C5 http://www.itu.dk/research/c5/
C5 is a library of generic collection classes for the upcoming version 2.0 of the .Net platform. C5 extends the standard .Net System.Collections.Generic namespace with tree data structures, heap based priority queues and hash indexed array lists and linked lists, making it more comprehensive than most collection class libraries on similar platforms. Unlike many other collection class libraries, C5 is designed with a strict policy of supporting "code to interface not implementation".
之所以看到那么多的Collection,我想还是因为.Net framework现在没有提供完整的数据结构.我的intern就向我抱怨说.Net framework 1.1里没有linkedList和Tree.