In the project column of the "Error List" window, I see DNX Core 5.0.

The type or namespace '[someclass]' could not be found.
In the project column of the "Error List" window, I see DNX Core 5.0.


solution:

In case some people are still struggling with this, you can dereference the DNX Core 5.0 by removing it from your framework references in the project.json file.

The section

  "frameworks": {
    "dnx451": { },
    "dnxcore50": { }
  },
Should become

  "frameworks": {
    "dnx451": { }
  },


now i got bellow error:
Error: Please run “dnu restore” to generate a new lock file

solution:
Go to the project where you are getting above error
Right click on "References" then select "Restore Packages"