Instances of the IConfigurationSectionHandler class must be thread safe and stateless. The Create method must be callable from multiple threads simultaneously.
Furthermore, the configuration object generated by the Create method must be thread safe and immutable. Because the configuration objects are cached by the configuration system, it is important not to modify the parent argument to the Create method. For example, if the return value of Create is only a small modification of the parent, actual modifications must be made on a clone of the parent, not the original.
http://msdn.microsoft.com/zh-cn/library/ms228056(v=vs.80).aspx
http://www.cnblogs.com/guozhijian/archive/2008/01/26/1054210.html
http://www.codeproject.com/Articles/10981/Understanding-Section-Handlers-App-config-File