I need an alert message when closing the browser window (when clicked on X button of browser)

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C#

VB

ASP.NET4

Dear,
All...

I need an alert message when I close the browser close(X) button. I googling on this issue, but answers are use onbeforeunload() method in body tag of the form. But when am using master page contained forms there is no body tag to call this method. I this case what i should do? please let me know, if anybody knows the solution. without using body tag in the form. when am using this onbeforeunload() in body tag, but when i refresh the page this alert firing. this alert message not fired when refresh the page.

Thanks in advance...