随机程序集引用失败(“您是否缺少使用指令或程序集引用?”)

My application has a mixture of 3.5 and 4.0-targeted assemblies. I'm working on a new Windows service targeting 4.0 and the project suddenly seems unable to see some of the other assemblies in the solution. Meaning, on build, all references to these certain other assemblies produce the following error:My application has a mixture of 3.5 and 4.0-tar