How to draw graphics/text on top of another application

There are no nice ways to do this, but one approach that may work for you is to hook the application in question using SetWindowsHookEx(...) to add a GetMsgProc, which draws your overlay in response to WM_PAINT messages. The basic idea is that you're drawing YOUR graphics right after the application finishes its own drawing.

In your main app:

....HMODULE hDllInstance = LoadLibrary("myFavoriteDll");HOOKPROC pOverlayHook = (HOOKPROC)GetProcAddress(hDllInstance, "OverlayHook");SetWindowsHookEx(WH_GETMESSAGE, pOverlayHook, hDllInstance, threadId);

Off in a DLL somewhere:

LRESULT CALLBACK OverlayHook(int code, WPARAM wParam, LPARAM lParam){  //Try and be the LAST responder to WM_PAINT messages;  //Of course, if some other application tries this all bets are off  LRESULT retCode = CallNextHookEx(NULL, code, wParam, lParam);  //Per GetMsgProc documentation, don't do anything fancy  if(code < 0) return retCode;  //Assumes that target application only draws when WM_PAINT message is  //removed from input queue.  if(wParam == PM_NOREMOVE) return retCode;  MSG* message = (MSG*)lParam;  //Ignore everything that isn't a paint request  if(message->message != WM_PAINT) return retCode;  PAINTSTRUCT psPaint;      BeginPaint(message->hwnd, &psPaint);  //Draw your overlay here  ...  EndPaint(message->hwnd, &psPaint);  return retCode;}

This is all win32 so your C# code will be p/invoke heavy and correspondingly quite ugly. Your DLL must be unmanaged as well (if you intend to inject into a process other than your own), making this an even nastier solution.

This would solve your issue with z-order and clipping issues, as you're rendering into the window itself. However, if the application you're targeting does any drawing outside of the WinProc responding to WM_PAINT things fall apart; this is not an entirely uncommon occurence.

You might want to draw it on top of directX for games

http://spazzarama.com/2011/03/14/c-screen-capture-and-overlays-for-direct3d-9-10-and-11-using-api-hooks/

There are problems with z order, clipping, and directing mouse clicks either to my application or the other application.

These are all tasks that the window manager was designed to deal with. You should create a layered window on top of the apps' windows.

See also: Prevent repainting of window in C++