Error while Fetch group from Active Directory

  DirectoryEntry obUser = new DirectoryEntry(res.Path);
// Invoke Groups method.                         object obGroups = obUser.Invoke("Groups");
fetch record group from LDAP gives an error as mention below
 threw an exception of type 'System.Runtime.InteropServices.COMException'    System.Guid {System.Runtime.InteropServices.COMException}
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