Лучше при помощи WMI (System.Management):

using System;
using System.Management;

namespace ConsoleApplication1
{
class Class1
{
static void Main(string[] args)
{
ManagementObjectSearcher searcher = new ManagementObjectSearcher
("SELECT * FROM Win32_NTDomain");
foreach (ManagementObject domain in searcher.Get())
Console.WriteLine("Domain= " + domain["DnsForestName"]);

Console.ReadLine();
}
}
}