vs 2017 intellisense not working C# and XAML

1. Close all open tabs in the project and quit VS, reopen the solution in VS and right click the XAML file in the Solution Explorer and then select Open With….> Source Code (Text) Editor.
2. Delete obj folder and clean project
3. Right click the XAML page>Properties>Build Action>change it to something else and back
4. Add a new content page under this specific project and check it works or not.

I would like to add that this worked for my .cs files only.

To get the XAML files Intellisense to work try to repeat the above steps on your .cs files or wait for the XAML files to gain Intellisense.

For ReSharper users: I found that having ReSharper's IntelliSense enabled for all languages can break Intellisense unexpectedly (especially for XAML files). Here's how I was able to fix the same problem that the asker had in Visual Studio 2017 with ReSharper installed:

1. In Visual Studio, go to the ReSharper menu and click on Options.
2. On the left side go to Environment > IntelliSense > General.
3. Select the Custom IntelliSense radio button.
4. Change any languages that have broken IntelliSense to Visual Studio (like XAML).
5. Click the Save button.

Just Exclude and Include Xaml Pages and It works again.