ViewState is a built-in structure for automatically 
retaining values among multiple requests for the same page. 
The view state is internally maintained as a hidden field 
on the page but is hashed, providing greater security than 
developer-implemented hidden fields do.
Following are the benefits of using ViewState:- 
	No server resources are required because state is 
contained in a structure in the page code.
	Simplicity.
	States are retained automatically.
	The values in view state are hashed, compressed, 
and encoded, thus representing a higher state of security 
than hidden fields.
	ViewState is good for caching data in Web farm 
configurations because the data is cached on the client. 
Following are limitation of using ViewState:- 
	Page loading and posting performance decreases when 
large values are stored because 
	View state is stored in the page.
	Although view state stores data in a hashed format, 
it can still be tampered with because it is stored in a 
hidden field on the page. The information in the hidden 
field can also be seen if the page output source is viewed 
directly, creating a potential security risk. 
	If page get destroyed or refresh information which 
is stored in view state gets vanished.

Below is sample of storing values in view state. 
this.ViewState["EnterTime"] = DateTime.Now.ToString();