CDO Emulation of VBasic value NOTHING

Problem/Question/Abstract:

I am currently writing a class that encapsulates the CDO (Collaboration Data Objects) in Delphi. For those of you who do not know what CDO is, it is a library that allows acces to MS-Exchange and Outlook folders,messages etc. much in the way of MAPI.

Answer:

One problem that kept cropping up from the examples in CDO.HLP was the use of something called "NOTHING" in the VB examples.

eg.

   Set objFolder = objSession.GetFolder(strFolderID)

    If objFolder Is Nothing Then
        Set objMessages = Nothing
        MsgBox "Unable to retrieve folder with specified ID"
        Exit Function
    End If

This is not "VARNULL",'VAREMPTY" or even "UNASSIGNED" in Delphi. Trying "if obFolder = VarNull" or any of above results in invalid typecast errors as do trying to compare to NIL.

What is it then ..... It is an IDispatch type which is set to NIL

The following 2 functions will emulate the behaviour of VB's NOTHING in Delphi.

// ===================================
// Emulate VB function IS NOTHING
// ===================================

function IsNothing(Obj: OleVariant): boolean;
begin
  Result := IDispatch(Obj) = nil;
end;

// ============================================
// Emulate VB function VarX := Nothing
// ============================================

function varNothing: IDispatch;
var
  Retvar: IDispatch;
begin
  Retvar := nil;
  Result := Retvar;
end;

--------------------------------------------------------------------------
Now the VBasic example can translate to ...

var
  objFolder, objMessages: OleVariant;

  objFolder := objSession.GetFolder(strFolderID);

if IsNothing(objFolder) then
begin
  objMessages := varNothing;
  ShowMessage('Unable to retrieve folder with specified ID');
  exit;
end;

For more information on CDO see CDO.HLP (From MS-Exchange CD) or Web Site
http://www.cdolive.com/exchange2000.htm