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TN039: MFC/OLE Automation Implementation

NoteNote:

The following technical note has not been updated since it was first included in the online documentation. As a result, some procedures and topics might be out of date or incorrect. For the latest information, it is recommended that you search for the topic of interest in the online documentation index.

The IDispatch interface is the means by which applications expose methods and properties such that other applications such as Visual BASIC, or other languages, can make use of the application's features. The most important part of this interface is the IDispatch::Invoke function. MFC uses "dispatch maps" to implement IDispatch::Invoke. The dispatch map provides the MFC implementation information on the layout or "shape" of your CCmdTarget-derived classes, such that it can directly manipulate the properties of the object, or call member functions within your object to satisfy IDispatch::Invoke requests.

For the most part, ClassWizard and MFC cooperate to hide most of the details of OLE automation from the application programmer. The programmer concentrates on the actual functionality to expose in the application and doesn't have to worry about the underlying plumbing.

There are cases, however, where it is necessary to understand what MFC is doing behind the scenes. This note will address how the framework assigns DISPIDs to member functions and properties. Knowledge of the algorithm MFC uses for assigning DISPIDs is only necessary when you need to know the IDs, such as when you create a "type library" for your application's objects.

Although the end-user of automation (a Visual Basic user, for example), sees the actual names of the automation enabled properties and methods in their code (such as obj.ShowWindow), the implementation of IDispatch::Invoke does not receive the actual names. For optimization reasons, it receives a DISPID, which is a 32-bit "magic cookie" that describes the method or property that is to be accessed. These DISPID values are returned from the IDispatch implementation through another method, called IDispatch::GetIDsOfNames. An automation client application will call GetIDsOfNames once for each member or property it intends to access, and cache them for later calls to IDispatch::Invoke. This way, the expensive string lookup is only done once per object use, instead of once per IDispatch::Invoke call.

MFC determines the DISPIDs for each method and property based on two things:

  • The distance from the top of the dispatch map (1 relative)

  • The distance of the dispatch map from the most derived class (0 relative)

The DISPID is divided into two parts. The LOWORD of the DISPID contains the first component, the distance from the top of the dispatch map. The HIWORD contains the distance from the most derived class. For example:

class CDispPoint : public CCmdTarget
{
public:
    short m_x, m_y;
    ...
    DECLARE_DISPATCH_MAP()
    ...
};

class CDisp3DPoint : public CDispPoint
{
public:
    short m_z;
    ...
    DECLARE_DISPATCH_MAP()
    ...
};

BEGIN_DISPATCH_MAP(CDispPoint, CCmdTarget)
    DISP_PROPERTY(CDispPoint, "x", m_x, VT_I2)
    DISP_PROPERTY(CDispPoint, "y", m_y, VT_I2)
END_DISPATCH_MAP()

BEGIN_DISPATCH_MAP(CDisp3DPoint, CDispPoint)
    DISP_PROPERTY(CDisp3DPoint, "z", m_z, VT_I2)
END_DISPATCH_MAP()

As you can see, there are two classes, both of which expose OLE automation interfaces. One of these classes is derived from the other and thus leverages the base class's functionality, including the OLE automation part ("x" and "y" properties in this case).

MFC will generate DISPIDs for class CDispPoint as follows:

property X    (DISPID)0x00000001
property Y    (DISPID)0x00000002

Since the properties are not in a base class, the HIWORD of the DISPID is always zero (the distance from the most derived class for CDispPoint is zero).

MFC will generate DISPIDs for class CDisp3DPoint as follows:

property Z    (DISPID)0x00000001
property X    (DISPID)0x00010001
property Y    (DISPID)0x00010002

The Z property is given a DISPID with a zero HIWORD since it is defined in the class that is exposing the properties, CDisp3DPoint. Since the X and Y properties are defined in a base class, the HIWORD of the DISPID is 1, since the class in which these properties are defined is at a distance of one derivation from the most derived class.

NoteNote:

The LOWORD is always determined by the position in the map, even if there exist entries in the map with explicit DISPID (see next section for information on the _ID versions of the DISP_PROPERTY and DISP_FUNCTION macros).

There are a number of additional features that ClassWizard does not support with this release of Visual C++. ClassWizard supports DISP_FUNCTION, DISP_PROPERTY, and DISP_PROPERTY_EX which define a method, member variable property, and get/set member function property, respectively. These capabilities are usually all that is needed to create most automation servers.

The following additional macros can be used when the ClassWizard supported macros are not adequate: DISP_PROPERTY_NOTIFY, and DISP_PROPERTY_PARAM.

DISP_PROPERTY_NOTIFY( 
   theClass, 
   pszName, 
   memberName, 
   pfnAfterSet, 
   vtPropType 
)

Parameters

theClass

Name of the class.

pszName

External name of the property.

memberName

Name of the member variable in which the property is stored.

pfnAfterSet

Name of member function to call when property is changed.

vtPropType

A value specifying the property's type.

This macro is much like DISP_PROPERTY, except that it accepts an additional argument. The additional argument, pfnAfterSet, should be a member function that returns nothing and takes no parameters, 'void OnPropertyNotify()'. It will be called after the member variable has been modified.

DISP_PROPERTY_PARAM( 
   theClass,
   pszName,
   pfnGet,
   pfnSet,
   vtPropType,
   vtsParams 
)

Parameters

theClass

Name of the class.

pszName

External name of the property.

memberGet

Name of the member function used to get the property.

memberSet

Name of the member function used to set the property.

vtPropType

A value specifying the property's type.

vtsParams

A string of space separated VTS_ for each parameter.

Much like the DISP_PROPERTY_EX macro, this macro defines a property accessed with separate Get and Set member functions. This macro, however, allows you to specify a parameter list for the property. This is useful for implementing properties that are indexed or parameterized in some other way. The parameters will always be placed first, followed by the new value for the property. For example:

DISP_PROPERTY_PARAM(CMyObject, "item", GetItem, SetItem, VT_DISPATCH,    VTS_I2 VTS_I2)

would correspond to get and set member functions:

LPDISPATCH CMyObject::GetItem(short row, short col)
void CMyObject::SetItem(short row, short col, LPDISPATCH newValue)

DISP_FUNCTION_ID( 
   theClass,
   pszName,
   dispid,
   pfnMember,
   vtRetVal,
   vtsParams 
) 
DISP_PROPERTY_ID( 
   theClass,
   pszName,
   dispid,
   memberName,
   vtPropType 
) 
DISP_PROPERTY_NOTIFY_ID( 
   theClass,
   pszName,
   dispid,
   memberName,
   pfnAfterSet,
   vtPropType 
) 
DISP_PROPERTY_EX_ID( 
   theClass,
   pszName,
   dispid,
   pfnGet,
   pfnSet,
   vtPropType 
) 
DISP_PROPERTY_PARAM_ID( 
   theClass,
   pszName,
   dispid,
   pfnGet,
   pfnSet,
   vtPropType,
   vtsParams 
)

Parameters

theClass

Name of the class.

pszName

External name of the property.

dispid

The fixed DISPID for the property or method.

pfnGet

Name of the member function used to get the property.

pfnSet

Name of the member function used to set the property.

memberName

The name of the member variable to map to the property

vtPropType

A value specifying the property's type.

vtsParams

A string of space separated VTS_ for each parameter.

These macros allow you to specify a DISPID instead of letting MFC automatically assign one. These advanced macros have the same names except that ID is appended to the macro name (e.g. DISP_PROPERTY_ID) and the ID is determined by the parameter specified just after the pszName parameter. See AFXDISP.H for more information on these macros. The _ID entries must be placed at the end of the dispatch map. They will affect the automatic DISPID generation in the same way as a non-_ID version of the macro would (the DISPIDs are determined by position). For example:

BEGIN_DISPATCH_MAP(CDisp3DPoint, CCmdTarget)
    DISP_PROPERTY(CDisp3DPoint, "y", m_y, VT_I2)
    DISP_PROPERTY(CDisp3DPoint, "z", m_z, VT_I2)
    DISP_PROPERTY_ID(CDisp3DPoint, "x", 0x00020003, m_x, VT_I2)
END_DISPATCH_MAP()

MFC will generate DISPIDs for class CDisp3DPoint as follows:

property X    (DISPID)0x00020003
property Y    (DISPID)0x00000002
property Z     (DISPID)0x00000001

Specifying a fixed DISPID is useful to maintain backward compatibility to a previously existing dispatch interface, or to implement certain system defined methods or properties (usually indicated by a negative DISPID, such as the DISPID_NEWENUM collection).

Retrieving the IDispatch Interface for a COleClientItem

Many servers will support automation within their document objects, along with the OLE server functionality. In order to gain access to this automation interface, it is necessary to directly access the COleClientItem::m_lpObject member variable. The code below will retrieve the IDispatch interface for an object derived from COleClientItem. You can include the code below in your application if you find this functionality necessary:

LPDISPATCH CMyClientItem::GetIDispatch()
{
    ASSERT_VALID(this);
    ASSERT(m_lpObject != NULL);

    LPUNKNOWN lpUnk = m_lpObject;

    Run();    // must be running

    LPOLELINK lpOleLink = NULL;
    if (m_lpObject->QueryInterface(IID_IOleLink, 
        (LPVOID FAR*)&lpOleLink) == NOERROR)
    {
        ASSERT(lpOleLink != NULL);
        lpUnk = NULL;
        if (lpOleLink->GetBoundSource(&lpUnk) != NOERROR)
        {
            TRACE0("Warning: Link is not connected!\n");
            lpOleLink->Release();
            return NULL;
        }
        ASSERT(lpUnk != NULL);
    }

    LPDISPATCH lpDispatch = NULL;
    if (lpUnk->QueryInterface(IID_IDispatch, &lpDispatch) 
        != NOERROR)
    {
        TRACE0("Warning: does not support IDispatch!\n");
        return NULL;
    }

    ASSERT(lpDispatch != NULL);
    return lpDispatch;
}

The dispatch interface returned from this function could then be used directly or attached to a COleDispatchDriver for type-safe access. If you use it directly, make sure that you call its Release member when through with the pointer (the COleDispatchDriver destructor does this by default).

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