Question

In Delphi 2009, I had no major problems with Generics so far (using Generics.Collections lists, with no special Generics features).

Now I found this code will cause a ‘Invalid pointer operation’ in the line which accesses MyList.Count. (MyList.Contains and MyList.IndexOf cause the same error).

The error disappears if I declare TMyList = class(TList<TMyEntry>);

Should I avoid TObjectList<T> or is something else in my code causing this error?

type
  TMyEntry = class(TStringlist);
  TMyList = class(TObjectList<TMyEntry>);

procedure TListTests.TestAV;
var
  Entry: TMyEntry;
  MyList: TMyList;
begin
  MyList := TMyList.Create;
  try
    Entry := TMyEntry.Create;
    try
      MyList.Add(Entry);
      Assert(MyList.Count = 1);  // <--- fails
    finally
      Entry.Free;
    end;
  finally
    MyList.Free;
  end;
end;

Answer 1

TObjectList named Object List because it owns objects wich it stores. so you have not to free objects wich list contains yourself. TObjectList does it for you.
I’ve tested you example code in Delphi2010. And I get AV in MyList.free() line. the reason is that you do Entry.Free. List doesn’t know about this, and tries to free object again.
TObjectLists constructor has boolean parametrer ownsObjects (default = true), you can use it and free objects, wich it contains manually.