HOWTO: Define the Display Size of an MFC ActiveX Control

ID: Q168326

The information in this article applies to:

The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++, 32-bit Editions, versions 4.2, 4.2b, 5.0, 6.0

SUMMARY

Sometimes you need to limit an ActiveX control's minimum or maximum size at design time. This article explains how to do this by overriding the virtual COleControl::OnSetExtent method.

MORE INFORMATION

The COleControl::OnSetExtent method is called by COleControl::XOleObject::SetExtent. COleControl::XOleObject is COleControl's IOleObject implementation. A container calls IOleObject::SetExtent when it needs to dictate to an embedded object the size at which it will be displayed.

COleControl::OnSetExtent takes one parameter, a pointer to a SizeL structure. The SizeL structure contains the new size that the container is requesting for the control in HIMETRIC units.

If a you need to limit a control to a certain size, you need to make it override COleControl::OnSetExtent and do the following:

Make any necessary unit conversions on the SizeL structure passed in.

Check the size in the SizeL structure that was passed in.

If the control cannot accomodate the request, return false.

Otherwise, pass the (unmodified) SizeL to the base class' implementation of OnSetExtent.

See the Sample Code section below for an example of how to override COleControl::OnSetExtent to limit the size of an ActiveX control.

Sample Code

The following code snippet shows how to override COleControl::OnSetExtent so that the height of the control will never be smaller than 20 pixels.


    BOOL CTestCtrl::OnSetExtent(LPSIZEL lpSizeL)
      {
         // This function limits the height of a control to be at
         // least 20 pixels.

         // Use the desktop window to get a DC so we can use
         // CDC::HIMETRICtoDP and CDC::DPtoHIMETRIC
         CWnd *pWnd = CWnd::FromHandle(::GetDesktopWindow());
         CClientDC dc(pWnd);

         CSize sz(lpSizeL->cx,lpSizeL->cy);
         dc.HIMETRICtoDP(&sz); //Convert the size to pixels

         if (sz.cy < 20)
            return false;

         return COleControl::OnSetExtent(lpSizeL);
      }

Note that the code is identical up to and including the line that says "if (sz.cy is greater than 20)". The new code also lacks the "TRACE" lines.

REFERENCES

Additional query words: ocx

Keywords : kbole kbMFC kbVC420 kbVC500 kbVC600 kbDSupport
Version : winnt:4.2,4.2b,5.0,6.0
Platform : winnt
Issue type : kbhowto