c# - GetFocus - Win32api help -

i trying grab selected text open form on users machine. have tried using getfocus defined as

    '[dllimport("user32.dll")]     static extern int getfocus();' 

in api says - retrieves handle window has keyboard focus, if window attached calling thread's message queue. explains why app can grab selected text window thats part of app, not 1 thats external, pdf example.

what alternative win32 method available me use fit purpose?

thanks.

edit: attempt @ moment

[dllimport("user32.dll")] static extern int getfocus();

[dllimport("user32.dll")] static extern bool attachthreadinput(uint idattach, uint idattachto, bool fattach);  [dllimport("kernel32.dll")] static extern uint getcurrentthreadid();  [dllimport("user32.dll")] static extern uint getwindowthreadprocessid(int hwnd, int processid);  [dllimport("user32.dll")] static extern int getforegroundwindow();  [dllimport("user32.dll", charset = charset.auto, setlasterror = false)] static extern int sendmessage(int hwnd, int msg, int wparam, stringbuilder lparam);   // second overload of sendmessage  [dllimport("user32.dll")] private static extern int sendmessage(intptr hwnd, uint msg, out int wparam, out int lparam);  const int wm_settext = 12; const int wm_gettext = 13;  private static string performcopy() {     try     {         //wait 5 seconds give chance give focus edit window,         //notepad example         system.threading.thread.sleep(1000);         stringbuilder builder = new stringbuilder(500);          int foregroundwindowhandle = getforegroundwindow();         uint remotethreadid = getwindowthreadprocessid(foregroundwindowhandle, 0);         uint currentthreadid = getcurrentthreadid();          //attachtrheadinput needed can handle of focused window in app         attachthreadinput(remotethreadid, currentthreadid, true);         //get handle of focused window           int focused = getfocus();             //now detach since got focused handle         attachthreadinput(remotethreadid, currentthreadid, false);          //get text active window stringbuilder         sendmessage(focused, wm_gettext, builder.capacity, builder);          return builder.tostring();     }     catch (system.exception oexception)     {         throw oexception;     } } 

check getforegroundwindow.