Delphi - Change Fields definitions of a TClientDataSet that has data

Question

I'm creating FieldDefs at runtime for a TClientDataSet. Still at runtime I want to remove all FieldDefs. I'm saving TClientDataSet physically to a disc file. I tried removing existing FieldDefs using the following code so I could add new ones. But it didn't work:

with fDataSet do begin
    Active := False;
    DisableControls;
    FieldDefs.Clear;
    Fields.Clear;
    EnableControls;
end;

After executing this code, FieldDefs and Fields count are 0, but if I close and reopen the disc file, FieldDefs and Fields are still there.

What is the right way to change FieldDefs and Fields?

Answer 1

Consecutive Open recreates fields from internal Dataset. Just clear old field defs, add new ones and recreated dataset:

...
CDS.FieldDefs.Clear;
CDS.Fields.Add(...);
...
CDS.Fields.Add(...);
CDS.CreateDataSet;
...

Answer 2

You'll have to pump the data from the old data set structure into the new along the following lines:

• Define a new client dataset with the new structure.
• Open your old client data set.
• Iterate the old data set, and insert a record in the new one, performing any calculations and default assignments as you go.

E.g.

while not FOldDataSet.Eof do
begin
  FNewDataSet.Insert;
  FNewDataSet['FIELD1'] := FOldDataSet['FIELD3'];
  FNewDataSet['FIELD2'] := CDefaultFIELD2;
  FNewDataSet.Post;

  FOldDataSet.Next;
end;