Programming the Preview Button

In this section, you ll add the preview feature to the preferences page. When the page is running, users can click the Preview button to see the effect of their settings. The preview is shown using the labelSampleCaption control. What you ll need, then, is a handler for the Preview button s Click event. In the handler, you ll examine the values of different controls the check box, the drop-down list, and the option buttons and based on the settings of these controls, you ll change the properties of the labelSampleCaption control. There is no magic, all-in-one way to do this; it s a matter of walking through each control one at a time, getting the control s value, and applying it to the property of the labelSampleCaption control.

To begin, you ll declare two String variables to store information about the current color name and the current font and one Boolean variable to store the true/false setting of the check box for showing captions. To declare these variables, click the Code tab and at the top of the file, insert the following statements:

Dim captionColorName As Strin Dim captionFontName As Strin Dim showCaption As Boolean

To add code for the Preview button, in Design view, double-click the Preview button to create the skeleton handler. Now I ll walk you through the code required to preview each individual setting. All of the code discussed in this section is part of the buttonPreview_Click handler.

Turning Captions On and Off

When the user unchecks the checkShowCaptions check box, you want to hide the sample caption label, which you do by setting the label s Visible property to false. Of course, if the user checks the check box again, you need to set the Visible property back to true. To toggle the label s visibility based on the check box setting, add the following code inside the buttonPreview_Click handler:

If checkShowCaptions.Checked = True The     labelSampleCaption.Visible = Tru Els     labelSampleCaption.Visible = Fals End If 

Previewing the Current Color

Next is to see what color the user chooses and apply it to the sample caption label. To use the color you get from the drop-down list, you need to go through a conversion process. Here s the code, which you should add right after the check box test you created in the previous section.

captionColorName = listColorNames.SelectedItem .Tex labelSampleCaption.ForeColor =      System.Drawing.Color.FromName(captionColor Name)

The current item in the drop-down list is available in its SelectedItem property, and what you want from that selected item is its Text property. The Text property gets you a color name such as Black or Red or whatever.

You can change the sample caption label s color by setting its ForeColor property. However, when you re setting a color in ASP.NET, you can t just use a color name. Colors are stored and manipulated internally in ASP.NET using a special format, namely a Color object. Therefore, you have to convert the color name from a string into a Color object. The FromName method of the Color object takes a color name, just what you happen to have, and converts it into the appropriate color format.

Namespaces

What s up with that System.Drawing business when you use the Color object? System.Drawing is a namespace. A namespace is like a library of programming objects. The Microsoft .NET Framework has hundreds of different namespaces.

Every object you work with belongs to a namespace. For example, the Label and DropDownList controls are in the System.Web.UI.Control namespace. When your page runs, it looks in the System.Web.UI.Control namespace for the definition of a Label control. As you ve probably deduced, the Color object is in the System.Drawing namespace.

You have to include the namespace when working with the Color object because the page doesn t know where to look for the definition. The page is designed to automatically know about some namespaces, such as System.Web.UI.Control and about a dozen others that contain objects commonly used in Web Forms pages. But the page doesn t know about every namespace in the .NET Framework, because that would be inefficient the page might have to look through hundreds of namespaces to look up an object definition.

There are two ways to work with objects that are not in namespaces that the page already knows about. One way is to use a fully qualified name by including the namespace name as part of the object, as you did with the Color object.

An alternative is to import, or reference, the namespace in the page. This reference tells the page to include that namespace when looking up objects. In that case, you don t have to use the fully qualified name, which can make your code a bit easier to type and to read.

To import a namespace, click the All tab of the designer window. You can add <%@ Import %> tags right after the <%@ Page %> tag. Your code might look like this:

<%@ Page Language="VB" Debug="true" %> <%@ Import namespace="System.Drawing" %>