FIX: Firing a KeyPress Event from an OLE Control Causes Beep

The information in this article applies to:

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

SYMPTOMS

While adding a KeyPress event to an OLE Control placed in an MFC-based OLE Control Container that handles the event, you will notice a beep for each key pressed that is not a mnemonic.

NOTE: The Grid OLE Control that ships with the product also displays this behavior.

CAUSE

If an OLE control is going to fire a KeyPress event, then when WM_GETDLGCODE is handled, it will need to specify DLGC_WANTCHARS as part of its return mask. In Visual C++ 4.0 and earlier, this is not being done and therefore the control never has an opportunity to process the WM_CHAR message that would normally fire the KeyPress event.

RESOLUTION

You need to handle WM_GETDLGCODE in your COleControl-derived class to inform the container that the control will handle WM_CHAR messages. For example, use this code:

UINT CKeyEventsCtrl::OnGetDlgCode() {

    // If you're firing KeyPress, prevent the container from
    // stealing WM_CHAR.
    return COleControl::OnGetDlgCode() | DLGC_WANTCHARS ;

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.

MORE INFORMATION

When developing OLE Controls, you may want the OLE control to fire a KeyPress event to the OLE control container. Usually, you would do this with ClassWizard by clicking the OLE Events tab and adding an event. With the KeyPress event, however, this is insufficient because the control is never given an opportunity to handle the WM_CHAR message, so it can't fire the KeyPress event.