coppy ds_list item to variable in GameMaker

function foo(_bar) constructor { bar = _bar; }

__foo = ds_list_create(); ds_list_add(__foo, new foo(5));

__bar = __foo[| 0]; __bar.bar++;

show_debug_message(string(__foo[| 0].bar)); show_debug_message(string(__bar.bar));

And I have in Output: 6 6 How I can __bar and __foo[| 0] as Difference.