The <asp:listbox> Control

The next HTML server control that we'll look at, <asp:listbox>, is very much related to <asp:dropdownlist>. In fact, we've almost mentioned it already. Remember the <select> HTML form control that creates drop-down listboxes? Well, the <asp:listbox> is a server-side equivalent of using that <select> tag with the size attribute set to the maximum number of options possible. In fact the only difference with this control is the fact that it doesn't drop down, and that it is capable of multiple selections.

The <asp:listbox> has the following format:

There is one point of interest, however: the selectionmode attribute is used to determine whether you can select multiple or only single items from the listbox. By default, it is set to single, but you have the option to change it.

Let's take a look at a quick example, where we alter our previous example to use a listbox instead of a drop-down list control. We'll also alter it to allow multiple selections.

1.       Enter the following code into your editor:

2.       Save this as listpage2.aspx.

3.       Run this page in your browser, and use the Ctrl, or shift keys to select multiple choices:

4.       Click on Submit Query to see the following:

How It Works

The controls in this example have hardly changed from the previous listpage.aspx example. All we've done is add a selectionmode attribute to allow us to make multiple selections:

However, we've had to completely overhaul our ASP.NET code:

In fact, we've introduced a whole new format to determine the options to display.

We use an If construct (we'll talk more about these in Chapter 6) to determine which items have been selected, and then use a variable assignment to create the display output. The first line in our code creates a String variable (again, we'll be looking at variables in depth very shortly). This simply declares a label 'msg' which refers to a portion in the computer's memory that we can use to hold a sequence of characters, or string. We set it up with a simple heading, and then add city names on the end, according to which list elements have been selected. Finally, we assign its final value (a long string of HTML) to the Text attribute of the Message label, so that it can be seen on the page.

That was a bit more complicated, wasn't it? Don't worry, it will become clearer once you've familiarized yourself with the topics in the following few chapters. For now, let's move on to some different types
of control.

Part 1 : HTML Server Controls Part 2 : Simple Web Theory Part 3 : HTML Forms Part 4 : How the <form> Tag Works in ASP.NET Part 5 : Control Examples Part 6 : Control Attributes Part 7 : The asp:dropdownlist Control Part 8 : The ASP.NET Code Part 9 : The asp:listbox Control Part 10 : The asp:dropdownlist Control Part 11 : The asp:radiobuttonlist Control Part 12 : The asp:checkbox and asp:checkboxlist Control Part 13 : Summary Buy Beginning ASP.NET with C# here © Copyright 2002 Wrox Press This chapter is written by David Sussman, et al and taken from "Beginning ASP.NET with C#" published by Wrox Press Limited in June 2002; ISBN 1861007345; copyright © Wrox Press Limited 2002; all rights reserved. No part of these chapters may be reproduced, stored in a retrieval system or transmitted in any form or by any means -- electronic, electrostatic, mechanical, photocopying, recording or otherwise -- without the prior written permission of the publisher, except in the case of brief quotations embodied in critical articles or reviews.