When using the following class in a console application, and having at least one instance of Notepad running, GetWindowThreadProcessId correctly returns a non-zero thread id. However, if the same code is included in a Windows Service, GetWindowThreadProcessId always returns 0 and no exceptions are thrown. Changing the user the service launches under to be the same as the one running the console application didn't alter the result. What causes GetWindowThreadProcessId to return 0 even if it is provided with a valid hwnd? And why does it function differently in the console application and the service? Note: I am running Windows 7 32-bit and targeting .NET 3.5.

public class TestClass
{
    [DllImport("user32.dll")]
    static extern uint GetWindowThreadProcessId(IntPtr hWnd, IntPtr ProcessId);

    public void AttachToNotepad()
    {
        var processesToAttachTo = Process.GetProcessesByName("Notepad")

        foreach (var process in processesToAttachTo)
        {
            var threadID = GetWindowThreadProcessId(process.MainWindowHandle, 
                IntPtr.Zero);

            ....
        }
    }
}

Console Code:

class Program
{
    static void Main(string[] args)
    {
        var testClass = new TestClass();

        testClass.AttachToNotepad();
    }
}

Service Code:

public class TestService : ServiceBase
{
    private TestClass testClass = new TestClass();

    static void Main()
    {
        ServiceBase.Run(new TestService());
    }

    protected override void OnStart(string[] args)
    {
        testClass.AttachToNotepad();

        base.OnStart(args);
    }

    protected override void OnStop()
    {
        ...
    }
}