PRB: MFC ActiveX Control with Subclassed ComboBox Does Not Drop Down Correctly in a VB Container

• The Microsoft Foundation Classes (MFC), included with:
  • Microsoft Visual C++, 32-bit Editions, version 6.0
• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

SYMPTOMS

An MFC ActiveX control with a subclassed ComboBox with the CBS_DROPDOWNLIST style, does not drop down correctly in a Visual Basic container. The control drops down and immediately retracts; it drops down correctly only if it is double-clicked.

CAUSE

The COleControl handlers for WM_LBUTTONDOWN and WM_LBUTTONUP call the SetCapture and ReleaseCapture functions, respectively. The call to ReleaseCapture in turn sends WM_CAPTURECHANGED notification, which in turn causes the list box portion of the combo box to hide itself.

RESOLUTION

Create a message handler for the WM_MOUSEACTIVATE message in your Control class. Call OnActivateInPlace in it:

int CCustomComboCtrl::OnMouseActivate(CWnd* pDesktopWnd, UINT nHitTest, UINT message)  
{ 	
     OnActivateInPlace (TRUE, NULL); 	
     return COleControl::OnMouseActivate(pDesktopWnd, nHitTest, message); 
}  

MORE INFORMATION

Steps to Reproduce Behavior

1. Use the AppWizard to create a new MFC ActiveX ControlWizard, and at step 2 of 2 click COMBOBOX where it says "Which window class, if any, should this control subclass?" and then click Finish.



2. In the PreCreateWindow of the Control class, add the CBS_DROPDOWNLIST style bit as shown below:

   BOOL CMySubComboCtrl::PreCreateWindow(CREATESTRUCT& cs)
   {
   	cs.style |= CBS_DROPDOWNLIST;
   	cs.lpszClass = _T("COMBOBOX");
   	return COleControl::PreCreateWindow(cs);
   } 




3. Open Visual Basic and select a new Standard EXE project.



4. Add a TextBox to the form.



5. On the Project menu, click Components, and then select the subclassed ComboBox control from the list. Click OK.



6. From the Controls toolbar, drop the control on the form below the edit control.



7. Run the sample and try to drop down the combo box by clicking the down arrow of the combo box.

© Microsoft Corporation 1999, All Rights Reserved.
Contributions by Vidyanand Rajpathak, Microsoft Corporation

Additional query words:

Keywords : kbComboBox kbContainer kbCtrl kbMFC kbVBp kbVC kbVC600 kbDSupport kbVB500 kbGrpMFCATL
Version : WINDOWS:5.0,6.0; winnt:6.0
Platform : WINDOWS winnt
Issue type : kbprb