See the question and my original answer on StackOverflow

You have to use the IVsSolutionEvents7.OnAfterOpenFolder Method that has been added for Visual Studio 2017.

Notifies listening clients that the folder has been opened.

public void OnAfterOpenFolder (string folderPath);

Since this is a native COM interface, you also have to make sure the implementing class is COM visible (through the ComVisible attribute that you can set on the assembly, on the class, on a base class, etc.).