function ChangeCheckBoxState(id, checkState) {
    var cb = document.getElementById(id);
    if (cb != null)
        cb.checked = checkState;
}

function ChangeAllCheckBoxStates(checkState) {
    // Toggles through all of the checkboxes defined in the CheckBoxIDs array
    // and updates their value to the checkState input parameter
    if (CheckBoxIDs != null) {
        for (var i = 0; i < CheckBoxIDs.length; i++)
            ChangeCheckBoxState(CheckBoxIDs[i], checkState);
    }
}

function ChangeHeaderAsNeeded() {
    // Whenever a checkbox in the GridView is toggled, we need to
    // check the Header checkbox if ALL of the GridView checkboxes are
    // checked, and uncheck it otherwise
    if (CheckBoxIDs != null) {
        // check to see if all other checkboxes are checked
        for (var i = 1; i < CheckBoxIDs.length; i++) {
            var cb = document.getElementById(CheckBoxIDs[i]);
            if (!cb.checked) {
                // Whoops, there is an unchecked checkbox, make sure
                // that the header checkbox is unchecked
                ChangeCheckBoxState(CheckBoxIDs[0], false);
                return;
            }
        }

        // If we reach here, ALL GridView checkboxes are checked
        ChangeCheckBoxState(CheckBoxIDs[0], true);
    }
}