Although creating a toolwindow and hosting a usercontrol in a Visual Studio package seems easy (thanks to the package wizard), initializing the usercontrol to receive an instance of your package (or the value of some property of your package) is somewhat tricky.
I have seen this question from time to time in the forums and until now my (wrong) answer was to use the ShowToolWindow event handler method of the package to get the instance of the usercontrol from the ToolWindowPane (using its Content property for WPF usercontrols and its Window property for Windows Forms usercontrols). Once you have the usercontrol instance, you can call a method of the usercontrol to pass it any piece of information that you want to initialize it. I won’t show the code because it is an incorrect approach due to this: when using a toolwindow in a package, if the last time that you closed Visual Studio the toolwindow was visible, the next time that you open Visual Studio the toolwindow will be shown automatically without the ShowToolWindow event handler method being called (so your usercontrol wouldn’t be initialized in that scenario).
This didn’t happen with add-ins (toolwindows were not open automatically in the next session if left open in the previous session). But with packages it happens, so the ShowToolWindow method of the package is not the proper place to initialize the usercontrol. Then, it must be done in the MyToolWindow class, which creates the usercontrol and it is always called, and fortunately it has a Package base property that you can use to get the instance of your package. Alas, that instance is null in the toolwindow constructor. My latest article shows a workaround:
HOWTO: Initialize the usercontrol of a toolwindow from a Visual Studio package