Checking All CheckBoxes in a GridView

A common website user interface pattern is a list of items with a checkbox in each row. The archetypical example is the web-based email client, which allows a series of messages to be selected by checking that email's checkbox. Once one or more emails have been selected, the user can apply some group action on those selected messages, such as deleting them, marking them read, or moving them to a different folder. In addition to having the ability to check each record, one at a time, many such user interfaces also provide a "check all" checkbox in the header row. This checkbox, if checked or unchecked, automatically checks or unchecks all items in the list below. (via Scott Guthrie's blog)