Visual Studio 2015 has been RTM for one month. However, it blows up very often, such as Razor editor. When I try to open any .cshtml file, it blows up with this error, then the syntax highlight is gone. I tried to uninstall all add-on, even repair install VS2015 can't get it working.

The detailed error message is:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentException: Item has already been added. Key in dictionary: 'RazorSupportedRuntimeVersion'

Someone has already filed a bug on "connect", and it is active:

https://connect.microsoft.com/VisualStudio/feedback/details/1572978/no-intellisense-in-razor-views-argumentexception-razorsupportedruntimeversion-already-added-to-dictionary

The solution is：

1. Close all instance of VS2015

2. Run cmd as administrator, go to VS2015 setup directory, and run:

devenv /resetuserdata

3. Go to your project folder, delete a hidden folder named ".VS"

4. Restart VS2015, it will work fine!