a stream of bytes. Serialization/Deserialization is mostly used to transport objects (e.g. during remoting), or to persist objects (e.g. to a file or database).
There are two separate mechanisms provided by the .NET class library - XmlSerializer and SoapFormatter/BinaryFormatter. Microsoft uses XmlSerializer for Web Services, and SoapFormatter/BinaryFormatter for remoting. Both are available for use in your own code.
There is a once-per-process-per-type overhead with XmlSerializer. So the first time you serialize or deserialize an object of a given type in an application, there is a significant delay. This normally doesn't matter, but it may mean, for example, that XmlSerializer is a poor choice for loading configuration settings during startup of a GUI application.
XmlSerializer will refuse to serialize instances of any class that implements IDictionary, e.g. Hashtable. SoapFormatter and BinaryFormatter do not have this restriction.