public string Collection
        {
            get 
            {
                List<String> sb = new List<String>();

                foreach (ListItem cBox in Collections.Items)
                {
                    if (cBox.Selected)
                    {
                        sb.Add(cBox.Value);
                    }
                }

                return string.Join(",", sb.ToArray());
            }
        }

comma separated list

C#