var
  I: Integer;
  Products: IList;
begin
  Products := MyClass.Products() as IList;

  TreeView.Items.Clear;
  TreeView.Items.BeginUpdate;
  try
    for I := 0 to (Products as ICollection).Count - 1 do
      TreeView.Items.Add(Products.Item[I]); // <- don't know what objects are you passing, so a cast to string might be wrong here
  finally
    TreeView.Items.EndUpdate;
  end;
end;