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Costruttore DropDownList

Inizializza una nuova istanza della classe DropDownList.

Spazio dei nomi: System.Web.UI.WebControls
Assembly: System.Web (in system.web.dll)

public DropDownList ()
public DropDownList ()
public function DropDownList ()

Utilizzare questo costruttore per creare e inizializzare una nuova istanza della classe DropDownList.

Nell'esempio di codice riportato di seguito viene illustrato come creare e inizializzare una nuova istanza della classe DropDownList.

<%@ Page Language="C#" AutoEventWireup="True" %>
<%@ Import Namespace="System.Data" %>

<html>
   <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>
  
<body>

   <form 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>
  
   </form>

</body>
</html>
 

Windows 98, Windows 2000 SP4, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

.NET Framework non supporta tutte le versioni di ciascuna piattaforma. Per un elenco delle versioni supportate, vedere Requisiti di sistema.

.NET Framework

Supportato in: 2.0 1.1 1.0

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