With AutoEventWireup which is turned on by default on a page you can just add methods prepended with Page_event and have ASP.NET connect to the events for you.
In the case of Unload the method signature is:
protected void Page_Unload(object sender, EventArgs e)
So you need to write:
protected void Page_Load(object sender, EventArgs e)
{
if(!IsPostBack)
{
}
}
protected void Page_UnLoad(object sender, EventArgs e)
{
}