Here Mudassar Ahmed Khan has explained with an example, how to build Country State City Cascading DropDownList in ASP.Net i.e. Country State City Cascading DropDownList populated from Database and dependent on each other.
First the Country DropDownList will be populated on Page Load event. Then when a Country is selected, the SelectedIndexChanged event will fire and the State DropDownList will be populated based on the selected Country.
In similar way, when the State is selected, the SelectedIndexChanged event will fire and the City DropDownList will be populated based on the selected State.