Ниже приведен пример:

using System;
using System.DirectoryServices;

namespace DirectoryServicesSamples
{
  class DomainListApp
  {
    [STAThread]
    static void Main(string[] args)
    {
      DirectoryEntry deRootDSE = new DirectoryEntry("LDAP://RootDSE");
      string sRootDomain = "LDAP://" + deRootDSE.Properties["rootDomainNamingContext"].Value.ToString();

      DirectoryEntry deRoot = new DirectoryEntry(sRootDomain);

      DirectorySearcher dsFindDomains = new DirectorySearcher(deRoot);
      dsFindDomains.Filter = "(objectClass=domainDNS)";
      dsFindDomains.SearchScope = SearchScope.Subtree;

      dsFindDomains.PropertiesToLoad.AddRange(new string[] {"distinguishedName", "name"} );

      foreach(SearchResult srDomain in dsFindDomains.FindAll())
      {
        Console.WriteLine(string.Format("{0} ({1})",
          srDomain.Properties["name"][0],
          srDomain.Properties["distinguishedName"][0]));
      }
      Console.ReadLine();
    }
  }
}