whata is the use of viewstate.

It's there to persist state across postbacks.
An ASP.NET Web page, its state is the property values of the controls that make up its control hierarchy.
These declarative properties are automatically reassigned on each postback when the control hierarchy is constructed, there's no need to store these property values in the view state.

View state is used by the ASP.NET page framework to automatically save the values of the page and of each control just prior to rendering to the page. When the page is posted, one of the first tasks performed by page processing is to restore view state. State management is the process by which you maintain state and page information over multiple requests for the same or different pages.
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