As a programmer, you have little control over what happens when the user clicks a hyperlink because the browser takes over. But what if you need to set a hyperlink's target on-the-fly when it's clicked, or if you want to ask the user to confirm that she wants to navigate to a new URL or execute some Visual Basic code when a link is clicked? For that, you use link buttons.
Link buttons look just like hyperlinks, but act like buttons, with both Click and Command events.
When the corresponding hyperlink is clicked, you can take some action in code,
not just let the browser automatically navigate to a new page. For example,
the LinkButton example in the code for this book makes a message appear in
a label with a dashed border when you click the link button, as you see in
Figure 14.14.
Figure 14.14
The LinkButton example.
Like hyperlink controls, link buttons are supported with HTML <a> elements,
but the code for link buttons is processed back at the server. Here's
what the HTML for the link button you see in Figure 14.13 looks like:
<a id="LinkButton1" href="javascript:__doPostBack('LinkButton1','')"
style="Z-INDEX: 101; LEFT: 193px; POSITION: absolute; TOP: 169px">
Click Me!</a>
Link buttons are supported with the System.Web.UI.WebControls.LinkButton class;
here's the hierarchy for this class:
System.Object
System.Web.UI.Control
System.Web.UI.WebControls.WebControl
System.Web.UI.WebControls.LinkButton
You can find the significant public properties of System.Web.UI.WebControls.LinkButton objects
in Table 14.7 and their significant public events in Table 14.8. (This class
has no noninherited methods.) Note that as with other Web server controls,
these tables do 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 14.7 Significant Public Properties of LinkButton Objects
| Property | Means |
|---|---|
CommandArgument
|
Returns or sets an (optional) argument holding text associated with
the command specified with the CommandName property. |
CommandName
|
Returns or sets the command name for this link button. |
Text
|
Returns or sets the text displayed in the link button. |
Table 14.8 Significant Public Events of LinkButton Objects
| Event | Means |
|---|---|
Click
|
Occurs when the link button was clicked. |
Command
|
Occurs when the link button was clicked and you've assigned text
to the CommandName property. |
The whole point of link buttons is that you can handle hyperlink events in
code. In particular, you can add code to the Click and Command event
handlers for link buttons, which you can't do for hyperlink controls.
Like hyperlink controls, you set the text in a link button with the Text property.
Unlike hyperlink controls, however, link buttons don't have a NavigateUrl property—instead,
you use code in the Click and Command events to handle link
button events.
You can see a link button at work in the LinkButton example in Figure
14.14. When the user clicks the hyperlink, a label with the text "Hello
from VB .NET!" and a dashed border becomes visible, as you see
in the figure. To make that label visible, all you need to do is to add code
to the link button's Click event like this:
Private Sub LinkButton1_Click(ByVal _
sender As System.Object, ByVal e As System.EventArgs) _
Handles LinkButton1.Click
Label1.Visible = True
End Sub
Here's another example; when the user clicks a link button in this case,
the code displays a button with the caption "Navigate to Microsoft's
Web Site?". If the user confirms her choice by clicking this button,
the code navigates the browser to http://www.microsoft.com:
Private Sub Page_Load(ByVal sender As _
System.Object, ByVal e As System.EventArgs) _
Handles MyBase.Load
Button1.Visible = False
End Sub
Private Sub LinkButton1_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles LinkButton1.Click
Button1.Text = "Navigate to Microsoft's Web Site?"
Button1.Visible = True
End Sub
Private Sub Button1_Click(ByVal _
sender As System.Object, ByVal e As System.EventArgs) _
Handles Button1.Click
Response.Redirect(" target="new">http://www.microsoft.com")
End Sub
Here's one more example, which sets the target of a link button on-the-fly,
depending on the current time of day (which you can find with the Month, Hour, Minute,
and Day functions if you pass them the current time, contained in
the Visual Basic Now property). When the user clicks this link button,
she will see a breakfast, lunch, or dinner menu, as appropriate (the Hour function
returns the current hour of the day in 0–23 format):
Private Sub LinkButton1_Click(ByVal _
sender As System.Object, ByVal e As System.EventArgs) _
Handles LinkButton1.Click
If Hour(Now) > 0 And Hour(Now) <= 11 Then
Response.Redirect("BreakfastMenu.html")
ElseIf Hour(Now) > 11 And Hour(Now) <= 17 Then
Response.Redirect("LunchMenu.html")
Else
Response.Redirect("DinnerMenu.html")
End If
End Sub
That's the whole idea behind link buttons—when the user clicks them, you can execute code, not just automatically jump to a new URL. And that's it for our look at Web server hyperlink and link button controls.
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