DropDownList Constructor
Definition
Important
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Initializes a new instance of the DropDownList class.
public:
DropDownList();public DropDownList ();Public Sub New ()Examples
The following code example demonstrates how to create and initialize a new instance of the DropDownList class.
<%@ Page Language="C#" AutoEventWireup="True" %>
<%@ Import Namespace="System.Data" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<script runat="server" >
void Selection_Change(Object sender, EventArgs e)
{
// Retrieve the DropDownList control from the Controls
// collection of the PlaceHolder control.
DropDownList DropList =
(DropDownList)Place.FindControl("ColorList");
// Set the background color for days in the Calendar control
// based on the value selected by the user from the
// DropDownList control.
Calendar1.DayStyle.BackColor =
System.Drawing.Color.FromName(DropList.SelectedItem.Value);
}
void Page_Load(Object sender, EventArgs e)
{
// Create a DropDownList control.
DropDownList DropList = new DropDownList();
// Set the properties for the DropDownList control.
DropList.ID = "ColorList";
DropList.AutoPostBack = true;
// Manually register the event-handling method for the
// SelectedIndexChanged event.
DropList.SelectedIndexChanged +=
new EventHandler(this.Selection_Change);
// Because the DropDownList control is created dynamically each
// time the page is loaded, the data must be bound to the
// control each time the page is refreshed.
// Specify the data source and field names for the Text and
// Value properties of the items (ListItem objects) in the
// DropDownList control.
DropList.DataSource = CreateDataSource();
DropList.DataTextField = "ColorTextField";
DropList.DataValueField = "ColorValueField";
// Bind the data to the control.
DropList.DataBind();
// Set the default selected item when the page is first loaded.
if(!IsPostBack)
{
DropList.SelectedIndex = 0;
}
// Add the DropDownList control to the Controls collection of
// the PlaceHolder control.
Place.Controls.Add(DropList);
}
ICollection CreateDataSource()
{
// Create a table to store data for the DropDownList control.
DataTable dt = new DataTable();
// Define the columns of the table.
dt.Columns.Add(new DataColumn("ColorTextField", typeof(String)));
dt.Columns.Add(new DataColumn("ColorValueField", typeof(String)));
// Populate the table with sample values.
dt.Rows.Add(CreateRow("White", "White", dt));
dt.Rows.Add(CreateRow("Silver", "Silver", dt));
dt.Rows.Add(CreateRow("Dark Gray", "DarkGray", dt));
dt.Rows.Add(CreateRow("Khaki", "Khaki", dt));
dt.Rows.Add(CreateRow("Dark Khaki", "DarkKhaki", dt));
// Create a DataView from the DataTable to act as the data source
// for the DropDownList control.
DataView dv = new DataView(dt);
return dv;
}
DataRow CreateRow(String Text, String Value, DataTable dt)
{
// Create a DataRow using the DataTable defined in the
// CreateDataSource method.
DataRow dr = dt.NewRow();
// This DataRow contains the ColorTextField and ColorValueField
// fields, as defined in the CreateDataSource method. Set the
// fields with the appropriate value. Remember that column 0
// is defined as ColorTextField, and column 1 is defined as
// ColorValueField.
dr[0] = Text;
dr[1] = Value;
return dr;
}
</script>
<head runat="server">
<title> DropDownList Constructor Example </title>
</head>
<body>
<form id="form1" runat="server">
<h3> DropDownList Constructor Example </h3>
Select a background color for days in the calendar.
<br /><br />
<asp:Calendar id="Calendar1"
ShowGridLines="True"
ShowTitle="True"
runat="server"/>
<br /><br />
<table cellpadding="5">
<tr>
<td>
Background color:
</td>
</tr>
<tr>
<td>
<asp:PlaceHolder id="Place"
runat="server"/>
</td>
</tr>
</table>
</form>
</body>
</html>
<%@ Page Language="VB" AutoEventWireup="True" %>
<%@ Import Namespace="System.Data" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<script runat="server" >
Sub Selection_Change(sender as Object, e As EventArgs)
' Retrieve the DropDownList control from the Controls
' collection of the PlaceHolder control.
Dim DropList As DropDownList = _
CType(Place.FindControl("ColorList"), DropDownList)
' Set the background color for days in the Calendar control
' based on the value selected by the user from the
' DropDownList control.
Calendar1.DayStyle.BackColor = _
System.Drawing.Color.FromName(DropList.SelectedItem.Value)
End Sub
Sub Page_Load(sender as Object, e As EventArgs)
' Create a DropDownList control.
Dim DropList As DropDownList = New DropDownList()
' Set the properties for the DropDownList control.
DropList.ID = "ColorList"
DropList.AutoPostBack = True
' Manually register the event-handling method for the
' SelectedIndexChanged event.
AddHandler DropList.SelectedIndexChanged, AddressOf Selection_Change
' Because the DropDownList control is created dynamically each
' time the page is loaded, the data must be bound to the
' control each time the page is refreshed.
' Specify the data source and field names for the Text and
' Value properties of the items (ListItem objects) in the
' DropDownList control.
DropList.DataSource = CreateDataSource()
DropList.DataTextField = "ColorTextField"
DropList.DataValueField = "ColorValueField"
' Bind the data to the control.
DropList.DataBind()
' Set the default selected item when the page is first loaded.
If Not IsPostBack Then
DropList.SelectedIndex = 0
End If
' Add the DropDownList control to the Controls collection of
' the PlaceHolder control.
Place.Controls.Add(DropList)
End Sub
Function CreateDataSource() As ICollection
' Create a table to store data for the DropDownList control.
Dim dt As DataTable = New DataTable()
' Define the columns of the table.
dt.Columns.Add(new DataColumn("ColorTextField", GetType(String)))
dt.Columns.Add(new DataColumn("ColorValueField", GetType(String)))
' Populate the table with sample values.
dt.Rows.Add(CreateRow("White", "White", dt))
dt.Rows.Add(CreateRow("Silver", "Silver", dt))
dt.Rows.Add(CreateRow("Dark Gray", "DarkGray", dt))
dt.Rows.Add(CreateRow("Khaki", "Khaki", dt))
dt.Rows.Add(CreateRow("Dark Khaki", "DarkKhaki", dt))
' Create a DataView from the DataTable to act as the data source
' for the DropDownList control.
Dim dv As DataView = New DataView(dt)
Return dv
End Function
Function CreateRow(Text As String, Value As String, dt As DataTable) As DataRow
' Create a DataRow using the DataTable defined in the
' CreateDataSource method.
Dim dr As DataRow = dt.NewRow()
' This DataRow contains the ColorTextField and ColorValueField
' fields, as defined in the CreateDataSource method. Set the
' fields with the appropriate value. Remember that column 0
' is defined as ColorTextField, and column 1 is defined as
' ColorValueField.
dr(0) = Text
dr(1) = Value
Return dr
End Function
</script>
<head runat="server">
<title> DropDownList Constructor Example </title>
</head>
<body>
<form id="form1" runat="server">
<h3> DropDownList Constructor Example </h3>
Select a background color for days in the calendar.
<br /><br />
<asp:Calendar id="Calendar1"
ShowGridLines="True"
ShowTitle="True"
runat="server"/>
<br /><br />
<table cellpadding="5">
<tr>
<td>
Background color:
</td>
</tr>
<tr>
<td>
<asp:PlaceHolder id="Place"
runat="server"/>
</td>
</tr>
</table>
</form>
</body>
</html>
Remarks
Use this constructor to create and initialize a new instance of the DropDownList class.
