DropDownList コンストラクタとは何？わかりやすく解説 Weblio辞書

.NET Framework クラスライブラリリファレンス

索引トップ用語の索引ランキング

DropDownList コンストラクタ

DropDownList クラスの新しいインスタンスを初期化します。

名前空間: System.Web.UI.WebControls
アセンブリ: System.Web (system.web.dll 内)
構文

解説

このコンストラクタを使用して、DropDownList クラスの新しいインスタンスを作成し、初期化します。

使用例

DropDownList クラスの新しいインスタンスを作成および初期化する方法を次のコード例に示します。

Visual Basic

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

<html>
   <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>
  
<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>

<%@ 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>