Port Forwarding by Using "HNetCfg.NATUPnP" Ole Object Failed

Question

I am using a code for forwarding a port. this code works fine on My Windows 7; but I can't use It on Windows XP.

Update 1 For Problem(2012-10-17 07:32:00Z)

This is my source code:

uses
  ActiveX, oleAuto;

Procedure AddUPnPEntry(Port: Integer; const Name: ShortString; LAN_IP: string);
Var
  Nat: Variant;
  Ports: Variant;
  SavedCW: Word;
Begin
  if NOT(LAN_IP = '127.0.0.1') then
  begin
    try
      Nat := CreateOleObject('HNetCfg.NATUPnP');
      Ports := Nat.StaticPortMappingCollection;

      // Error Raized From Here!!!
      ShowMessage(inttostr(Ports.count));

      Ports.Add(Port, 'TCP', Port, LAN_IP, True, name);
    except
      ShowMessage('An Error occured with adding UPnP Ports. The ' + name +
        ' port was not added to the router. Please check to see if  your ' +
        'router supports UPnP and has it enabled or disable UPnP.');
    end;
  end;
End;

procedure TForm1.Button2Click(Sender: TObject);
begin
  AddUPnPEntry(1234, 'Hello3', '192.168.1.1');
end;

AV Error Message:

Project Project1.exe raised exception class $C0000005 with message 'access violation at 0x00504876: read of address 0x00000000'.

Answer 1

If you are getting an access violation, when you access the count property, this means which the IStaticPortMappingCollection interface returned by the IUPnPNAT.get_StaticPortMappingCollection method is nil, this can be caused by many reasons your device doesn't supports UPnP, The UPnP is not enabled on the device, The UPnP User Interface is not installed/active, and so on.

Anyway to prevent this kind of exceptions (the access violation) you must check the value returned by the property or method before to use it, in this case you can use the VarIsClear function like so :

try
  Nat := CreateOleObject('HNetCfg.NATUPnP');
  Ports := Nat.StaticPortMappingCollection;

  if not VarIsClear(Ports) then
  begin
    //do something
    ShowMessage(inttostr(Ports.count));
    Ports.Add(Port, 'TCP', Port, LAN_IP, True, name);
  end;

except on E:Exception do
  ShowMessage('An Error occured with adding UPnP Ports. '+E.Message);
end;