Browser close event in code behind?

 Hi All,
 Whenever the browser is closed using the "X" button, I need to call a function.
 I used

$(window).bind("beforeunload", function(e) {
MY function here........
} );
But, this beforeunload event is called everytime when the request is given to server.
How can the browser close event be detected using C# code?

 
 
 
...