Setting EnableViewStateMac=true is a security measure that allows ASP.NET to ensure

that the viewstate for a page has not been tampered with. If on Postback, theASP.NET

framework detects that there has been a change in the value of viewstate that was sent to

the browser, it raises an error – Validation of viewstate MAC failed.

Use <%@ Page EnableViewStateMac=”true”%> to set it to true (the default value, if this

attribute is not specified is also true) in an aspx page.