if (userAccount.Properties["manager"].Value != null)
{
  DirectorySearcher search2 = new DirectorySearcher(domain.GetDirectoryEntry())
  {
    Filter = string.Format("(distinguishedName={0})", userAccount.Properties["manager"].Value)
  };
  search2.PropertiesToLoad.Add("displayName");
  search2.PropertiesToLoad.Add("mail");
  search2.PropertiesToLoad.Add("manager");
  DirectoryEntry mgrAcc = search2.FindOne()?.GetDirectoryEntry();
}