Controls You Can Use on Web Forms   ASP.NET Standard Controls   ListBox Control

The information in this topic applies to all list Web server controls: ListBox, DropDownList, CheckBoxList, and RadioButtonList.

Normally, users select items in a list Web server control to indicate their choice. However, you might need to preselect items at design time, or dynamically select items at run time based on some condition.

• Include the Selected attribute in the opening tag of the list item that is to be initially selected.

<asp:ListBox id="myListBox" runat="server">
   <asp:ListItem text="Item 1" />
   <asp:ListItem text="Item 2" />
   <asp:ListItem text="Item 3" selected />
   <asp:ListItem text="Item 4" />
   <asp:ListItem text="Item 5" />
</asp:ListBox>

• Set the control’s SelectedIndex property to the index value of the item to select. The index is zero-based. To reset ( no selection ), set SelectedIndex to -1.

myList.SelectedIndex = 2;    // selects the third item

myList.SelectedIndex = 2     ' selects the third item

• Loop through the control’s Items collection and set the Selected property of every individual item.

NOTE: You can only select multiple items in ListBox controls if the control’s SelectionMode property is set to Multiple.

<asp:ListBox id="myList" multiple runat="server">

The following example shows how you can set the selections in a multi-selection ListBox control. In this case, the code selects even-numbered items.

void setSelected ( object src, EventArgs e ) {
   int i = 0;
   foreach ( ListItem item in myList.Items ) {
      if ( i % 2 == 0 ) {
         item.Selected = true;
      }
      i++;
   }
}

Sub setSelected ( ByVal src As Object, ByVal e As EventArgs )
   Dim i As Integer=0
   For Each item As ListItem In myList.Items
      If ( i Mod 2=0 ) Then
         item.Selected = True
      End If
      i=i + 1
   Next
End Sub

ListBox4.aspx Run Sample | View Source

Setting Web Forms Server Control Properties   Setting Web Server Control Properties Programmatically