change DropDownCheckboxes property SelectBoxCaption dynamic

Question

When user select multiple values from DropDownCheckboxes, and How can you show the selected values at the SelectBoxCaption.

How can i do it.

my .aspx code:

<%@ Register Namespace="Saplin.Controls" Assembly="DropDownCheckBoxes" TagPrefix="asp"%> 
<asp:DropDownCheckBoxes ID="dropdown1" runat="server" UseSelectAllNode="true" 
UseButtons="true" OnSelectedIndexChanged="dropdown1_SelectedIndexChanged"
AutoPostBack="true"><Style DropDownBoxBoxWidth="200"/> </asp:DropDownCheckBoxes> 

.cs

protected void dropdown1_SelectedIndexChanged(object sender, EventArgs e)
 {
 List<String> checkedList = new List<string>();
 foreach (ListItem item in dropdown1.Items)
 { 
if (item.Selected)
 {       
  checkedList.Add(item.Text);
 } 
}   
  dropdown1.Texts.SelectBoxCaption = String.Join(",", checkedList); 
}

Can anyone help me 
  
Thank you