This has nothing to do with Code Analysis, which is why its generated code setting isn't relevant. The C# compiler is generating the warning (which you are treating as an error) for the lack of xmldoc comment on the class and constructor.
Unfortunately, the approach used for generating the .g.cs file is hardcoded in Microsoft.Build.Tasks.Xaml.ClassGenerator (in XamlBuildTask.dll). It does not use a template, and there's no way to override just part of its behavior.
I can see only two relatively simple approaches that don't involve replacing the entire class generation mechanism:
- Generate your activity class with internal instead of public visibility if that works for your scenario (see http://msdn.microsoft.com/en-us/library/ms754029(v=vs.110).aspx for details).
- Move just your activities to a separate project that disables the CS1591 compiler warning.
Either way, you might want to submit a request at http://visualstudio.uservoice.com/ for the .g.cs file generation to be moved to a templated approach.