What makes SwitchDesktop not work right after a user unlocks it session?

Question

I have a program that switches desktop and start a new process on it. When the process exits, the parent process restores the original desktop.

For testing purposes, I put a button in a plain win32 app that triggers the switch. It works, and closing the launched process (notepad), I go back to the original desktop.

In that same program, I have called WTSRegisterSessionNotification to receive a notification when a session is unlocked (WTS_SESSION_UNLOCK). I receive it.

But when I try to switch desktops in WTS_SESSION_UNLOCK message handler, SwitchDesktop fails and GetLastError is 0. The documentation says that last error is usually not set by SwitchDesktop.

Funny enough, if I put my call to switch desktop in a for loop, it works on the 5th iteration.

In short, this does not work :

    case WM_WTSSESSION_CHANGE:
      if(wParam == WTS_SESSION_UNLOCK)          
      {
          SwitchDesktop(a_valid_desktop_handle);
      }
    break;

But this ugly hack works :

    case WM_WTSSESSION_CHANGE:
      if(wParam == WTS_SESSION_UNLOCK)          
      {
         for(int i=0; i<10; ++i)
         {
            if(SwitchDesktop(a_valid_desktop_handle))
            {
                //This will work when i == 5, maybe 6.
                break;
            }
         }
      }
    break;

Setting a timer (to exit the message loop) also works, but it is just a more convoluted form of loop with regards to this problem. SwitchDesktop will work on after a handfull of WM_TIMER messages. It looks like constant time, although I did not measure it.

MSDN documentation for SwitchDesktop mentions that this will fail with a custom Userinit process, which I use. But getting the name of the current desktop just before the switch :

wchar_t name[512];
GetUserObjectInformation(GetThreadDesktop(GetCurrentThreadId()), UOI_NAME, name, sizeof(name)/sizeof(*name), 0);
OutputDebugString(name);

Gives me default all the time. And since GetLastError is 0, not 5 (access denied) I am pretty sure the secure desktop is gone before I receive the WTS_SESSION_UNLOCK notification.

I known I can't switch desktop while the screen is locked, but is there a "grace period" after the desktop is unlocked in which I can't call SwitchDesktop ?

Answer 1

When the desktop gets locked, it switches to another desktop which is reserved for this purpose. It is quite possible that when you receive the message, that desktop is still in control and you're not allowed to switch because you're not running in the current desktop.

Answer 2

I can't test it right now but I would put the call to SwitchDesktop not on WTS_SESSION_UNLOCK but on WTS_CONSOLE_CONNECT. From what I gather WTS_SESSION_UNLOCK occurs first and then your get WTS_CONSOLE_CONNECT which would correspond to what you see with the "constand time"...