HOWTO: Implement Visual C++ COM Objects Returning Recordsets

• Active Server Pages
• ActiveX Data Objects (ADO), versions 1.5, 2.0
• Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

SUMMARY

This article describes, by example, how to implement a Visual C++ Component Object Model (COM) object that returns a recordset to Active Server Pages (ASP).

Implementing this incorrectly can result in memory leaks or one of the following errors:

The operation requested by the application is not allowed if the object is closed.

-or-

Type Mismatch

-or-

error 'ASP 0115' - A trappable error occured in an external object

MORE INFORMATION

Use the following steps to implement a method that returns a recordset from a Visual C++ COM object to Active Server Pages.

1. Create an ATL DLL project called PassRs.



2. Insert an ATL object named PassRsObj.



3. Add a method with the following information:
   

   
   Method Name: TestMethod
   Parameters : [out, retval] LPDISPATCH* ppRecordset 




4. Include the following line in the object's implementation file:

   
   #import "msado15.dll" no_namespace rename("EOF", "adoEOF") 




5. Implement the method as follows:
   

   
   STDMETHODIMP CPassRsObj::TestMethod(LPDISPATCH *ppRecordset )
   {
       _ConnectionPtr pConn;
       _RecordsetPtr pRs;
   	
       pConn.CreateInstance(__uuidof(Connection));
       pRs.CreateInstance(__uuidof(Recordset));
   
       pConn->Open("DSN=pubs;uid=sa;pwd=;", (BSTR) NULL, (BSTR) NULL, -1);
   
       //Client side cursor is required for disconnected recordsets
       pRs->CursorLocation = adUseClient;
   
       pRs->Open( "select * from authors",
                  pConn.GetInterfacePtr(),
                  adOpenKeyset, adLockOptimistic, -1);
   
       // Disconnect the recordset
       pRs->PutRefActiveConnection(NULL);
   
       //Clone the recordset. 
       //NOTE: Recordset to be cloned must support bookmarks
       pRs->Clone(adLockOptimistic)->QueryInterface(IID_IDispatch, (void**) ppRecordset);
   
       pRs->Close();
       pConn->Close();
   
       pRs = NULL;
       pConn = NULL;	
   
       return S_OK;
   } 




6. Create an ASP page with the following code:

   
   <%
   Dim rsTest, oTestPassRs
   
   Set oTestPassRs = Server.CreateObject("PassRs.PassRsObj")
   Set rsTest = oTestPassRs.TestMethod()
   
   Do 
      Response.Write ( "Value in Record = " & rsTest(1) & "<BR>" )
      rsTest.MoveNext
   Loop until rsTest.EOF
   
   rsTest.Close
   Set rsTest = Nothing
   Set oTestPassRs = Nothing
   %> 

REFERENCES

For additional information, please see the following articles in the Microsoft Knowledge Base:

Q184397 HOWTO: Getting ADO Disconnected Recordsets in VBA/C++/Java

Q193515 PRB: Update of Same Record 2x w/Disconnect ADO Recordset Fails

Additional query words:

Keywords          : kbADO150 kbADO200 kbCOMt kbVC500 kbVC600 kbGrpASP 
Version           : WINDOWS:1.5,2.0; winnt:5.0,6.0
Platform          : WINDOWS winnt 
Issue type        : kbhowto 

Last Reviewed: August 8, 1999