private void lookUpEntry ( object source,  ServerValidateEventArgs args ) {

   DataView dv;
   dv = myDataSet.Tables [ 0 ].DefaultView;
   string txtEmail;
   args.IsValid = false;    // Assume False
   // Loop through table and compare each record against user’s entry
   foreach ( DataRowView datarow in dv ) {
      // Extract e-mail address from the current row
      txtEmail = datarow [ "email_address" ].ToString ( );
      // Compare e-mail address against user’s entry
      if ( txtEmail == args.Value ) {
         args.IsValid = true;

      }
   }
}

Protected Sub lookUpEntry ( ByVal source As Object, _
      ByVal args As ServerValidateEventArgs )
   Dim dv As DataView
   dv = myDataSet.Tables ( 0 ) .DefaultView
   Dim txtEmail As String
   args.IsValid = False    ' Assume False
   ' Loop through table and compare each record against user’s entry
   For Each datarow As DataRowView In dv
      ' Extract e-mail address from the current row
      txtEmail = datarow.Item ( "email_address" ) .ToString ( )
      ' Compare e-mail address against user’s entry
      If txtEmail = args.Value Then
         args.IsValid = True
         Exit For
      End If
   Next
End Sub