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



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