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 filescommon filessystemadomsado15.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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