Creating a Disconnected Recordset in C++ For VB

Environment: Visual C++ 6.0 and VB 6.0, ADO 1.5

This article assumes that you are familiar with ADO, Visual C++ and ATL.

This article shows how to create a Visual C++ COM object that creates a disconnected ADO Recordset. With a disconnected ADO recordset, your ATL COM object can connect to the database, create the Recordset, and disconnect without having to keep a connection open, while allowing the client to read the data in the Recordset. This works great with MTS where database connection pooling requires this functionality.

Resources: HOWTO: Getting ADO Disconnected Recordsets in VBA/C++/Java
SAMPLE: ATL2ADO.exe Returns Disconnected ADO Recordset

In your project make sure you import dll to create your ADO classes.


#import "c:\program files\common files\system\ado\msado15.dll" no_namespace rename("EOF", "adoEOF") implementation_only

Next create a method with an [out, retval] property of type _Recordset **

[id(1), helpstring("method GetADORecordset")] HRESULT GetADORecordset( [out, retval] _Recordset **ppRecordset);

Your source code should look something like this:

STDMETHODIMP CReturnADO::GetADORecordset(_Recordset **ppRecordset)
{
	
	_ConnectionPtr	pConnection;
	_RecordsetPtr	pRecordset;
	
	try
	{
		pConnection.CreateInstance(__uuidof(Connection));
		pConnection->CursorLocation = adUseClient; // Important for disconnected sets
		pConnection->Open("DSN=SQLSource","loginnmae","password",-1);


		pRecordset.CreateInstance(__uuidof(Recordset));
		pRecordset->CursorLocation = adUseClient; // Important for disconnected sets
		
		pRecordset->PutRefActiveConnection(pConnection.GetInterfacePtr()); // Associate the connection to this recordset

		pRecordset->Open("SELECT DISTINCT name FROM TBLWHATEVER WHERE name IS NOT NULL",vtMissing,
			adOpenForwardOnly, adLockReadOnly,
			adCmdText);

        pRecordset->PutRefActiveConnection(NULL); // Disassociate the connection from the recordset.

		if(pConnection->GetState() == adStateOpen)
			pConnection->Close();
		pConnection = NULL;
		
		*ppRecordset = pRecordset.Detach();

	}
	catch(_com_error *e)
	{
		return AtlReportError(CLSID_ReturnADO,e->ErrorMessage(),IID_IReturnADO,E_FAIL);
	}
	catch(...)
	{
		return AtlReportError(CLSID_ReturnADO,"Unknown Error.",IID_IReturnADO,E_FAIL);
	}

	return S_OK;
}

Now your interface descriptions and CLSID's will be different, but go ahead and use this code to build your own version. Your VB code might look something like this:

    Dim adoset As ADODB.Recordset
    Dim engine As New TESTRETURNADOLib.ReturnADO
    
    Set adoset = engine.GetADORecordset
    While Not adoset.EOF
        List1.AddItem adoset.Fields(0)
        adoset.MoveNext
    Wend
    
    Set adoset = Nothing



Comments

  • There are no comments yet. Be the first to comment!

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Intelligent N+X Redundancy, Placement Affinities, & Future Proofing in the Virtualized Data Center Virtualization brought about the ability to simplify business continuity management in IT. Workload portability and data replication capabilities mean that physical infrastructure failures no longer need impact application services, and they can rapidly be recovered even in the event of complete site failure. However, Enterprises and Service Providers face new challenges ensuring they have enough compute …

  • The nature of the financial services industry places a myriad of international compliance requirements on a company's IT team, as well as an expectation by its customers to deliver the high test levels of performance and reliability. To survive and thrive, businesses in the industry must not only keep pace with customer demand but gain competitive advantage. Those demands mean the IT team must be at the forefront of adopting emerging technologies

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date