static methods and sessions

Hi there If you use WCF and set the host to create sessions, I am wondering what happens when two sessions simultaneously use a static method? for example, if session A calls static dosomething which creates, for example, an SQL string, and then session B calls it, is session B blocked until A finishes? or can session B corrupt A's data? What if the method is not static, but there is only one object to use (a singleton)? Is it any different? Thanks