I recently answered a post where the user was using ASP .Net 3.5 but not binding data to a DropDownList but instead using hard coded data to load the drop down in the HTML. So I created this quick example to show Linq-To-Xml (Linq) loading of an Asp.Net DropDownList and handling the automatic postback selection changes.
As mentioned example shows how to bind data to a dropdown list using Linq, specifically Linq-To-Xml, but you could use any other of the Linq methodologies because we are using the anonymous types to work with the needs of the DropDownList.
First we will place the Asp.Net DropDownListon our page along with a label which will used to show state changes.
<asp:DropDownList ID="ddlMain"
runat="server"
AutoPostBack="true"
OnSelectedIndexChanged="SelectionMade" />
<asp:Label ID="lblWhat" runat="server"/>
Nothing earth shattering here, we have AutoPostBack because we want the control to tell our code behind on the server when a selection has been made. Following that we have the method which will be called for that event of SelectionMade.
The next thing is to load the DropDownList with values to choose from. We will use an XElement loaded dynamically to show how to do do this in Linq. The other thing of note is that we only want load this once, so we check the postback status. If its a postback, we do not reload.
protected void Page_Load( object sender, EventArgs e )
{
if ( Page.IsPostBack == false ) // Only do this once
{
// Simulate an Linq-To-SQL call with an Linq-To-SQL
// So anyone can use this demo.
XElement dataForList
= new XElement(
"DropDownData",
new XElement( "Node",
new XElement( "Text", "Selection 1" ),
new XElement( "Data", "1Selection" ) ),
new XElement( "Node",
new XElement( "Text", "Selection 2" ),
new XElement( "Data", "2Selection" ) ));
var toTheScreen = from x in dataForList.Descendants( "Node" )
select new
{
Text = x.Descendants( "Text" ).First().Value,
Value = x.Descendants( "Data" ).First().Value
};
ddlMain.DataTextField = "Text";
ddlMain.DataValueField = "Value";
ddlMain.DataSource = toTheScreen;
ddlMain.DataBind();
}
}
As shown above we work through all the elements named Node. For each of those nodes we will create a new entity with two properties Text and Value. Text will be shown to the user and value is what is associated with the selection. In the binding operations we specify the Text and Value field for the DropDownList to use and simply bind it to our anonymous projection.
Now all we have to do is handle the selection change event
protected void SelectionMade( object sender, EventArgs e )
{
DropDownList target = sender as DropDownList;
if ( target != null )
lblWhat.Text =
string.Format(
"Item {0} selected with Value of {1}",
target.SelectedItem.Text,
target.SelectedValue );
}
Nothing magical here, we get the DropDownList from the sender argument, divine the selection and update the label. Here is what it should look like after a change:
