Beginning ASP.NET 4.5 in CSharp and VB Opsylum

242 x CHAPTER 6 CREATING CONSISTENT LOOKING WEBSITES
6. Save all changes and then request Default.aspx in your browser again. The drop-down list in the
sidebar should display the first item in the list as selected. Select the other option from the list and
the page will reload. The item you chose last in the drop-down list should now be preselected
in the drop-down list. Close your browser and then browse to Default.aspx again. The theme
you chose should still be selected in the drop-down list. Notice that you keep seeing the same
theme because you haven’t written any code yet that applies the selected theme. You see how to do
that in a later exercise.
COMMON MISTAKES If you get an error, make sure you have no typos in the
code. If nothing seems to happen (for example, the page doesn’t post back),
check if you set the AutoPostBack attribute on the DropDownList control to
True. Also, check the spelling of the name of the cookie (PreferredTheme) in
both code blocks. Finally, if the correct item is not preselected after a postback
or after you close and reopen your browser, check you browser’s or computer’s
security settings. You may have confi gured your browser to delete cookies
when you close the browser, or you may have security software running on your
machine that blocks cookies altogether. If you can’t make it work in one browser,
try it in another to rule out problems with the code.
How It Works
You made three important changes to the master page. First, you added the drop-down list and set
AutoPostBack to True. This causes the page to submit itself back to the server as soon as you choose
a new item in the list. When that happens, the code in the SelectedIndexChanged handler fires. This
code creates a cookie that can be stored on the user’s computer. To make the cookie last between
browser sessions, you need to set the Expires property. In the code example, the cookie is set to expire
three months from now, which means the browser will discard it automatically after that period. Every
time the user chooses a new theme, this date is extended, setting it for another three months:
VB.NET
Dim preferredTheme As HttpCookie = New HttpCookie("PreferredTheme")
preferredTheme.Expires = DateTime.Now.AddMonths(3)
C#
HttpCookie preferredTheme = new HttpCookie("PreferredTheme");
preferredTheme.Expires = DateTime.Now.AddMonths(3);
After the cookie has been created, you can set its Value property. In the example, the SelectedValue
of the DropDownList (containing the name of the theme) is stored in the cookie. The cookie is then
added to the Cookies collection using Response.Cookies.Add:
VB.NET
preferredTheme.Value = ThemeList.SelectedValue
Response.Cookies.Add(preferredTheme)