C# - How to Make the ComboBox ' Values Distinct (Unique)?

using (SqlConnection conn = new SqlConnection("Your Connectionstring"))
               {
                   conn.Open();
                   SqlCommand cmd = new SqlCommand("Your select Query", conn);

                   SqlDataReader dr = cmd.ExecuteReader();

                   IList<string> listName = new List<string>();
                   while (dr.Read())
                   {
                       listName.Add(dr[0].ToString());
                   }
                   listName = listName.Distinct().ToList();
                   ComboBox1.DataSource = listName;
               }</string></string>