Update: User Control in Tool Window of a Visual Studio Addin

After some more experimentation it seems that the last parameter of CreateToolWindow2() is null, if the hosted control is in a separate assembly / project. For instance: Suppose you have a solution containing two projects, let's call them MyAddIn and MyControls. The first one is your add-in itself and the second one contains some controls you want to have in a separate assembly as you want to use them also for other solutions, e.g. a stand-alone application. Now, if you have a UserControl in the project MyControls, let's call it MyUserControl1, then you can still host that control in your add-in. However, in this case the method CreateToolWindow2() returns null as the value of the last parameter. If on the other hand you add a UserControl, let's call it MyUserControl2, to the add-in project 'MyAddIn', and then host MyUserControl2 in the add-in, the last parameter of CreateToolWindow2() will contain a reference to your MyUserControl2 instance. Of course MyUserControl2 can in turn contain one or more instances of MyUserControl1. Strange that whether or not a UserControl is contained in the add-in assembly/project or not makes a difference regarding the last parameter of CreateToolWindow2(). Am I missing something?