Simple example, that explains everything
Core WebApi functionality is located in System.Web.Http.dll from Microsoft.AspNet.WebApi.Core Nuget.
Marks HttpGetAttribute, HttpPostAttribute and other Http*Attributes with MeansImplicitUseAttribute.
So ReSharper no longer suggests to remove unused WebApi methods.
Also marks ApiController with UsedImplicitlyAttribute(ImplicitUseKindFlags.Access) to prevent warning class MyController is never used when MyController is inherited from ApiController.
Tested with:
- Resharper 9.0 and Visual Studio 2013,
- Resharper 10.0 and Visual Studio 2015.
- Clean ReSharper project cache Resharper->Options->Environment->General->Clear Caches.
- Restart Visual Studio.
https://resharper-plugins.jetbrains.com/packages/WebApi.Annotations/

