Title: CDO Emulation of VBasic value NOTHING
Question: 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.
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.
What is it then ..... The following 2 functions will emulate the behaviour of VB's NOTHING in Delphi.
Answer:
// ===================================
// 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;
Exit Function
end;
For more information on CDO see CDO.HLP (From MS-Exchange CD) or Web Site
http://www.cdolive.com/exchange2000.htm