Gracefully Exit Explorer (Programmatically)

I debugged this out of curiosity. All it does is post a message to one of explorer's windows:

BOOL ExitExplorer(){    HWND hWndTray = FindWindow(_T("Shell_TrayWnd"), NULL);    return PostMessage(hWndTray, 0x5B4, 0, 0);}

Of course this is an undocumented WM_USER message so the behavior could quite possibly change in the future.

@Luke: first of all, thanks for the detailed analysis and the hint about the 0x5B4 user message to the Shell_TrayWnd!

Unfortunately, the method has two drawbacks; First, it uses an undocumented user message, which may change in future Windows versions, and second, it does not work under Windows XP, since the 'magic procedure' to exit windows is different (open the shutdown dialog, then cancel it pressing SHIFT-CTRL-ALT-ESC) and no message posting is involved there.

It would be nice to have a reliable and portable way to terminate explorer cleanly from another process regardless of the windows version. So I continued debugging into the disassembly of the code which terminates explorer cleanly in order to find a hint about how I could achieve this. I still don't have the perfect solution but I made some interesting observations (on Windows 7 and Windows XP) which I want to share with whoever might be interested:

Windows 7

The 0x5B4-message is eventually handled by the method CTray::_DoExitExplorer. If you have symbol server enabled, then you can set a breakpoint in

{,,explorer.exe}CTray::_DoExitExplorer (visual studio syntax)

resp.

explorer!CTray::_DoExitExplorer (windbg syntax)

Windows XP

In WinXP, you have to set your breakpoint at

{,,explorer.exe}CTray::_ExitExplorerCleanly (visual studio syntax)

resp.

explorer!CTray::_ExitExplorer (windbg syntax)

before you enter the 'magic keystrokes' (SHIFT-CTRL-ALT-ESC) at the shutdown dialog. Both methods are very similar, as you can see from the disassembly (see followup post). The pseudo code is

    if (bUnnamedVariable == FALSE) {        g_fFakeShutdown = TRUE;  // (1)        PostMessage(hWndProgMan, WM_QUIT, 0, TRUE);   // (2)        if (PostMessage(hWndTray, WM_QUIT, 0, 0)) {    // (3)            bUnnamedVariable = TRUE;        }    }

Note that the first PostMessage() call passes TRUE as lParam, which is officially unused by WM_QUIT. The meaning of the lParam seems to be bShutdown == TRUE.

Of course it is impossible (or not feasible) to set g_fFakeShutdown from another application. So I tested different combinations of PostMessage(hWndProgMan, WM_QUIT, 0, TRUE/FALSE) followed or not by PostMessage(hWndTray, WM_QUIT, 0, FALSE). It seems that explorer shows different behaviour under Windows XP and Windows 7.

The following two methods seem to be good candidates to terminate explorer under windows XP. Unfortunately they don't work under Windows 7:

    BOOL ExitExplorer1() {        HWND hWndProgMan = FindWindow(_T("Progman"), NULL);        PostMessage(hWndProgMan, WM_QUIT, 0, TRUE);   // <=  lParam == TRUE !        HWND hWndTray = FindWindow(_T("Shell_TrayWnd"), NULL);        PostMessage(hWndTray, WM_QUIT, 0, 0);         return TRUE;    }     BOOL ExitExplorer2() {        HWND hWndProgMan = FindWindow(_T("Progman"), NULL);        PostMessage(hWndProgMan, WM_QUIT, 0, FALSE);   // <=  lParam == FALSE !        return TRUE;    } 

Behaviour in Windows XP

In both cases the shell (explorer.exe) terminates and before terminating it sets the registry key

    HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\CleanShutdown = TRUE

as can be observed using Sysinternals Process Monitor, or by setting a breakpoint at {,,explorer}_WriteCleanShutdown@4 (resp. explorer!_WriteCleanShutdown).

Behaviour in Windows 7

Both methods don't work: although it appears that the shell terminated, the explorer.exe process is still running.

Remark

If I only post a WM_QUIT to hWndProgMan with lParam = TRUE without posting a message to hWndTray, i.e.,

    BOOL ExitExplorer3() {        HWND hWndProgMan = FindWindow(_T("Progman"), NULL);        PostMessage(hWndProgMan, WM_QUIT, 0, TRUE);         return TRUE;    } 

then I get an interesting behaviour (both Win7 and WinXP): The shutdown dialog appears. If you cancel it, everything appears to be normal, but after two or three (!) seconds, explorer terminates.

Conclusion

Maybe the best solution is to use ExitExplorer() with the undocumented WM_USER function for Windows 7 and either ExitExplorer1() or ExitExplorer2() for Windows XP. Does any one of the two XP-methods have advantages over the other? I don't know.

Appendix

Disassembly of CTray::_DoExitExplorer (Windows 7) and CTray::_ExitExplorerCleanly (Windows XP)

Windows 7

    {,,explorer.exe}CTray::_DoExitExplorer:    explorer!CTray::_DoExitExplorer:    00fdde24 833df027020100  cmp     dword ptr [explorer!g_fInSizeMove+0x4 (010227f0)],0 ds:0023:010227f0=00000000    00fdde2b 53              push    ebx    00fdde2c 8bd9            mov     ebx,ecx    00fdde2e 7535            jne     explorer!CTray::_DoExitExplorer+0x41 (00fdde65)    00fdde30 56              push    esi    00fdde31 8b35ec14f700    mov     esi,dword ptr [explorer!_imp__PostMessageW (00f714ec)]    00fdde37 57              push    edi    00fdde38 33ff            xor     edi,edi    00fdde3a 47              inc     edi    00fdde3b 57              push    edi    00fdde3c 6a00            push    0    00fdde3e 6a12            push    12h    00fdde40 ff35e8000201    push    dword ptr [explorer!v_hwndDesktop (010200e8)]    00fdde46 893ddc270201    mov     dword ptr [explorer!g_fFakeShutdown (010227dc)],edi    00fdde4c ffd6            call    esi    00fdde4e 6a00            push    0    00fdde50 6a00            push    0    00fdde52 6a12            push    12h    00fdde54 ff7304          push    dword ptr [ebx+4]    00fdde57 ffd6            call    esi    00fdde59 85c0            test    eax,eax    00fdde5b 7406            je      explorer!CTray::_DoExitExplorer+0x3f (00fdde63)    00fdde5d 893df0270201    mov     dword ptr [explorer!g_fInSizeMove+0x4 (010227f0)],edi    00fdde63 5f              pop     edi    00fdde64 5e              pop     esi    00fdde65 a1f0270201      mov     eax,dword ptr [explorer!g_fInSizeMove+0x4 (010227f0)]    00fdde6a 5b              pop     ebx    00fdde6b c3              ret

('bUnnamedVariable' is a module global variable at address g_fInSizeMove+4)

Windows XP

    {,,explorer.exe}CTray::_ExitExplorerCleanly:    01031973 8B FF            mov         edi,edi     01031975 57               push        edi      01031976 8B F9            mov         edi,ecx     01031978 83 BF 40 04 00 00 00 cmp         dword ptr [edi+440h],0     0103197F 75 35            jne         CTray::_ExitExplorerCleanly+43h (10319B6h)     01031981 53               push        ebx      01031982 56               push        esi      01031983 8B 35 94 17 00 01 mov         esi,dword ptr [__imp__PostMessageW@16 (1001794h)]     01031989 33 DB            xor         ebx,ebx     0103198B 43               inc         ebx      0103198C 53               push        ebx      0103198D 6A 00            push        0        0103198F 6A 12            push        12h      01031991 FF 35 8C 60 04 01 push        dword ptr [_v_hwndDesktop (104608Ch)]     01031997 89 1D 48 77 04 01 mov         dword ptr [_g_fFakeShutdown (1047748h)],ebx     0103199D FF D6            call        esi      0103199F 6A 00            push        0        010319A1 6A 00            push        0        010319A3 6A 12            push        12h      010319A5 FF 77 04         push        dword ptr [edi+4]     010319A8 FF D6            call        esi      010319AA 85 C0            test        eax,eax     010319AC 74 06            je          CTray::_ExitExplorerCleanly+41h (10319B4h)     010319AE 89 9F 40 04 00 00 mov         dword ptr [edi+440h],ebx     010319B4 5E               pop         esi      010319B5 5B               pop         ebx      010319B6 8B 87 40 04 00 00 mov         eax,dword ptr [edi+440h]     010319BC 5F               pop         edi      010319BD C3               ret              

('bUnnamedVariable' seems to be a member of CTray at relative offset 440h)

RemarkIt seems that WM_QUIT is used here in a very non-standard way, compare the following excerpt from MSDN WM_QUIT on MSDN

This message does not have a return value because it causes the message loop to terminate before the message is sent to the application's window procedure.

Remarks The WM_QUIT message is not associated with a window and therefore will never be received through a window's window procedure. It is retrieved only by the GetMessage or PeekMessage functions.

Do not post the WM_QUIT message using the PostMessage function; use PostQuitMessage.

On Windows Vista and above, you can use RestartManager API to gracefully shutdown explorer.

In pseudocode it will look like this:

RmStartSession(...); RM_UNIQUE_PROCESS[] processes = GetProcesses("explorer.exe"); // get special handles to process you want to close    RmRegisterResources(processes); // register those processes with restart manager session    RmShutdown(RM_SHUTDOWN_TYPE.RmForceShutdown);     RmRestart(...); // restart them back, optionally    RmEndSession(...);