Is it possible to display selected items in dropdowncheckbox

Question

In asp.net web application, i have dropdowncheckboxes control in that whatever items i selected i need to show that selected items in  dropdown textfield as separated with comma.

 

I tried this code but i am not getting result:

 

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.DataTextField = String.Join(",", checkedList);
}

}

 My screen shots for this:

in My dropdown1 has many items i need to select two items i.e., shown in below figure

 

what i select items from dropdown it will display on text field as like below figure

 

 

Can anyone please tell me how to do this.

thank you