总结：
1. 对于native c++ world来说，Dynamic-Link Library Search Order 说的很清楚了。就是还有一个疑问：如果在任意目录建立一新folder，把一个application需要的plugin之类的dll及dependency dlls都放在里面，在load这个plugin需要的dependency dlls时 （If a DLL with dependencies is loaded by specifying a full path, the system searches for the DLL's dependent DLLs as if they were loaded with just their module names.）这个新的folder会被search到吗？
2. 对于dotnet world，首先理解有两个application级的文件：app.exe.manifest, app.exe.config。 然后，关于runtime CLR如何locate assembly，这篇How the Runtime Locates Assemblies 是全面的介绍。The simple version is that the GAC is searched firstly, and then the app path tree. %PATH% is never used. 相关基础知识：
1）In dotnet world, A private assembly is stored in the application’s directory and used by a single application. A share assembly can be used by multiple applications and is stored in the Global assembly cache, a repository of assemblies maintained by the .Net Framework.
2）GAC 和sidebyside的区别，本blog里有详细介绍。
3）如果你确实需要把一个app的private assembly（一定程度上算，但绝对算不是放在GAC里的资格）搬出app path来放，MS还提供了一套机制来帮助你定位assembly成功：AssemblyResolver。具体介绍，参见本blog AssemblyResolver。
参考: In what order are locations searched to load referenced DLLs?