When you get this Error?
Two things to consider here.
1. ViewState
2. Message Authentication Code (MAC)
1. What is ViewState
> When postback happens on webpages, the information of the
> webpage will be stored on the ViewState.
> After postback, the page controls would get the data from the ViewState.
2. What is Message Authentication Code (MAC)
> When doing the page redirection or postback, there is a possibility that
> the data in the ViewState can be tampered.
> Can be called as
HACKED ...!!!!!! ...
> In oreder to protect this tampering, the Viewstate data
> is protected by
Message Authentication Code (MAC).
> This Error mainly occurs When the MAC validation for the Viewstate fails.
> When there is not provided any machine key on the web.config of each IIS application then
> Asp.Net automatically generate cryptographic key for each application and
> stores the key in the
HKCU registry.
> When mismatch happens between the applications on the auto generated key, this error happens.