Lluís Franco on Geeks.ms

using System;

using System.Collections.Generic;

using System.DirectoryServices;

using System.DirectoryServices.ActiveDirectory;

using System.Linq;

using System.Net.NetworkInformation;

using System.Runtime.InteropServices;

using System.Security.Principal;

using System.Text;

namespace Alpha.Code

{

    public static class SecurityExtensions

    {

        public static IOrderedEnumerable<IGrouping <string, NTAccount>> 

            GetGroupsUnderDomains(this WindowsIdentity identity)

        {

            var groups =

                        from grIdentity in identity.Groups

                        where grIdentity.IsValidTargetType(typeof(NTAccount))

                        select grIdentity.Translate(typeof(NTAccount)) as NTAccount into ntAccounts

                        let domainName = ntAccounts.GetDomainName()

                        let groupName = ntAccounts.GetAccountName()

                        orderby domainName

                        group ntAccounts by domainName into domainGroups

                        orderby domainGroups.Key

                        select domainGroups;

            return groups;

        }

        public static string GetDomainName(this NTAccount account)

        {

            string[] split = account.Value.Split('\\');

            return split.Length == 1 ? string.Empty : split[0];

        }

        public static string GetAccountName(this NTAccount account)

        {

            string[] split = account.Value.Split('\\');

            return split[split.Length - 1];

        }

    }

}

var groups = WindowsIdentity.GetCurrent().GetGroupsUnderDomains();

foreach (var dg in groups)

{

    Console.WriteLine(string.Format("Groups under: {0}", dg.Key));

    foreach (var g in dg)

    {

        Console.WriteLine(string.Format("  - Group name: {0}", g.GetAccountName()));

    }

}