public static void AddFieldToList(SPList list, SPField field)
        {
            list.RequireNotNull("list");
            field.RequireNotNull("field");

            if (!list.Fields.Contains(field.Id))
            {
                list.Fields.Add(field);
                list.Update();
            }
        }
 public static void AddFieldToContentType(SPContentType cType, SPField field)
 {
     cType.RequireNotNull("cType");
     field.RequireNotNull("field");
     var matchingLinks = from SPFieldLink fl in cType.FieldLinks
                         where fl.DisplayName.Equals(field.Title)
                         select fl;
     if (!cType.Fields.Contains(field.Id) && matchingLinks.Count() == 0)
     {
         SPFieldLink fieldLink = new SPFieldLink(field);
         cType.FieldLinks.Add(fieldLink);
         cType.Update(true);
     }
 }