I have a class that looks like this:
public class MyService
{
private MyService(){}
public static string GetStuff()
{
var stuffDid = new
Maybe your Test project is referencing an older Core binary, rather than the Core (source code) project?
Try re-adding the reference in your Test project:
Go to your Test project and remove the reference to the Core project.
Now select the References folder and right click it and select the menu option to add a new reference. When in the Reference Manager dialog, make sure you are selecting Solution and then Projects on the left. Then in the middle of the Reference Manager dialog, select (check) the Core project.
Try debugging again and see if that helps.