How to Implement the Type Property of a 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

The type of an OLE control is the coclass name for the OLE control as defined in its ODL file. For example, the type of Textbox control is TextBox, the type of the Data Bound Grid control from Apex is DBGrid, and so on. In Visual Basic, the type of an OLE Control appears to the right of the control name in the Editbox portion of the property browser. This article shows how you can implement a custom Type property for a custom OLE control that returns its type.

MORE INFORMATION

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. Use the Visual C++ MFC ClassWizard to add the following Property to your control class. The external name of this Property is Type. It is implemented as a Get method only (called GetControlType), so it is a read-only property. It takes no parameters and returns the type as a BSTR. The implementation of this Get method calls an internal private member function of the control class called GetCtrlType() that does all the work.

   You will also need to add a protected member variable m_bstrType (to actually hold the type name as a BSTR) and a protected member function GetCtrlType() to the control class in the header file as follows:

         Class CTestControl::COleControl
            {
             .
             .
             .
   
             protected:
             BSTR m_bstrType;
             BSTR GetCtrlType (void);
             }
   
   

3. 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_bstrType = NULL;
   
            .
            .
            .
          }
   
   
   
   //////////////////////////////////////////////////////////////////////
         // CTestCtrl::~CTestCtrl - Destructor
   
         CTestCtrl::~CTestCtrl()
         {
            // TODO: Cleanup your control's instance data here.
   
            if (m_bstrType)
               SysFreeString(m_bstrType);
   
            .
            .
            .
          }
   
          BSTR CTestCtrl::GetCtrlType (void)
          {
   
              IProvideClassInfo *pProvideClassInfo;
              LPTYPEINFO pTypeInfo;
   
              if(GetIDispatch(FALSE)->QueryInterface(IID_IProvideClassInfo,
                (LPVOID   *)&pProvideClassInfo) ==  NOERROR)
             {
                if (pProvideClassInfo->GetClassInfo(&pTypeInfo) == NOERROR)
                {
                   if (m_bstrType)
                      SysFreeString(m_bstrType);
   
                   pTypeInfo->GetDocumentation(MEMBERID_NIL, &m_bstrType,
                                               NULL, NULL, NULL);
                   pTypeInfo->Release();
                }
   
                pProvideClassInfo->Release();
             }
   
             return m_bstrType;
          }
   
          BSTR CTestCtrl::GetControlType()
          {
             CString strResult;
             // TODO: Add your property handler here
   
             strResult = GetCtrlType();
             return strResult.AllocSysString();
          }
   
   

4. Build the Test OLE control from Visual C++ 4.x, which should automatically register the control if everything compiles properly.

5. 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.

6. Add an instance of the Test OLE control to Form1. This will be named Test1.

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

      Private Sub Form_Click()
         Debug.Print Test1.Type
      End Sub
   
   

8. Run the Visual Basic program, and click the form. You should see Test printed in the Debug window