How to: Determine the Selection in List Web Server Controls

Artikkel
10/22/2014

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

One of the most common tasks in working with a list Web server control is to determine what item or items users have selected. The procedure varies depending on whether the list control allows single or multiple selections.

Use the following procedure when working with the DropDownList control, the RadioButtonList control, and a single-selection ListBox control.

To determine the selection in a single-selection list control

Use one of the following methods:

To get the index value of the selected item, read the value of the SelectedIndex property. The index is zero-based. If nothing has been selected, the value of the property is -1.

To get the contents of the selected item, get the control's SelectedItem property. This property returns an object of type ListItem. You can get the contents of the selected item by getting the Text or Value property of the object.

Security Note:
Controls in a Web Forms page can include potentially malicious client script. By default, the Web Forms page validates that user input does not include script or HTML elements. For more information, see How to: Protect Against Script Exploits in a Web Application by Applying HTML Encoding to Strings

The following code example shows how you can test which item is selected in a RadioButtonList control. The code first determines whether there is a selection at all by reading the value of the SelectedIndex property, which is set to -1 until the user selects an item. It then gets the SelectedItem object and displays that object's Text property.

Protected Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click
   ' Is anything selected? The index is -1 if nothing is selected.
   If RadioButtonList1.SelectedIndex > -1 Then
      Label1.Text="You chose: " & RadioButtonList1.SelectedItem.Text
   End If
End Sub

Protected void Button1_Click (object sender, System.EventArgs e)
{
   // Is anything selected? The index is -1 if nothing is selected.
   if (RadioButtonList1.SelectedIndex > -1) {
      Label1.Text="You chose: " + 
          RadioButtonList1.SelectedItem.Text;
   }
}

If the list control supports multiple selections, you must loop through the control and check for selected items one by one.

To determine the selection in a multi-selection list control

Loop through the control's Items collection and test the Selected property of every individual item.

The following code example shows how you can test selections in a multi-selection ListBox control called ListBox1. The code displays a list of selected items in a label.

Protected Sub Button1_Click(ByVal sender As System.Object, _
       ByVal e As System.EventArgs) Handles Button1.Click
   Dim msg As String
   Dim li As ListItem
   msg = ""
   For Each li In ListBox1.Items
      If li.Selected = True Then
          msg = msg & "<br>" & li.Text & " selected."
      End If
   Next
   Label1.Text = msg
End Sub

Protected void Button1_Click(object sender, System.EventArgs e)
{
   string msg = "" ;
   foreach(ListItem li in ListBox1.Items)
   {
      if(li.Selected == true)
         {
            msg += "<BR>" + li.Text + " is selected.";
         }
   }
   Label1.Text = msg;
}

Del via

How to: Determine the Selection in List Web Server Controls

To determine the selection in a single-selection list control

To determine the selection in a multi-selection list control

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Reference

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