Q138411: Deriving an OLE Control from a Base Control
Article: Q138411
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s):
Keyword(s): kbfile
Last Modified: 06-MAY-2001
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The information in this article applies to:
- The Microsoft Foundation Classes (MFC), used with:
- Microsoft OLE Control Developer's Kit (CDK)
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This is the Visual C++ 1.5x and 2.x version of this sample. There is an
equivalent Visual C++ 4.x 32-bit sample available under the name SHAPES32.
SUMMARY
=======
The SHAPES sample illustrates how to derive an OLE control from a base control
thus allowing the derived control to take advantage of the base control's
methods, properties, and events.
The following files are available for download from the Microsoft Download
Center:
Shapes2.exe
For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:
Q119591 How to Obtain Microsoft Support Files from Online Services
Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.
MORE INFORMATION
================
The Shapes2 sample implements a base control class called CBaseShapeCtrl and two
derived control classes called CCircleCtrl and CRectangleCtrl. CCircleCtrl and
CRectangleCtrl draw themselves using the properties provided by the base
CBaseShapeCtrl class. CCircleCtrl and CRectangleCtrl also allow access to the
methods and events implemented in the base CBaseShapeCtrl class.
Properties, Methods, and Events
-------------------------------
The CBaseShapeCtrl class provides the base functionality of a simple shape
control. It implements the following properties, events, and methods:
Name Type Use
FillColor Property An OLE_COLOR value that represents the color
used to fill the shape.
LineColor Property An OLE_COLOR value that represents the color
used for the shape's outline.
LineWidth Property A short value that represents the line width
in pixels of the shape's outline.
BaseMethod1 Method A test method that, when invoked, fires the
BaseEvent1 event. Takes a single parameter of
type long.
BaseMethod2 Method A test method that, when invoked, fires the
BaseEvent2 event. Takes a single parameter of
type BSTR.
BaseEvent1 Event A test event. Returns a long.
BaseEvent2 Event A test event. Returns a BSTR.
The CCircleCtrl class provides the functionality of a simple circle control. It
implements the following properties, events, and methods:
Name Type Use
CircleShape Property A boolean value. If TRUE, the control draws
itself as a circle. If FALSE, it draws itself
as an ellipse.
CircleOffset Property A short value that represents the number of
pixels offset from the center of the bounding
rectangle where the control will draw itself.
CircleMethod1 Method A test method that, when invoked, fires the
CircleEvent1 event. Takes a single parameter
of type long.
CircleMethod2 Method A test method that, when invoked, fires the
CircleEvent2 event. Takes a single parameter
of type long.
CircleEvent1 Event A test event. Returns a long.
CircleEvent2 Event A test event. Returns a long.
The CRectangleCtrl class provides the functionality of a simple rectangle
control. It implements the following properties, events, and methods:
Name Type Use
RoundedCorners Property A boolean value. If TRUE, the control draws
itself with rounded corners. If FALSE, it
draws itself with square corners.
RectangleInset Property A short value that represents the number of
pixels inside the control's bounding
rectangle where the control will inset itself.
RectMethod1 Method A test method that, when invoked, fires the
RectEvent1 event. Takes no parameters.
RectMethod2 Method A test method that, when invoked, fires the
RectEvent2 event. Takes no parameters.
RectEvent1 Event A test event. Returns void.
RectEvent2 Event A test event. Returns void.
Modifying the Base Control Class
--------------------------------
Several changes need be made to the code generated by ControlWizard to allow a
derived control to cleanly inherit the functionality provided by a base control.
The following changes were made to the files generated by ClassWizard:
1. The ON_OLEVERB entry for AFX_IDS_VERB_PROPERTIES in the CBaseShapeCtrl
message map was removed:
BEGIN_MESSAGE_MAP(CBaseShapeCtrl, COleControl)
//{{AFX_MSG_MAP(CBaseShapeCtrl)
//}}AFX_MSG_MAP
//ON_OLEVERB(AFX_IDS_VERB_PROPERTIES, OnProperties)
END_MESSAGE_MAP()
If this isn't done, control containers will list the Properties verb twice
when the the user clicks Control Object on the Edit menu.
2. The CBaseShapeCtrl::CBaseShapeCtrlFactory::UpdateRegistry function was
changed to simply return TRUE:
BOOL CBaseShapeCtrl::CBaseShapeCtrlFactory::UpdateRegistry(
BOOL bRegister)
{
return TRUE;
}
If this isn't done, the base control is registered and containers will list it
along with the derived controls when the user attempts to insert a new
control.
3. The CBaseShapePropPage::CBaseShapePropPageFactory::UpdateRegistry function
was changed to simply return TRUE:
BOOL CBaseShapePropPage::CBaseShapePropPageFactory::UpdateRegistry(
BOOL bRegister)
{
return TRUE;
}
If this isn't done, the base class control's Property page is registered, so
it will appear in the registry.
4. The call to InitializeIIDs in the base control class constructor was
removed:
CBaseShapeCtrl::CBaseShapeCtrl()
{
//InitializeIIDs(&IID_DBaseShape, &IID_DBaseShapeEvents);
// TODO: Initialize your control's instance data here.
}
If this isn't done, there will be a memory leak upon termination when the
cached type information for the base class is not freed.
5. The code in the base control's section of the project's .odl file was changed
to comments (commented out). If this isn't done, some containers (for
example, Visual Basic version 4.0) won't load the control.
6. The CBaseShapeCtrl::DoPropExchange method was modified to not call the
COleControl::ExchangeVersion function:
void CBaseShapeCtrl::DoPropExchange(CPropExchange* pPX)
{
// ExchangeVersion(pPX, MAKELONG(_wVerMinor, _wVerMajor));
COleControl::DoPropExchange(pPX);
...
}
If this isn't done, saving the state of the derived OLE control will generate
an assertion on line 1169 of the Propset.cpp file. The assertion is generated
because the _Version property has already been serialized by a call to the
ExchangeVersion method in the DoPropExchange method of the derived control.
7. The DoPropExchange method of the derived OLE controls was changed to make a
call to CBaseShapeCtrl::DoPropExchange instead of
COleControl::DoPropExchange:
void CCircleCtrl::DoPropExchange(CPropExchange* pPX)
{
ExchangeVersion(pPX, MAKELONG(_wVerMinor, _wVerMajor));
CBaseShapeCtrl::DoPropExchange(pPX);
...
}
Manually Updating the Project's .Odl File
-----------------------------------------
ClassWizard does not provide any support for developing OLE controls that are
derived from another control. As a result, the property, method, and event
dispatch ID's used by a derived control class need to be manually updated. MFC
Technical Note #39 discusses the dispatch ID numbering scheme used by MFC.
Basically, MFC divides a 32-bit dispatch ID (DISPID) into two parts. The LOWORD
of the DISPID contains the distance from the top of the dispatch map (1
relative). The HIWORD contains the distance of the dispatch map from the most
derived class (0 relative).
The CCircleCtrl and CRectangleControl classes use this technique with their
DISPIDs. For example, the CCircleCtrl derived control inherits the FillColor,
LineColor, and LineWidth properties from CBaseShapeCtrl. The DISPIDs for these
properties need to be manually added to the CCircleCtrl properties section of
the project's .odl file and the HIWORD of the DISPIDs needs to be adjusted:
// Primary dispatch interface for CCircleCtrl
[ uuid(A7EC6760-BFED-11CE-8250-524153480001),
helpstring("Dispatch interface for sample Circle Control"), hidden ]
dispinterface _DCircle
{
properties:
// NOTE - ClassWizard will maintain property information here.
// Use extreme caution when editing this section.
//{{AFX_ODL_PROP(CCircleCtrl)
[id(0x10001)] OLE_COLOR FillColor;
[id(0x10002)] OLE_COLOR LineColor;
[id(0x10003)] short LineWidth;
[id(1)] boolean CircleShape;
[id(2)] short CircleOffset;
//}}AFX_ODL_PROP
...
The FillColor, LineColor, and LineWidth properties implemented in the base class
have DISPIDs of 1, 2, and 3 respectively. Once the HIWORD portion of these
DISPIDs has been adjusted, they become 0x10001 (65537), 0x10002 (65538), and
0x10003 (65539).
The DISPIDs for events are handled differently from those for properties and
methods. Event DISPIDs do not have their HIWORD portion adjusted. Instead, they
are sequential. For example, CCircleCtrl inherits the BaseEvent1 and BaseEvent2
events from the base control. The DISPIDs for the base events need to be
manually added to the CCircleCtrl events section of the project's .odl file.
Also, the DISPIDs assigned by ClassWizard for the events provided by CCircleCtrl
(CircleEvent1 and CircleEvent2) need to have their DISPIDs manually updated:
[ uuid(A7EC6761-BFED-11CE-8250-524153480001),
helpstring("Event interface for sample Circle Control") ]
dispinterface _DCircleEvents
{
properties:
// Event interface has no properties
methods:
// NOTE - ClassWizard will maintain event information here.
// Use extreme caution when editing this section.
//{{AFX_ODL_EVENT(CCircleCtrl)
[id(1)] void BaseEvent1(long lParam);
[id(2)] void BaseEvent2(BSTR pszString);
[id(3)] void CircleEvent1(long lParam);
[id(4)] void CircleEvent2(long lParam);
//}}AFX_ODL_EVENT
};
Manually updating the DISPIDs can be problematic because ClassWizard may get
confused when it sees the entries for the events in the base class. As a result,
more than one event may be assigned the same DISPID. After adding new events to
a derived class, inspect the project's .odl file, and fix any conflicts.
ClassWizard also maintains DISPID values for properties, methods, and events, in
an enum member of the COleControl derived class. Again, because ClassWizard does
not support deriving an OLE control from another control, it may generate
conflicting values for the different DISPIDs. If this happens, manually edit the
values so that they are correct, and match the values as specified in the .odl
file. Following is the portion of the CCircleCtrl class that shows the DISPIDs
used for both the base class and derived class properties, events, and methods:
class CCircleCtrl : public CBaseShapeCtrl
{
...
// Dispatch and event IDs
public:
enum {
//{{AFX_DISP_ID(CCircleCtrl)
//dispidFillColor = 65537L,
//dispidLineColor = 65538L,
//dispidLineWidth = 65539L,
//dispidBaseMethod1 = 65540L,
//dispidBaseMethod2 = 65541L,
dispidCircleShape = 1L,
dispidCircleOffset = 2L,
dispidCircleMethod1 = 3L,
dispidCircleMethod2 = 4L,
//eventidBaseEvent1 = 1L,
//eventidBaseEvent2 = 2L,
eventidCircleEvent1 = 3L,
eventidCircleEvent2 = 4L,
//}}AFX_DISP_ID
};
the Shapes2 sample is based on a ControlWizard-generated OLE control. Files
included with the sample that are directly related to deriving an OLE control
from a base control are:
Shapes.odl -
Shows the modified DISPIDs used by the derived control classes.
Basectl.cpp -
Provides the implementation of the base CBaseShapeCtrl class.
Circctl.cpp -
Provides the implementation of the derived CCircleCtrl class.
Rectctl.cpp -
Provides the implementation of the derived CRectangleCtrl class.
REFERENCES
==========
MFC Technical Note #39.
Additional query words: 2.00 2.10 2.20 2.50 2.51 2.52 3.00 3.10 3.20 shapes32
======================================================================
Keywords : kbfile
Technology : kbAudDeveloper kbMFC
Version : winnt:
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