Delphi 2006 TClientDataSet index issue

Question

I use a ClientDataSet with a DataSetProvider linked to a local DataSet. When I want to edit the data in the DataSet I open the ClientDataSet and add some indexes to it. After I'm done editing the data I close the ClientDataSet. All works fine, except that when I open the ClientDataSet again and I select an index it's throwing an exception with the message "index 'xxx' not found". What I'm doing wrong?

Here is the code for opening the ClientDataSet:

 Application.CreateForm (TfrmCardDep, frmCardDep);
 try
  with DM.tblCCardDep do
   begin
    IndexDefs.Clear;
    if not Active then Open;
    AddIndex ('iDepID', 'DepID', []);
    AddIndex ('iDep', 'Dep', []);
    IndexName := 'iDep';
    FieldByName('Dep').DisplayLabel := 'Departament';
    FieldByName('Dep').DisplayWidth := 50;
    FieldByName('DepID').Visible := false;
   end;

  frmCardDep.ShowModal;
 finally
  if DM.tblCCardDep.Active then DM.tblCCardDep.Close;
  frmCardDep.Free; frmCardDep := nil;
 end;

DM.tblCCardDep is the ClientDataset

Answer 1

After the first round you have IndexName set on the ClientDataSet. When IndexDefs are discarded the index it refers becomes invalid. Clear IndexName before re-opening the dataset, i.e. modify your code to read:

 [..]
 try
  with DM.tblCCardDep do
   begin
    IndexDefs.Clear;
    IndexName := '';      // <- here
    if not Active then Open;
    [..]

Or use something like this: [..]

 try
  with DM.tblCCardDep do
   begin
    if not Active then Open;
    if IndexDefs.Count = 0 then
     begin
      AddIndex ('iDepID', 'DepID', []);
      AddIndex ('iDep', 'Dep', []);
      IndexDefs.Update;             // Update IndexDefs
      IndexName := 'iDep';
     end;
    FieldByName('Dep').DisplayLabel := 'Departament';
    [..]

Answer 2

Client dataset indexes are always discarded when you close the client dataset. "Persistent index" in the context of the client dataset means that it stays in memory as long as the client dataset is open:

Understanding ClientDataSet Indexes

TClientDataset indexes: temporary or persistent?