To get the data you use the name
of the element as:
Request.Form["test[0].myitem"]
Request.Form["test[0].quantity"]
to see all the posted data you use the Request.Form.ToString()
Now, you say that this is not work for you - you have wrong, or some other security of the asp.net did not accept your post back as valid and you get an error.
I just try it on a simple page and works to me.