Hidden field is client side state management technique in asp.net. As you know asp.net pages are state less that means whenever round trip happens all the page data get lost. To retain the page data during postback we use state management and Hidden Field is one of client side technique.
A HiddenField control stores a single variable in its Value property and must be explicitly added to the page.
In order for hidden-field values to be available during page processing, you must submit the page using an HTTP POST command. If you use hidden fields and a page is processed in response to a link or an HTTP GET command, the hidden fields will not be available.
: Do not store sensitive information in hidden field.
For more information :
please refer : http://wiki.asp.net/page.aspx/298/hiddenfield/
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