how to get a selected column's data in string from Gridviw

I want to get that ID data to a string variable in a button click present inside the Gridview itself 

it is a forum, if the user clicks on button present in the side of every particular question that has been asked previously, he can view the replies given by other users

so for every question posted there is a unique id will be generated behind and will be posted in gridview..please see the image attached

if the View Button is clicked the corresponding question's replies should be shown on another Gridview in another page with question on the top and replies on the below as like the picture..so for tracking the question i need to get the question ID [column is hidden] in string in the VIEW BUTTON click event

i think you understand my requirement better now