How to Get Container hWnd from Windowless Custom OLE Control

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The information in this article applies to:

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic for Windows, 16-bit and 32-bit, version 4.0
• Microsoft Visual C++, 32-bit Edition, version 4.x

SUMMARY

An OLE control that is invisible at run time does not have a window handle. Hence the GetParent() Windows API function cannot be used to obtain the hWnd of the container's window. This article shows how you can implement a read-only hWndParent property for a custom OLE control that returns the container window's hWnd.

MORE INFORMATION

The window handle of the container's window can be obtained by calling IOleInPlaceSite::GetWindow(). The IOleInPlaceSite interface pointer can be obtained by QueryInterfacing for it from the ClientSite pointer in the override of OnSetClientSite(). Note that this technique can also be used for OLE controls that have a window handle at run-time.

Step-by-step example

1. Create a rudimentary 32-bit OLE Control using the MFC CDK that ships with Visual C++ 4.x. This article assumes that this custom OCX is called Test (coclass name) and that the control class name is CTestCtrl.

2. In the .CPP file for your control class, add the OLEMISC_INVISIBLEATRUNTIME flag to your control. It should look something like this:

   //////////////////////////////////////////////////////////////////////
         // Control type information
   
         static const DWORD BASED_CODE _dwTestOleMisc =
         OLEMISC_INVISIBLEATRUNTIME |
         OLEMISC_ACTIVATEWHENVISIBLE |
         OLEMISC_SETCLIENTSITEFIRST |
         OLEMISC_INSIDEOUT |
         OLEMISC_CANTLINKINSIDE |
         OLEMISC_RECOMPOSEONRESIZE;
   
   

3. Use the Visual C++ MFC ClassWizard to add the following Property to your control class. The external name of this Property is hWndParent. It is implemented as a Get method only (called GetHWndParent), so it is a read-only property. It takes no parameters and returns the hWnd as an OLE_HANDLE.

   You will also need to add a protected member variable m_hWndParent (to actually hold the window handle as an OLE_HANDLE) and override the virtual function OnSetClientSite in the header file for the control class as follows:

         Class CTestControl::COleControl
         {
          .
          .
          .
   

         // Overrides
         virtual void OnSetClientSite();
   

          .
          .
   

          // Implementation
          protected:
             OLE_HANDLE m_hWndParent;
   
             .
             .
          };
   
   

4. Add the following code to the implementation file (.CPP) of the control class:

         /////////////////////////////////////////////////////////////////////
         // CTestCtrl::CTestCtrl - Constructor
   

         CTestCtrl::CTestCtrl()
         {
          InitializeIIDs(&IID_DTest, &IID_DTestEvents);
   

          // TODO: Initialize your control's instance data here.
   

          m_hWndParent = NULL;
   

          .
          .
          .
          }
   

          void CTestCtrl::OnSetClientSite()
          {
           if (m_pClientSite)
           {
            IOleInPlaceSite *pOleInPlaceSite = NULL;
   
            if (NOERROR == m_pClientSite >QueryInterface(IID_IOleInPlaceSite,
                                             (LPVOID *)&pOleInPlaceSite))
                 {
                  pOleInPlaceSite->GetWindow((HWND *)&m_hWndParent);
                  pOleInPlaceSite->Release();
                 }
            else
               m_hWndParent = NULL;
            }
            else
            m_hWndParent = NULL;
           }
   
           OLE_HANDLE CTestCtrl::GetHWndParent()
           {
            // TODO: Add your property handler here
   
            return m_hWndParent;
           }
   
   

5. Build the Test OLE control from Visual C++ 4.x to automatically register the control if everything compiles properly.

6. From Visual Basic 4.0 32-bit, open a new project, and add the Test OLE control to the Visual Basic toolbox by choosing it from the Tools Custom Controls menu.

7. Add an instance of the Test OLE control to Form1. This will be named Test1. Also add a Picture Box control (Picture1) to Form1, and then draw another Test OLE control (Test2) directly inside the PictureBox.

8. Add the following code to the general declarations portion of Form1:

      Private Sub Form_Click()
         Debug.Print Hex(Test1.hWndParent), Hex(Form1.hWnd)   'Both are same
         Debug.Print Hex(Test2.hWndParent), Hex(picture1.hWnd)'Both are same
         End Sub
   
   

9. Run the Visual Basic program, and click the form. You should see the same values for the Window handles printed on the same line in the Debug window in each of the two cases.