传说中的HACK…
靠看文档是不知道的
http://wekempf.spaces.live.com/blog/cns!d18c3ec06ea971cf!468.entry?wa=wsignin1.0&sa=837408022
http://blogs.msdn.com/b/johngossman/archive/2008/07/28/how-to-initialize-an-attached-dependencyproperty-of-type-collection.aspx
