Get SPUser Object from ListObject by key

GetSPUser.cs

private static SPUser GetSPUser(SPListItem item, string key)
{
  SPFieldUser field = item.Fields[key] as SPFieldUser;

  if (field != null && item[key] != null)
  {
    SPFieldUserValue fieldValue = field.GetFieldValue(item[key].ToString()) as SPFieldUserValue;
    if (fieldValue != null)
    {
      return fieldValue.User;
    }
  }
  return null;
}