Web pages can contain hyperlinks, and in Visual Basic .NET Web applications,
you use the HyperLink control to create hyperlinks. Such hyperlinks
can link to another page in your Web application or anywhere on the Web. You
can specify the location of the linked page in an absolute way, where you use
the linked page's complete URL, or in a relative way, with respect to
the current page.
You can see both types of links in Figure
14.11, in the HyperLink example in the code for this book—this example
supports hyperlinks both to Microsoft's Web site (using an absolute
URL: "http://www.microsoft.com")
and to an HTML page in the same project (using a relative URL: "HTMLPage1.htm").
In that figure, the link to the other page in the same project (an HTML page,
HTMLPage1.htm, that's been added to the project) has been clicked, bringing
up that page.
Figure 14.11
The HyperLink example.
The HTML used to support hyperlinks is, of course, the <a> element;
here's the HTML generated for the two hyperlinks you see in Figure
14.11:
<a id="HyperLink1" href="HTMLPage1.htm" target="_blank"
style="Z-INDEX: 102; LEFT: 174px; POSITION: absolute; TOP: 103px">
The Other Page</a>
<a id="HyperLink2" href="http://www.microsoft.com"
style="Z-INDEX: 103; LEFT: 158px; POSITION: absolute; TOP: 157px">
Microsoft's Web Site</a>
You use the System.Web.UI.WebControls.HyperLink class to create hyperlinks
in Web applications. Here is the hierarchy of that class:
System.Object
System.Web.UI.Control
System.Web.UI.WebControls.WebControl
System.Web.UI.WebControls.HyperLink
You can find the significant public properties of the System.Web.UI.WebControls.HyperLink objects
in Table 14.6. (This class has no non-inherited methods or events.) Note that
as with other Web server controls, this table does not list the significant
properties, methods, and events this class inherits from the Control and WebControl classes—you
can find them in Tables 12.1 to 12.5.
Table 14.6 Significant Public Properties of HyperLink Objects
| Property | Means |
|---|---|
ImageUrl
|
Returns or sets the URL of an image for use in the hyperlink. |
NavigateUrl
|
Returns or sets the URL to navigate to when the hyperlink is clicked. |
Target
|
Returns or sets the target window or frame to display the linked-to content in when the hyperlink is clicked. |
Text
|
Returns or sets the clickable text displayed in the Web page for the hyperlink. |
You create hyperlink controls by dragging them from the toolbox at design time, and, as with any other control, you can also create them in code.
When you create a hyperlink, you need to set the text it displays, as well
as the URL it links to. The text in the hyperlink control is specified with
the Text property. You can also display an image, which you specify
with the ImageUrl property. You set the URL that the link navigates
to with the NavigateUrl property.
Tip - If both the Text and ImageUrl properties
are set, the ImageUrl property is used. If the image to display
is not available, the text in the Text property is displayed
instead. |
You can set the Text property at design time simply by entering
the hyperlink's text into the Text property in the Properties window.
When you click the NavigateUrl property's ellipsis (...)
button in the Properties window, the Select URL dialog opens, as you see in
Figure 14.12. You can select the type of URL you want to specify with the URL
Type drop-down list box; the various types are Absolute, Document Relative,
or Root Relative. In Figure 14.12, we're setting a hyperlink to a relative URL to the HTML page
added to the project, HTMLPage1.htm. To connect to an absolute URL on the Internet,
set the URL Type to absolute and type the URL into the URL text box.
Figure 14.12
The Select URL dialog.
There's more to this story—you can also specify how you want to
display the new page when its URL is clicked, using the Target property.
The target specifies where the linked-to content will be displayed. By default,
when you click a hyperlink control, the linked-to content replaces the current
page in the browser, and no new browser window is opened. However, you can
open a new window if you prefer by setting the Target property to
the name of a window or frame or one of these HTML values:
-
_blank—Will display the linked-to content in a new window, without frames. -
_parent—Will display the linked-to content in the frameset parent of the current window. -
_self—Will display the linked-to content in the frame with focus. -
_top—Will display the linked-to content in the top window without frames.
You can also create hyperlinks in code, of course, as you see in the CreateHyperLink example in the code for the book. You can see this example at work in Figure 14.13; when you click the Create HyperLink button, a new HyperLink control is created and added to a panel control.
Figure 14.13
The CreateHyperLink example.
You can create a new hyperlink control as we've created other Web server
controls; here, we'll use the Dim statement to create the control
in code, customize its properties, and add it to a panel using the panel's Controls collection's Add method:
Dim HyperLink1 As New HyperLink
Private Sub Button1_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Button1.Click
HyperLink1.Text = "Microsoft's Web Site"
HyperLink1.NavigateUrl = "http://www.microsoft.com"
Panel1.Controls.Add(HyperLink1)
End Sub
To make sure that the new control reappears each time the page is reloaded,
you can add the control to the panel in the Page_Load event handler.
First, we create a new Boolean variable, blnHyperLinkVisible, and
set it to True after the button has been clicked:
Dim HyperLink1 As New HyperLink
Dim blnHyperLinkVisible As Boolean = False
Private Sub Button1_Click(ByVal
sender As System.Object, _
ByVal e As System.EventArgs) Handles Button1.Click
HyperLink1.Text = "Microsoft's
Web Site"
HyperLink1.NavigateUrl = "http://www.microsoft.com"
Panel1.Controls.Add(HyperLink1)
blnHyperLinkVisible = True
End Sub
In the Page_Load event handler, all you have to do is to check blnHyperLinkVisible and
display the hyperlink if it's True:
Private Sub Page_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
blnHyperLinkVisible = Me.ViewState("blnHyperLinkVisible")
If blnHyperLinkVisible Then
HyperLink1.Text = "Microsoft's Web Site"
HyperLink1.NavigateUrl = "http://www.microsoft.com"
Panel1.Controls.Add(HyperLink1)
End If
End Sub
Note that hyperlinks like the ones we've been working with are really
just HTML <a> controls made into Web server controls—besides
customizing their text, targets, and URLs, you can't do much with them
in code. They don't even have a Click event. But link buttons
do.
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