Performant XML (I): Dynamic XPath expressions compilation

"Everybody" knows that precompiling your XPath expressions ahead of time, usually in static .ctors, improves execution performance. But what's actually happening at that "compilation" time? Unlike what many believe, compiled IL is NOT being generated, neither on-the-fly assemblies or anything like it. It's NOT like RegEx compilation, sadly. But what's going on is important for performance because it involves parsing, AST (abstract syntax tree) building, etc.

http://weblogs.asp.net/cazzu/archive/2003/10/07/30888.aspx