Windows is trying hard to ensure that nobody is trying to steal the keyboard focus while a user is working. Apart from Windows Update, which is known for stealing the focus at the most annoying time, modern versions of Windows effectively don’t do anything when calling SetActiveWindow() on an window which does not belong to the already active application.

But my use case is legitimate

In my case, I was trying to find a solution, where two applications (a main app and a companion app) have to switch back and forth between their windows.

The main app has a button which brings up a window of the companion app, and should change the focus so that the user can directly type into the companion app.

The solution found on StackOverflow Is there a reliable way to activate / set focus to a window using C#? did not work out for me.

A clean solution - if you can modify both apps

I’ve come up with the following solution:

• The main app starts the companion app and retrieves the window handle of the companion window.
• The main app sends the companion app its main window handle.
• The companion app sets the owner window to the main window of the main app, using the native Win32 API, SetWindowLong(hwnd, GWLP_HWNDPARENT, other).
• Now, the main app can effectively activate the companion app’s window by calling SetActiveWindow().

When the companion app gets minimized, I have to reset its owner window, and restore it when the window gets restored, or else the main app seems not to be able to use ShowWindow() to restore the minimized window.

I’ve put up a C# demo on GitHub which shows how this can be solved with .NET.

What if my Companion App is an electron instance?

Apparently, the same kind of magic can be done thanks to node-ffi with electron. See this StackOverflow question for inspiration.