Question

In this case I have some code which is working without problem in an existing program, but throws an excecption when I use it in a new program.

It may not be the best code, but it is working in every day use …

Function  DoSQlCommandWithResultSet(const command : String; 
                                    AdoConnection : TADOConnection; 
                                    resultSet : TStringList): Boolean;
  var i : Integer;
      AdoQuery : TADOQuery;
begin
  Result := True;
  resultSet.Clear();

  AdoQuery := TADOQuery.Create(nil);
  try
    AdoQuery.Connection := AdoConnection;
    AdoQuery.SQL.Add(command);
    AdoQuery.Open();

    i := 0;
    while not AdoQuery.eof do
    begin
      resultSet.Add(ADOQuery.Fields[i].Value);
      AdoQuery.Next;
      Inc(i);
    end;

  finally
    AdoQuery.Free();
  end;
end;

Yes, it probably needs a try/catch and the boolean result isn’t used, but it works …

…. in the previous program, but in a new one it thows an exception when called …

procedure TForm1.FormCreate(Sender: TObject);
   var my_stringlist : TStringList;
       i : integer;
begin
   AdoConnection := TADOConnection.Create(nil);

   if ConnectToDefaultDatabase(AdoConnection) = False then
      MessageDlg('Agh !', mtError, [mbOK], 0);

   my_stringlist := TStringList.Create();
   if DoSQlCommandWithResultSet('show databases', AdoConnection, my_stringlist) = False then
      MessageDlg('Urk !', mtError, [mbOK], 0);

   for i := 0 to Pred(my_stringlist.Count) do
      memo1.Lines.Add(my_stringlist.Strings[i]);
end;  // FormCreate()

Now, here’s the interesting part … it throws the exception on Inc(i) and, if I replace that while loop with a for loop …

    for i := 0 to Pred(ADOQuery.Fields.count) do
      resultSet.Add(ADOQuery.Fields[i].Value);

it works just fine.

I suppose that I could just use the for loop & move on, but I would like to understand what is going wrong …. can someone explain to me? Thanks

Answer 1

The first thing that jumps out at me is that

i := 0;
while not AdoQuery.eof do
begin
  resultSet.Add(ADOQuery.Fields[i].Value);
  AdoQuery.Next;
  Inc(i);
end;

and

for i := 0 to Pred(ADOQuery.Fields.count) do
  resultSet.Add(ADOQuery.Fields[i].Value);

are not semantically equivalent! When you call Next, you’re advancing the current record in the dataset. A loop until you hit EOF will run through each record in the dataset once. But the second loop never calls Next and doesn’t check for EOF; it’s grabbing all the fields from one record.

If I had to guess what’s causing the exception in the first loop, I’d say that you’ve got more records (rows) than fields (columns) in your dataset, and so after enough iterations, i ends up at ADOQuery.Fields.Count and you get an index out of bounds error.

What exactly are you trying to do here?