Why do I get error message ‘NewPageIndex is not a member of ‘System.EventArgs’.’

You probably declared the event handler for OnPageIndexChanged event incorrectly. Make sure that the event handler looks like the following.

<asp:DataGrid id='DataGrid1' runat='server'... OnPageIndexChanged='<EventName>'.../>

VB.NET

protected Sub <Eventname>(ByVal source As Object, ByVal e  As System.Web.UI.WebControls.DataGridPageChangedEventArgs)  
’...
end sub

C#

protected void <EventName>(object sender , System.Web.UI.WebControls.DataGridPageChangedEventArgs)
{
//...
}