The Login.Authenticate event is raised when a user uses the System.Web.UI.WebControls.Login control to log in to a Web site. Custom authentication schemes can use the Login.Authenticate event to authenticate users.
When a user uses the System.Web.UI.WebControls.Login control to log in to a Web site, all data in the view state and all post data is lost. Do not perform actions in the Login.Authenticate event that rely on the view state.
For more information about handling events, see Handling and Raising Events.