Unable to get all DirectoryEntry in a Forest

I'm trying to get all directory entries in a forest.

My code snippet is shown below:

DirectoryContext dc = new DirectoryContext(DirectoryContextType.DirectoryServer, "xx.x.xxx.40", "w28\\administrator", "pwd");

Forest forest = Forest.GetForest(dc);

Console.WriteLine("Domain count in forest: " + forest.Domains.Count);
DomainCollection domains = forest.Domains;

foreach (Domain d in domains)
{
    Console.WriteLine(d.Name);

    //It doesn't work
    DirectoryEntry entry = d.GetDirectoryEntry();
    foreach (DirectoryEntry child in entry.Children)
    {
        Console.WriteLine(" - " + child.Name);
    }
}

However, I get an exception:

System.DirectoryServices.ActiveDirectory.ActiveDirectoryOperationException: Unknown error (0x80005000) ---> System.Runtime.InteropServices.COMException (0x80005000): Unknown error (0x80005000)
   at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
   at System.DirectoryServices.DirectoryEntry.Bind()
   at System.DirectoryServices.DirectoryEntry.get_AdsObject()
   at System.DirectoryServices.PropertyValueCollection.PopulateList()
   at System.DirectoryServices.PropertyValueCollection..ctor(DirectoryEntry entry, String propertyName)
   at System.DirectoryServices.PropertyCollection.get_Item(String propertyName)
   at System.DirectoryServices.ActiveDirectory.PropertyManager.GetPropertyValue(DirectoryContext context, DirectoryEntry directoryEntry, String propertyName)
   --- End of inner exception stack trace ---
   at System.DirectoryServices.ActiveDirectory.PropertyManager.GetPropertyValue(DirectoryContext context, DirectoryEntry directoryEntry, String propertyName)
   at System.DirectoryServices.ActiveDirectory.DirectoryEntryManager.ExpandWellKnownDN(WellKnownDN dn)
   at System.DirectoryServices.ActiveDirectory.Domain.GetDirectoryEntry()

I've got the same error in case when several of my domain controllers are turned off and became unavailable.

I have changed the way I'm getting the DirectoryEntry instance for the domain:

entry = new DirectoryEntry(string.Format("LDAP://{0}", d.Name), UserNameFull, password);

This works fine because the error "The server is not operational" is raised, so it gets the points to the user that something wrong with its DC.