PRB: Messages Not Received by Dynamically Created Control

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2

SYMPTOMS

When you create ActiveX Controls dynamically via CWnd::CreateControl(), Windows messages are not sent to your CWnd-derived class. For example, if you create a handler for WM_KILLFOCUS, it is not called.

CAUSE

CWnd::CreateControl() doesn't subclass the HWND associated with the control.

STATUS

This problem is by design.

WORKAROUND

As a workaround, use a CFormView or CDialog and add the control to the dialog box resource at design time via the Resource Editor. You can then add a member variable using Class Wizard. The C++ class wrapper created by Component Gallery doesn't have a message map, so you must add this manually.

The following steps illustrates how to insert an ActiveX Control and create a message handler for it:

1. While editing the dialog box resource, on the Visual C++ menu, click Insert/Component to insert a control and generate a C++ wrapper class.

2. Add the control to your dialog box resource by dragging it from the Controls toolbar.

3. Add a member variable for it using Class Wizard

4. In the header file generated by Component Gallery, add these lines at the end of the class declaration:

         //{{AFX_MSG(CMyClass)
         //}}AFX_MSG
         DECLARE_MESSAGE_MAP()
   

   Replace CMyClass with the actual class name.

5. In the .cpp file generated by Component Gallery, add these lines after the #include section:

         BEGIN_MESSAGE_MAP(CMyClass, CWnd)
             //{{AFX_MSG_MAP(CMyClass)
             //}}AFX_MSG_MAP
         END_MESSAGE_MAP()
   
      Again, replace CMyClass with the actual class name.
   
   

6. Start Class Wizard, click Add Class/From A File, and add this class.

7. Add your message handler.

MORE INFORMATION

When you insert a control via Component Gallery, the C++ class wrapper has Create() functions that also call CWnd::CreateControl().

The workaround above works because adding a member variable adds a call to DDX_Control(). DDX_Control() calls SubclassWindow() for you.

If you try to call SubclassWindow() after calling CreateControl(), you will get several ASSERTs, because the HWND associated with the control has already been attached to a different MFC class (CWnd member of COleControlSite).