• The Old New Thing

    How to host an IContextMenu, part 6 - Displaying menu help

    • 14 Comments

    One of the subtleties of context menus is showing help in the status bar. Now, the program we've been developing doesn't have a status bar, so we'll fake it by putting the help text in the title bar.

    The key method for this task is IContextMenu::GetCommandString, which allows communication with a context menu handler about the verbs in the menu. We'll have to stash yet another interface in our "instance variables disguised as globals".

    IContextMenu *g_pcm;
    

    (Remember, in a "real program", these would be per-window instance variables, not globals.)

    We also need to update that variable during menu tracking.

          g_pcm = pcm;
          int iCmd = TrackPopupMenuEx(hmenu, TPM_RETURNCMD, pt.x, pt.y, hwnd, NULL);
          g_pcm = NULL;
    

    With that out of the way, we can now provide feedback as the user browses the popup menu.

    [Introduction of g_pcm variable added 29 September.]

    // This code is buggy - see below.
    void OnMenuSelect(HWND hwnd, HMENU hmenu,
                      int item, HMENU hmenuPopup, UINT flags)
    {
      if (g_pcm && item >= SCRATCH_QCM_FIRST &&
          item <= SCRATCH_QCM_LAST) {
        TCHAR szBuf[MAX_PATH];
        if (FAILED(g_pcm->GetCommandString(item - SCRATCH_QCM_FIRST,
                                           GCS_HELPTEXT, NULL,
                                           (LPSTR)szBuf, MAX_PATH))) {
          lstrcpyn(szBuf, TEXT("No help available."), MAX_PATH);
        }
        SetWindowText(hwnd, szBuf);
      }
    }
    

    This function checks whether the menu selection is in the range of items that we allowed the context menu to own. If so, we ask for the help string (or use fallback text if the context menu handler didn't provide a help string) and display it as our window title.

    Finally, we insert this function into our window procedure. We want to update the menu selection status even if the context menu handlers do something with it, so we need to call OnMenuSelect before dispatching to the context menu handlers.

    LRESULT CALLBACK
    WndProc(HWND hwnd, UINT uiMsg, WPARAM wParam, LPARAM lParam)
    {
        if (uiMsg == WM_MENUSELECT) {
            HANDLE_WM_MENUSELECT(hwnd, wParam, lParam, OnMenuSelect);
        }
        if (g_pcm3) {
    ...
    

    Wait a second, there was a comment up there that said that the OnMenuSelect function is buggy. Where's the bug?

    Well, technically there is no bug, but if you run this program as-is (and I suggest that you do), you'll find that what you get is rather erratic.

    That's because there are a lot of buggy context menu handlers out there.

    Some context menu handlers don't support Unicode; others don't support Ansi. What's really fun is that instead of returning E_NOTIMPL, they return S_OK but don't actually do anything. Other context menus have buffer overflow problems and write to the buffer beyond the actual size you specified.

    Welcome to the world of application compatibility.

    Let's write a helper function that tries to hide all of these weirdnesses.

    HRESULT IContextMenu_GetCommandString(
        IContextMenu *pcm, UINT_PTR idCmd, UINT uFlags,
        UINT *pwReserved, LPWSTR pszName, UINT cchMax)
    {
      // Callers are expected to be using Unicode.
      if (!(uFlags & GCS_UNICODE)) return E_INVALIDARG;
    
      // Some context menu handlers have off-by-one bugs and will
      // overflow the output buffer. Let's artificially reduce the
      // buffer size so a one-character overflow won't corrupt memory.
      if (cchMax <= 1) return E_FAIL;
      cchMax--;
    
      // First try the Unicode message.  Preset the output buffer
      // with a known value because some handlers return S_OK without
      // doing anything.
      pszName[0] = L'\0';
    
      HRESULT hr = pcm->GetCommandString(idCmd, uFlags, pwReserved,
                                         (LPSTR)pszName, cchMax);
      if (SUCCEEDED(hr) && pszName[0] == L'\0') {
        // Rats, a buggy IContextMenu handler that returned success
        // even though it failed.
        hr = E_NOTIMPL;
      }
    
      if (FAILED(hr)) {
        // try again with ANSI - pad the buffer with one extra character
        // to compensate for context menu handlers that overflow by
        // one character.
        LPSTR pszAnsi = (LPSTR)LocalAlloc(LMEM_FIXED,
                                          (cchMax + 1) * sizeof(CHAR));
        if (pszAnsi) {
          pszAnsi[0] = '\0';
          hr = pcm->GetCommandString(idCmd, uFlags & ~GCS_UNICODE,
                                      pwReserved, pszAnsi, cchMax);
          if (SUCCEEDED(hr) && pszAnsi[0] == '\0') {
            // Rats, a buggy IContextMenu handler that returned success
            // even though it failed.
            hr = E_NOTIMPL;
          }
          if (SUCCEEDED(hr)) {
            if (MultiByteToWideChar(CP_ACP, 0, pszAnsi, -1,
                                    pszName, cchMax) == 0) {
              hr = E_FAIL;
            }
          }
          LocalFree(pszAnsi);
    
        } else {
          hr = E_OUTOFMEMORY;
        }
      }
      return hr;
    }
    

    The shell has lots of strange functions like this.

    [pszAnsi comparison fixed, 29 September.]

    With this helper function, we can fix our help text function.

    void OnMenuSelect(HWND hwnd, HMENU hmenu,
                      int item, HMENU hmenuPopup, UINT flags)
    {
      if (g_pcm && item >= SCRATCH_QCM_FIRST &&
          item <= SCRATCH_QCM_LAST) {
        WCHAR szBuf[MAX_PATH];
        if (FAILED(IContextMenu_GetCommandString(g_pcm,
                                           item - SCRATCH_QCM_FIRST,
                                           GCS_HELPTEXTW, NULL,
                                           szBuf, MAX_PATH))) {
          lstrcpynW(szBuf, L"No help available.", MAX_PATH);
        }
        SetWindowTextW(hwnd, szBuf);
      }
    }
    

    This new version displays help texts for all the context menu handlers that support it, in spite of the attempts of many of those context menu handlers to get it wrong or even create a buffer overflow security vulnerability.

    Okay, that was quite a long digression from part 1 of this series. Let's return to the subject of invoking the default verb next time.

  • The Old New Thing

    How to host an IContextMenu, part 5 - Handling menu messages

    • 2 Comments

    One bug that was called out immediately in our first attempt at displaying the context menu to the user is that the Open With and Send To submenus don't work.

    The reason for this is that these submenus are delay-generated (which explains why they don't contain anything interesting when you expand them) and owner-drawn (which you can't notice yet because of the first problem, but trust me, they are).

    This is where the IContextMenu2::HandleMenuMsg and IContextMenu3::HandleMenuMsg2 methods are used.

    Historical note: IContextMenu2::HandleMenuMessage is on its own interface rather than being merged with the base interface IContextMenu because it was added late in Windows 95 development, so it was considered safer to add a derived interface than to make everybody who had been writing Windows 95 shell extensions go back and rewrite their code. IContextMenu3::HandleMenuMessage2 was added in Internet Explorer 4 (I think) when it became clear that the ability for a context menu extension to override the default message return value was necessary in order to support keyboard accessibility in owner-drawn context menus.

    In a "real program", these two variables would be class members associated with the window, but this is just a sample program, so they are globals. When you write your own programs, don't use global variables here because they will result in mass mayhem once you get a second window, since both of them will try to talk to the interface even though only the window displaying the context menu should be doing so.

    IContextMenu2 *g_pcm2;
    IContextMenu3 *g_pcm3;
    

    These two new variables track the IContextMenu2 and IContextMenu3 interfaces of the active tracked popup menu. We need to initialize and uninitalize them around our call to TrackPopupMenuEx:

          pcm->QueryInterface(IID_IContextMenu2, (void**)&g_pcm2);
          pcm->QueryInterface(IID_IContextMenu3, (void**)&g_pcm3);
          int iCmd = TrackPopupMenuEx(hmenu, TPM_RETURNCMD, pt.x, pt.y, hwnd, NULL);
          if (g_pcm2) {
            g_pcm2->Release();
            g_pcm2 = NULL;
          }
          if (g_pcm3) {
            g_pcm3->Release();
            g_pcm3 = NULL;
          }
    

    And finally we need to invoke the HandleMenuMessage/HandleMenuMessage methods in the window procedure:

    LRESULT CALLBACK
    WndProc(HWND hwnd, UINT uiMsg, WPARAM wParam, LPARAM lParam)
    {
        if (g_pcm3) {
            LRESULT lres;
            if (SUCCEEDED(g_pcm3->HandleMenuMsg2(uiMsg, wParam, lParam, &lres))) {
              return lres;
            }
        } else if (g_pcm2) {
            if (SUCCEEDED(g_pcm2->HandleMenuMsg(uiMsg, wParam, lParam))) {
              return 0;
            }
        }
    
        switch (uiMsg) {
        ....
    

    In the window procedure, we ask the context menu whether it wishes to handle the menu message. If so, then we stop and return the desired value (if HandleMenuMsg2) or just zero (if HandleMenuMsg).

    With these changes, run the scratch program again and observe that the Open With and Send To submenus now operate as expected.

    Next time: Getting menu help text.

  • The Old New Thing

    The unrecognized assistants on those do-it-yourself shows

    • 28 Comments

    Some people from my knitting group have been among those assisting in the preparation of today's episode of the knitting show Knitty Gritty on The Do It Yourself Network.

    Stop reading if you don't want the television illusion shattered, or at least cracked a bit.

    Each volunteer was assigned a section of a sweater to knit. The show's host will take those pieces and do some short demonstration with it, perhaps sewing pieces together or illustrating one of the trickier parts.

    After each volunteer made their assigned part, they also had to start working on the same sweater again, but not to finish it. They will be in the background knitting while the host does the feature demonstration.

    What's more, each volunteer has already been told what problem they will be having with the sweater, so that the host can help extricate them from whatever mess they got themselves into. Nevermind that they already made the sweater that the host was using in the demonstration just a few minutes ago. Clearly, they know what they're doing; they did all the knitting!

    If you thought the hosts of these shows actually did the cooking/sewing/knitting/whatever, well now you know better.

  • The Old New Thing

    How to host an IContextMenu, part 4 - Key context

    • 6 Comments

    Another of the bugs you may have noticed in our first attempt at displaying the context menu to the user is that the Delete command doesn't alter its behavior depending on whether you hold the shift key. Recall that holding the shift key changes the behavior of the Delete command, causing it to delete a file immediately instead of moving it to the Recycle Bin. But in our sample program, it always offers to move the file to the Recycle Bin, even if you have the shift key down.

    (You can see the difference in the wording of the dialog and in the icon. If the operation is to move the item into the Recycle Bin, you get a Recycle Bin icon and the text asks you to confirm sending the item to the Recycle Bin. If the operation will delete the item permanently, then you get an icon that shows a file and a folder fading away and the text asks you to confirm deleting the item.)

    To convey this information to the context menu, you need to pass the key states in the CMINVOKECOMMANDINFOEX structure.

              CMINVOKECOMMANDINFOEX info = { 0 };
              info.cbSize = sizeof(info);
              info.fMask = CMIC_MASK_UNICODE | CMIC_MASK_PTINVOKE;
              if (GetKeyState(VK_CONTROL) < 0) {
                info.fMask |= CMIC_MASK_CONTROL_DOWN;
              }
              if (GetKeyState(VK_SHIFT) < 0) {
                info.fMask |= CMIC_MASK_SHIFT_DOWN;
              }
    

    Make this change and observe that the dialogs you get from the Delete option now respect your shift key state.

    Warning: Before playing with this, make sure that you have enabled delete confirmation warnings or you will end up deleting your clock.avi file for real! If you want to play around with the Delete option, you may want to tweak the program so it operates on a file you don't mind losing.

    Exercise: There's another place where key context influences the context menu, namely the convention that holding the shift key while right-clicking enables "extended verbs". These are verbs that are lesser-used and therefore do not appear on the conventional context menu to avoid creating clutter. For homework, incorporate the extended verb convention into the sample program.

    [Sorry today's entries are late. Had problems connecting to the blog server.]

  • The Old New Thing

    Penguins do not fall over!

    • 20 Comments

    Everyone has read the story about penguins falling over backwards while observing low-flying aircraft. The Annals of Improbable Research reports that Dr. Richard Stone has been researching the ecological effects of helicopter overflights on King penguins and his preliminary results show that King penguins do not, in fact, fall over backwards (or forwards or even sideways) when you fly over them. On the other hand, low-flying aircraft do create distress among the penguins; Dr. Stone recommends a minimum overflight altitude of 1000 feet.

    [Typo fixed, 24 September.]

  • The Old New Thing

    How to host an IContextMenu, part 3 - Invocation location

    • 5 Comments

    One of the bugs you may have noticed in our first attempt at displaying the context menu to the user is that the Properties dialog doesn't show up at the point you clicked. The Properties dialog isn't psychic; it doesn't know where the original mouse click occurred. You have to tell it.

              CMINVOKECOMMANDINFOEX info = { 0 };
              info.cbSize = sizeof(info);
              info.fMask = CMIC_MASK_UNICODE | CMIC_MASK_PTINVOKE;
              info.hwnd = hwnd;
              info.lpVerb  = MAKEINTRESOURCEA(iCmd - SCRATCH_QCM_FIRST);
              info.lpVerbW = MAKEINTRESOURCEW(iCmd - SCRATCH_QCM_FIRST);
              info.nShow = SW_SHOWNORMAL;
              info.ptInvoke = pt;
    

    You tell it by setting the CMIC_MASK_PTINVOKE flag in the fMask and specifying the invocation point in the ptInvoke member of the CMINVOKECOMMANDINFOEX structure.

    Make this change and observe that now the Properties dialog appears at the point you clicked the mouse rather than in a random location on the screen.

    Next time, we'll clean up another minor problem with our sample program.

  • The Old New Thing

    How to contact Raymond

    If your subject matter is a suggestion for a future topic, please post it to the Suggestion Box (using the Suggestion Box link on the side of the page). If you send it to me directly, I will probably lose track of it.

    If your subject matter is personal (for example "Hi, Raymond, remember me? We went to the same high school..."), you can send email using the Contact page. (Update 25 March 2007: The contact page has been disabled because over 90% of the messages that arrive are spam.)

    I do not promise to respond to your message.

    If you are making a suggestion for a future version of a Microsoft product, submit it directly to Microsoft rather than sending it to me, because I won't know what to do with it either.

    I do not provide one-on-one technical support. If your question is of general interest, you can post it to the Suggestion Box. If you are looking for technical support, there are a variety of options available, such as newgroups, bulletin boards, forums (fora?), and Microsoft's own product support web site.

  • The Old New Thing

    How to host an IContextMenu, part 2 - Displaying the context menu

    • 6 Comments

    Instead of invoking a fixed verb, we'll ask the user to choose from the context menu and invoke the result.

    Make these changes to the OnContextMenu function:

    #define SCRATCH_QCM_FIRST 1
    #define SCRATCH_QCM_LAST  0x7FFF
    
    #undef HANDLE_WM_CONTEXTMENU
    #define HANDLE_WM_CONTEXTMENU(hwnd, wParam, lParam, fn) \
        ((fn)((hwnd), (HWND)(wParam), GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam)), 0L)
    
    // WARNING! Incomplete and buggy! See discussion
    void OnContextMenu(HWND hwnd, HWND hwndContext, int xPos, int yPos)
    {
      POINT pt = { xPos, yPos };
      if (pt.x == -1 && pt.y == -1) {
        pt.x = pt.y = 0;
        ClientToScreen(hwnd, &pt);
      }
    
      IContextMenu *pcm;
      if (SUCCEEDED(GetUIObjectOfFile(hwnd, L"C:\\Windows\\clock.avi",
                       IID_IContextMenu, (void**)&pcm))) {
        HMENU hmenu = CreatePopupMenu();
        if (hmenu) {
          if (SUCCEEDED(pcm->QueryContextMenu(hmenu, 0,
                                 SCRATCH_QCM_FIRST, SCRATCH_QCM_LAST,
                                 CMF_NORMAL))) {
            int iCmd = TrackPopupMenuEx(hmenu, TPM_RETURNCMD,
                                        pt.x, pt.y, hwnd, NULL);
            if (iCmd > 0) {
              CMINVOKECOMMANDINFOEX info = { 0 };
              info.cbSize = sizeof(info);
              info.fMask = CMIC_MASK_UNICODE;
              info.hwnd = hwnd;
              info.lpVerb  = MAKEINTRESOURCEA(iCmd - SCRATCH_QCM_FIRST);
              info.lpVerbW = MAKEINTRESOURCEW(iCmd - SCRATCH_QCM_FIRST);
              info.nShow = SW_SHOWNORMAL;
              pcm->InvokeCommand((LPCMINVOKECOMMANDINFO)&info);
            }
          }
          DestroyMenu(hmenu);
        }
        pcm->Release();
      }
    }
    

    The first change addresses the first issue brought up in the discussion of the WM_CONTEXTMENU message and fixes the HANDLE_WM_CONTEXTMENU message.

    The second change addresses the second issue, and that's the special handling of keyboard-invoked context menus. When we receive a keyboard-invoked context menu, we move it to the (0, 0) position of our client area. This keeps the context menu displayed in a vaguely sane position. (If we were a container with objects, it would have been better to display the context menu over the selected sub-object.)

    The third change actually does what we're talking about: Displaying the context menu to the user, collecting the result, and acting on it.

    You are certainly familiar with the TrackPopupMenuEx function. Here we use the TPS_RETURNCMD flag to indicate that the item the user selected should be returned by the function instead of being posted as a WM_COMMAND to our window.

    This highlights the importance of the fact that SCRATCH_QCM_FIRST is 1 and not zero. If it were zero, then we wouldn't be able to distinguish between the user selecting item zero and the user cancelling the menu.

    Once we are confident that the user has selected an item from the menu, we fill out a CMINVOKECOMMANDEX structure, specifying the user's selection in the two verb fields and indicating the invocation point via the ptInvoke member.

    Note that when you invoke a command by menu ID, you must specify the offset of the menu item relative to the starting point passed to IContextMenu::QueryContextMenu. That's why we subtracted SCRATCH_QCM_FIRST.

    When you run this program, you may notice that some things don't quite work. Most obviously, the Open With and Send To submenus don't work, but there are more subtle bugs too. We'll address them over the next few days.

  • The Old New Thing

    Swedes struggle with the meaning of sick leave

    • 7 Comments

    As part of the continuing campaign to shed their hard-working stereotype, perhaps taking a cue from their more well-adjusted Norwegian neighbors, Swedes have been taking dubious sick leave in record numbers.

    [A] study showed 40 percent believe it is enough to feel tired to stay home and draw benefits.

    A survey of 1,002 Swedes by the board also showed 65 percent believed they could go on sick leave if they felt stressed at work and 41 percent thought a conflict with their boss or workmates was a good enough reason.

    One fifth thought a strike at the child care center also made them eligible for the benefits and 71 percent said family problems entitled them always or sometimes to sick leave.

    Reuters claims Dagens Nyheter as the source, but if you punch "Anna Hedborg" into DN:s search engine, asking for articles within the last week, it finds nothing. But if you ask Google, the article is right there: Trötta sjukskriver sig (The tired call in sick). Once again, Google finds a web page that the site itself cannot.

    On a somewhat related note, a different type of sick-leave abuse has been detected.

    According to the Social Insurance Board, several people have been drawing sick-leave pay for several years, while simultaneously working another job and receiving unemployment benefits.

    Some cases are of ordinary people who defraud the Board in small amounts but over a long period of time. In the other cases, there are links to serious economic crime where, for example, money was paid out to fictitious employees at nonexistent companies.

    (Raymond's bad translation.)

    [Typo fixed 25 September.]

  • The Old New Thing

    Pitfalls in handling the WM_CONTEXTMENU message

    • 29 Comments

    Before we continue with our IContextMenu discussion, I need to take a little side trip and discuss the subtleties of the WM_CONTEXTMENU message.

    First, a correction to the existing <windowsx.h> header file:

    #undef HANDLE_WM_CONTEXTMENU
    #define HANDLE_WM_CONTEXTMENU(hwnd, wParam, lParam, fn) \
        ((fn)((hwnd), (HWND)(wParam), GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam)), 0L)
    

    Apparently, HANDLE_WM_CONTEXTMENU was overlooked when the <windowsx.h> header file gained multimonitor support.

    The second subtlety of the WM_CONTEXTMENU message is the recognition that context menus can be invoked from the keyboard, not just by the mouse. If you have a 104-key keyboard, you will probably have a menu key to the right of your space bar. (Laptop owners: You're on your own. Laptop keyboards are hardly standardized.) Alternatively, you can type Shift+F10 to get the same effect.

    When the user invokes a context menu from the keyboard, the x and y coordinates are both -1. In this case, you should display the context menu for the currently-selected item (or items, if a multiple selection is active). If you miss this detail, then you will end up hit-testing against (-1, -1) and probably not find anything.

    Okay, now that these remarks on the WM_CONTEXTMENU message are out of the way, we can return to our discussion of the IContextMenu interface next time.

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