HOWTO: Define the Display Size of an MFC ActiveX Control

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

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:

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

2. Check the size in the SizeL structure that was passed in and change it if necessary.

3. If necessary, reconvert the SizeL structure to HIMETRIC units.

4. Pass the SizeL structure to the base class implementation of OnSetExtent.

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)
      {
         TRACE("%ld\n",sz.cy);

         //size is less than 20, set it to 20
         sz.cy = 20;
         dc.DPtoHIMETRIC(&sz);//re-convert to HIMETRIC
         lpSizeL->cy = sz.cy;
      }
      else
      {
         TRACE("%ld\n",sz.cy);

         //no conversion necessary
      }

      return COleControl::OnSetExtent(lpSizeL);
   }

REFERENCES

An ActiveX control may be defined to be the size of the dialog template from which it was created. For additional information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q155973
   TITLE     : SAMPLE: Create an ActiveX Control with a Dialog Resource