I ran into the following intellisense error while adding a new proto file:
This was one of many errors that were shown and appeared to be an issue with Intellisense mistaking the file for a normal class definition. The following appears to have fixed it:
1) Right click on the file and select “exclude from project”
2) Clean the solution and rebuild
3) Re-include the file in the project (you may have to toggle ‘Show All Files’ at the top of solution explorer to see the excluded item).