help for my problem(invalid post back or callback.......)

I have a big
problem that I can't solve it .here is my code for shopping cart,I wrote a class for my shopping cart and
storing list in session.i have a gridview in basket page that I bind that list
to it.in this gridview when I press edit button ,gridview go to edit
mode and I can change the quantity, but after that when I press update I
see the error (  invalid post back or callback argument.........).


in the page load of basket page I bind gridview, if I put
it into if(!page.ispostback) I don't see this error but when i press edit
, after that the gridview be invisible.when I set the


enableEventValidation="false" ,I don't see the error, but when i press update

I see the old quantity in textbox and gridview don't go to
normal mode and still in edit mode.


I went to the post &nbs ...