dynamic create TDataset

pele

I try to create dynamic dataset

var DynamicDataset : TDataset;
begin
   DynamicDataset := TDataset.Create(nil);
   DynamicDataset .FieldDefs.Add('ID',ftInteger, 0, True);
   DynamicDataset .Open;                 <-- Abstract Error    
end;

but I get Abstract Error on Open line.
Why?
What is proper way to do it?