I observed something last week that I did not expect, and will describe below. I am curious as to why this happens. Is it something internal to the TDataSet class, an artifact of the TDBGrid, or something else?
The order of the fields in an open ClientDataSet changed. Specifically, I created a ClientDataSet in code by calling CreateDatatSet after defining its structure using FieldDefs. The first field in this ClientDataSet's structure was a Date field named StartOfWeek. Only moments later, code that I had also written, which assumed that the StartOfWeek field was in the zeroeth position, ClientDataSet.Fields[0], failed, since the StartOfWeek field was no longer the first field in the ClientDataSet.
After some investigation, I learned that it was possible that every single field in the ClientDataSet might, at a given moment, appear in some position different from the original structure at the time that the ClientDataSet was created. I was unaware that this could happen, and a search on Google didn't turn up any mention of this effect either.
What happened wasn't magic. The fields didn't change position by themselves, nor did they change based on anything I did in my code. What caused the fields to physically appear to change position in the ClientDataSet was that the user had changed the order of the Columns in a DbGrid to which the ClientDataSet was attached (through a DataSource component, of course). I replicated this effect in Delphi 7, Delphi 2007, and Delphi 2010.
I created a very simple Delphi application that demonstrates this effect. It consists of a single form with one DBGrid, a DataSource, two ClientDataSets, and two Buttons. The OnCreate event handler of this form looks like the following
procedure TForm1.FormCreate(Sender: TObject);
begin
with ClientDataSet1.FieldDefs do
begin
Clear;
Add('StartOfWeek', ftDate);
Add('Label', ftString, 30);
Add('Count', ftInteger);
Add('Active', f