How can I get the MainWindowHandle of a Windows 7 application running as user from within a service running as Local System?

Question

I've created a service that runs as the Local System user. This service launches and monitors a Silverlight Out-of-browser application using native interop and the CreateProcessAsUser() method (to run it as the currently logged-in user, rather than Local System). I'm able to get a handle on the spawned Process and do things like Kill() it, however, I've become aware that the service is unable to get a handle to the main window of the child application because the child application is running as a different user. I'm running on Windows 7.

My end goal is to respond properly to when the Process stops responding (i.e. Process.Responding == false) so that I can kill the application and restart it. However, Process.Responding requires a handle to the main window of the process (Process.MainWindowHandle, to be exact), however, in this scenario, Process.MainWindowHandle always returns 0.

I'm stumped here. Is there any way for one user to get a window handle to a process running as another user in Win 7?

Thanks in advance for any and all help.

Answer 1

No, that's not possible. Windows Services are completely isolated from user-mode applications for security reasons. If you could get the handle to a window, you could manipulate and otherwise interact with that window, leaving open a huge security vulnerability.

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Strictly speaking, what you're using the Windows Service for in the first place is bad design. It shouldn't be creating or launching any user-mode processes, or interacting with the user in any way. Remember that services run even when there is no user logged in! Thus, they shouldn't be starting applications.

A better solution is a simple background application, set to launch automatically when the user logs in. This background application could then launch the Silverlight application, monitor its state, and interact with it as necessary, because both would be running under the context of the same local user account. The effect is similar to a service, but without any of the drawbacks of isolation. The easiest way to do this in Visual Studio is to create a WinForms application (or possibly a WPF application, I have less experience in that area) that simply doesn't show any forms/windows.