Detect change of focus and find it C#

I need an underlying process to gather information about other applications used by Windows. I suspect this would be done using WAPI hooks. What I wish to do is for my program to detect when windows changes focus from one program to another and tell me which one currently has focus.

First I need an event that triggers each time Windows swaps focus between two applications. All events I've found so far only handles changes made from or to the program it's being used by, but I need to find all focus-changes, even if it's between two other programs.

I'll also need a function that gives me the window in focus. Would this work, or is this only internally (windows within the current application and not other programs)?

Cheers

Depending on how accurate you need your focus change detection system to be you might be able to get away with a service that polls for the foreground window using the API function you described GetForegroundWindow (yes this is system-wide, not process specific).

You can then use the handle of that function to determine which process is the current active/focused process. Then retrieve the focused element (child window) of that process..

HWND hwnd = GetForegroundWindow();
DWORD remoteThreadId = GetWindowThreadProcessId(hwnd, NULL);
DWORD currentThreadId = GetCurrentThreadId();
AttachThreadInput(rThreadId, curThreadId, TRUE);
HWND focusElement = GetFocus();
AttachThreadInput(rThreadId, curThreadId, FALSE);

Keep doing this.. and do whatever you need to do with focusElement

UPDATE

Well, apparently, as @Kenneth K. posted in a comment, there is a global EVENT_SYSTEM_FOREGROUND event that you can hook so that you application gets notified when the foreground (focused) window changes. This way you don't need to loop continuously to detect these changes.

EVENT_SYSTEM_FOREGROUND = 3;
WINEVENT_OUTOFCONTEXT = 0;

You can follow the example in this answer to see how to hook this event and get the notifications. Then whenever the foreground (focused) window changes you can hook that window's message loop and look form focus changed events withing that window using the SetWindowsHookEx function.

Another options is to consult the list of system events on MSDN and see if there is one you can use instead of the EVENT_SYSTEM_FOREGROUND, or along with it to detect control-level focus events. Perhaps the EVENT_OBJECT_FOCUS might be useful.

Please let me know if this is still unclear..