What is the use of Viewstatemac.. What will happen if I set the value to True

ViewStateMAC property is used for security purpose. It is used to maintain data integrity of viewstate across postbacks.

Basically it is used to check if user has changed (tempered) the viewState and making request.

If we will set EnableViewStateMac to True then, the encoded and encrypted view state is checked to verify that it has not been tampered with on the client.

You can find more information at MSDN
ViewStateMac Property


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Vikas Vaidya

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Hello,
A view state MAC is an encoded version of the hidden variable that a page's view state is persisted to when sent to the browser.
When you set the EnableViewStateMac attribute to true, the encoded and encrypted view state is checked to verify that it has not been tampered with on the client.
It is recommended to set the EnableViewStateMac attribute to true using the @ Page directive in the .aspx file.
I hope this information is enough for you.