How can i show the selected items as comma separated in the Multi select dropdown text field in asp.net
 
In my web application, i am trying to do show the selected items as comma separated in the Multi select drop down text field is it possible to do and how can i do this can any one tell me. 
I tried:
      protected void dropdown1_SelectedIndexChanged(object sender, EventArgs e)
            {    
               string element= string.Empty;
              foreach(ListItem item in dropdown1.Items)
              {
                  if (item.Selected)
                      element += "'" + item.Value + "',";
              }          
              if (element.Length > 0)
              {
                 element = element.Substring(0, element.Length - 1);
                  string qry = "select * from Selections where col1= '" + dropdowncol1.SelectedValue.ToString() + "' and col2 in (" +  element+ ")";
                  SqlCommand cmd = new SqlCommand(qry, con);
                  DataTable dt = new DataTable();
                  SqlDataAdapter da = new SqlDataAdapter(cmd);
                  da.Fill(dt);
                 dropdown1.DataTextField =  element;
                             
              }
Can anyone help me to how to do this.  
Thank you