for some time now, on my 2nd development machine VS2012 is not showing any (XML-)documentation for system/GAC/built-in assemblies. E.g., when I jump to a built-in Type that resides in mscorlib.dll, there is no documentation (see screenshot). If I include my own assemblies (that are accompanied by their respective XMLs) I can see the full doc. I tried resetting Intellisense but that didn't help. This is how it looks:
(The pluses over each member that would expand the comment are totally missing and ctrl+space doesn't show anything either)
The most likely cause for this problem is the XML documentation for mscorlib is not installed in the same location as the mscorlib reference assembly. The best way to check for this
- Expand the region at the top of the file
- It will contain the full path of the assembly C# is using. For example
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.5\mscorlib.dll
- Open that directory in explorer
- Check for mscorlib.xml
If the file is not there then it looks like the reference assemblies failed to properly install documentation. You could just copy it from another machine or repair the Visual Studio installation
来源：https://stackoverflow.com/questions/22028236/missing-documentation-for-system-assemblies-in-vs2012