Here Mudassar Ahmed Khan has explained with an example, how to implement check uncheck all (select unselect all) CheckBoxes functionality in ASP.Net DataList control using JavaScript and jQuery.
When the header Checkbox is checked (selected), all the CheckBoxes in DataList will be checked (selected) and vice versa. When any CheckBox in DataList is unchecked (unselected), the header Checkbox is also unchecked (unselected).