Author: Mike Heydon

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.
1   
2   // ===================================
3   // Emulate VB function IS NOTHING
4   // ===================================
5   
6   function IsNothing(Obj: OleVariant): boolean;
7   begin
8     Result := IDispatch(Obj) = nil;
9   end;
10  
11  // ============================================
12  // Emulate VB function VarX := Nothing
13  // ============================================
14  
15  function varNothing: IDispatch;
16  var
17    Retvar: IDispatch;
18  begin
19    Retvar := nil;
20    Result := Retvar;
21  end;
22  
23  -------------------------------------------------------------------------- 
24  Now the VBasic example can translate to ... 
25  
26  var
27    objFolder, objMessages: OleVariant;
28  
29    objFolder := objSession.GetFolder(strFolderID);
30  
31  if IsNothing(objFolder) then
32  begin
33    objMessages := varNothing;
34    ShowMessage('Unable to retrieve folder with specified ID');
35    exit;
36  end;


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