该问题原来是GAC中安装的一组旧程序集，这些旧程序集是从Web API的Beta版安装中遗留下来的.我能够使我的项目引用新的程序集，并且问题得以解决.

The problem turned out to be a set of old assemblies installed in the GAC that were left over from a beta installation of the Web API. I was able to get my project to reference the new assemblies, and the problem was resolved.