Build Your Own ASP.NET 3.5 Website Using C# & VB (75 page)

its Click event. Not this time! The button is located
inside
the DataList—it’s part

of its ItemTemplate—so it’s not directly visible to your code. Also, when the code

executes, the page will contain more instances of this button, so on the server side,

you’ll need a way to know which of them was clicked!

Luckily, ASP.NET provides an ingenious means of handling this scenario.

Whenever a button inside a DataList generates a Click event, the DataList generates

itself an ItemCommand event. The DataList control
is
accessible in your code, so

you can handle its ItemCommand event, whose arguments will give us information

about which control was clicked.

Within the ItemCommand event handler, we can retrieve the data contained in the

LinkButton’s CommandName and CommandArgument properties. We use these properties

to pass the employee ID to the ItemCommand event handler, which can use the ID to

get more data about that particular employee.

Take another look at the button definition from the DataList’s ItemTemplate:

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425

Visual Basic

Dorknozzle\VB\03_EmployeeDirectory.aspx
(excerpt)

Text=<%#"View more details about " & Eval("Name")%>

CommandName="MoreDetailsPlease"

CommandArgument=<%#Eval("EmployeeID")%>
/>

Here, you can see that we’re using CommandArgument to save the ID of the employee

record with which it’s associated. We’re able to read this data from the DataList’s

ItemCommand event handler.

Let’s use Visual Web Developer to generate the ItemCommand event handler. Open

EmployeeDirectory.aspx
in
Design
view, select the DataList, and hit
F4
to open its
Properties
window. There, click the yellow lightning symbol to open the list of

events, and double-click the ItemCommand event in that list. Visual Web Developer

will generate an empty event handler, and take you to the event handler’s code in

the code-behind file.

If you were to open the DataList’s properties again, you’d see the event handler

name appearing next to the event name, as depicted in Figure 10.3.

Figure 10.3. The ItemCommand event in the
Properties
window

If you’re using C# you will also notice that as well as the generation of the empty

event handler in the code-behind file, the onitemcommand property has been added

to the DataList element, as shown here:

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Build Your Own ASP.NET 3.5 Web Site Using C# & VB

C#

onitemcommand="employeesList_ItemCommand"
>

⋮

Modify the code in employeesList_ItemCommand like so:

Visual Basic

Dorknozzle\VB\04_EmployeeDirectory.aspx.vb
(excerpt)

Protected Sub employeesList_ItemCommand(ByVal source As Object,

➥ ByVal e As System.Web.UI.WebControls.DataListCommandEventArgs)

➥ Handles employeesList.ItemCommand

If e.CommandName = "MoreDetailsPlease" Then

Dim li As Literal

li = e.Item.FindControl("extraDetailsLiteral")

li.Text = "Employee ID: " & e.CommandArgument & _

"
"

End If

End Sub

C#

Dorknozzle\CS\04_EmployeeDirectory.aspx.cs
(excerpt)

protected void employeesList_ItemCommand(object source,

DataListCommandEventArgs e)

{

if (e.CommandName == "MoreDetailsPlease")

{

Literal li;

li = (Literal)e.Item.FindControl("extraDetailsLiteral");

li.Text = "Employee ID: " + e.CommandArgument +

"
";

}

}

Our code is almost ready to execute, but we should make one more minor tweak

before we execute this page. At the moment, the Page_Load method will data bind

the DataList every time the page loads, which will put unnecessary load on our

database server. Let’s change this code so that the data binding only takes place

when the page is being loaded for the first time. We’ll also move the data binding

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427

code into its own function, so that we can make use of it later. Modify the code as

shown below, moving the current contents of Page_Load into a new method called

BindList:

Visual Basic

Dorknozzle\VB\05_EmployeeDirectory.aspx.vb
(excerpt)

Protected Sub Page_Load(ByVal sender As Object, _

ByVal e As System.EventArgs) Handles Me.Load

If Not IsPostBack Then

BindList()

End If

End Sub

Protected Sub BindList()

Dim conn As SqlConnection

Dim comm As SqlCommand

Dim reader As SqlDataReader

⋮

End Sub

C#

Dorknozzle\CS\05_EmployeeDirectory.aspx.cs
(excerpt)

protected void Page_Load(object sender, EventArgs e)

{

if (!IsPostBack)

{

BindList();

}

}

protected void BindList()

{

SqlConnection conn;

SqlCommand comm;

SqlDataReader reader;

⋮

}

Execute the project and click the
View more details
links to see the employee ID, as

shown in Figure 10.4
.

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Build Your Own ASP.NET 3.5 Web Site Using C# & VB

Figure 10.4. The Employee Directory showing employee IDs

The code in employeesList_ItemCommand shows how you can work with controls

inside a DataList template, and how to handle their events. We determine which

control was clicked by checking the value of e.CommandName in the event handler;

it will be populated with the value of the CommandName property of the control that

was clicked. Since our LinkButton has the CommandName value MoreDetailsPlease,

we check for this value in the ItemCommand event handler, as shown below:

Visual Basic

Dorknozzle\VB\05_EmployeeDirectory.aspx.vb
(excerpt)

If
e.CommandName = "MoreDetailsPlease"
Then

C#

Dorknozzle\CS\05_EmployeeDirectory.aspx.cs
(excerpt)

if (
e.CommandName == "MoreDetailsPlease"
)

{

Once we know the
View more details
button was clicked, we want to use the extraDetailsLiteral control from our template to display the employee ID. But, given that this control is inside our template, how can we access it through code?

To use a control inside a DataList, we use the FindControl method of the object

e.Item. Here, e.Item refers to the template that contains the control; the FindControl

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Displaying Content Using Data Lists

429

method will return a reference to any control within the template that has the supplied ID. So, in order to obtain a reference to the control with the ID extraDetailsLiteral, we use FindControl like this:
Visual Basic

Dorknozzle\VB\05_EmployeeDirectory.aspx.vb
(excerpt)

Dim li As Literal

li =
e.Item.FindControl("extraDetailsLiteral")

Note that FindControl returns a generic Control object. If you’re using VB, the returned Control is automatically converted to a Literal when you assign it to an object of type Literal. In C#, we need an explicit cast, or conversion, as shown

here:

C#

Dorknozzle\CS\05_EmployeeDirectory.aspx.cs
(excerpt)

Literal li;

li =
(Literal)
e.Item.FindControl("extraDetailsLiteral");

Finally, once we have access to the Literal control in a local variable, setting its

contents is a piece of cake:

Visual Basic

Dorknozzle\VB\05_EmployeeDirectory.aspx.vb
(excerpt)

li.Text = "Employee ID: " & e.CommandArgument & _

"
"

C#

Dorknozzle\CS\05_EmployeeDirectory.aspx.cs
(excerpt)

li.Text = "Employee ID: " + e.CommandArgument +

"
";

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Build Your Own ASP.NET 3.5 Web Site Using C# & VB

Disabling View State

If you take a look at the definition of the extraDetailsLiteral control in
Em-

ployeeDirectory.aspx
, you’ll see that we set its EnableViewState property to

False:

Dorknozzle\VB\03_EmployeeDirectory.aspx
(excerpt)

EnableViewState="false"
/>

When this property is False, its contents aren’t persisted during postback events.

In our case, once the visitor clicks another
View more details
button, all the instances of that Literal control lose their values. This way, at any given moment, no

more than one employee’s ID will be displayed. If you change EnableViewState

to True (the default value), then click the
View more details
button, you’ll see that

all employee IDs remain in the form, as they’re persisted by the view state mechanism.

Editing DataList Items

and Using Templates

Continuing our journey into the world of the DataList, let’s learn a little more about