Tadoblobstream.Create出错？请帮忙解释一下

[chdaner]

procedure Tfrmretailupdate.Button1Click(Sender: TObject);
var filename:string;
bs:Tadoblobstream;
begin
with adoquery1 do begin
close;
sql.Clear;
sql.Add('select f_name,f_program from T_retailversionupdate where f_name='''+'retail.exe'+'''');
open;
filename:=adoquery1.fieldbyname('f_name').AsString;
showmessage('test');
bs:=Tadoblobstream.Create(Tblobfield(adoquery1.FieldByName('f_name')),bmRead);
bs.SaveToFile(filename);
bs.Free;
end;
end;

执行到这一步bs:=Tadoblobstream.Create(Tblobfield(adoquery1.FieldByName('f_name')),bmRead);出错，提示‘invalid variant type’ 是什么意思？

[dinoalex]

历史一般性是:TBlobField(A_ADO_Field).SaveToStream(ms); //MS: TMemoryStream
ms.SaveToFile(....);