role of machine key when using view state read full article here http://www.vishalpatwardhan.com/2011/05/state-management-technique-view-state.html

Answer:

ViewState is a state management technique build in ASP.NET.
ViewState basically maintains the state of the pages between postbacks.
ViewState maintain the session within the same page.
ViewState allows the state of objects to be stored in a hidden field on the page.
 

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