GAC解决的问题是dotnet assembly如何共享assembly的问题，比如dotnet framework，肯定要部署到GAC中。注：dotnet assembly locate rule是先找GAC，再找当前app所在的目录树（CLR will firstly search in GAC for assembly loading and then get into app specific directory / path tree）。所以，对于公用的assembly，只有一条路：就是放在GAC中。
sidebyside assembly解决的是dotnet assembly 版本冲突的问题。两方面的工作借助OS作为媒介来实现：一方面，那些sidebyside assembly，比如donet framework assemblies，MFC，ATL，CRT dlls，需要一个manifest文件来描述它自己给OS； 另一方面，使用它们的app，需要一个manifest文件来告诉OS他要使用的是哪个版本的assembly。这个样子下来，OS就了了，就能在app跑起来的时候，知道app需要哪个版本的assembly并知道去哪里去找到啦。(DLL hell designates a group of problems that arise from the use of dynamic-link libraries in Microsoft Windows. Problems include version conflicts, missing DLLs, duplicate DLLs, and incorrect or missing registration. In SxS, Windows stores multiple versions of a DLL in the WinSXS subdirectory of the Windows directory, and loads them on demand. This reduces dependency problems for applications that include an SxS manifest.)
