PRB: Assert Error in BARTOOL.CPP line 398

The information in this article applies to:

• The Microsoft Foundation Classes included with: Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1

SYMPTOMS

An Assert error occurs in BARTOOL.CPP, line 398 if a command handler for a toolbar button destroys the window that owns the toolbar.

CAUSE

The MFC implementation of CToolbar::OnLButtonUp() performs additional processing after invoking the command handler implemented for that particular toolbar button.

Specifically, the CToolbar::OnLButtonUp() function executes this code:

   GetOwner()->SendMessage(WM_COMMAND, nIDCmd)

If the execution of the command handler causes the toolbar to be destroyed, then OnLButtonUp() attempts a call to SetButtonStyle(iButtonCapture, nNewStyle) on a toolbar that no longer exists. This action leads to the assertion failure.

RESOLUTION

Implement the command handler to post the message that eventually destroys the toolbar. This way the toolbar is not destroyed until the execution of OnLButtonUp is complete.

Example Scenario and Resolution

A popular scenario that leads to this assertion is implementing a toolbar button that is intended to close a CFrameWnd window. For example, you implement a command handler in the class derived from CFrameWnd to execute the code SendMessage(WM_CLOSE). Executing this code immediately causes a WM_CLOSE message to be sent to the frame window, which in turn destroys that window and all the windows it owns, including the toolbar, thereby causing the assertion.

A better implementation is to execute PostMessage(WM_CLOSE) in the toolbar button's command handler. Then the WM_CLOSE message is posted at the end of the message queue, and the OnLButtonUp() completes before the WM_CLOSE message is handled.

MORE INFORMATION

This problem is specific to MFC versions 3.x, shipped with Visual C++, 32-bit Edition, versions 2.x. The CToolBar object of MFC versions previous to 2.5 was not as sophisticated and did not provide the OnLButtonUp() member function. The implementation of CToolbar::OnLButtonUp() in MFC version 2.5 (16-bit) is slightly different from that of MFC version 3.0 (32-bit); in MFC version 2.5, GetOwner()->SendMessage(WM_COMMAND, nIDCmd) is the last line of code in CToolbar::OnLButtonUp(). In MFC version 4.0 (shipped with Visual C++ version 4.0), the CToolBar object has been redefined significantly and no longer supports the OnLButtonUp() function. In addition, this redefinition produced a more robust CToolBar that does not assert in the example scenario discussed above.

REFERENCES

For more information about this issue, see \MSVC20\MFC\SRC\BARTOOL.CPP.