There is no property named 'TriggerEvent' under updatepanel.
- UpdateMode: Gets or sets a value that indicates when an UpdatePanel control's content is updated.
- Triggers: Gets an UpdatePanelTriggerCollection object that contains AsyncPostBackTrigger and PostBackTrigger objects that were registered declaratively for the UpdatePanel control.
- RequiresUpdate: Gets a value that indicates whether the content of the UpdatePanel control will be updated.
- ChildrenAsTriggers: Gets or sets a value that indicates whether postbacks from immediate child controls of an UpdatePanel control update the panel's content.