Edward Wilde

Okay so its pretty easy to get the fully qualified domain name (FQDN).

using System.DirectoryServices;
using System.DirectoryServices.ActiveDirectory;

Console.WriteLine(Domain.GetCurrentDomain().Name)

But what happens if you want to get the old-skool netbios name. The quickest way is to use System.Environment:

using System

Console.WriteLine(System.Environment.UserDomainName)

This gets the network domain name associated with the current user. So not much good if your code is running as a service that could well be configured to run at Network Account. Surely you can do this using DirectoryServices? Bit of Googling and coding and we have a nice little extension method which does just that

/// <summary>
    /// Defines extentions made to the <see cref="Domain"/> class.
    /// </summary>
    public static class DomainExtensions
    {
        public static string GetNetbiosName(this Domain domain)
        {

            // Bind to RootDSE and grab the configuration naming context
            DirectoryEntry rootDSE = new DirectoryEntry(@"LDAP://RootDSE");
            DirectoryEntry partitions = new DirectoryEntry(@"LDAP://cn=Partitions," + rootDSE.Properties["configurationNamingContext"].Value);

            DirectoryEntry domainEntry = domain.GetDirectoryEntry();

            //Iterate through the cross references collection in the Partitions container
            DirectorySearcher directorySearcher = new DirectorySearcher(partitions)
               {
                    Filter = "(&(objectCategory=crossRef)(ncName=" +
                         domainEntry.Path
                             .Replace("LDAP://", string.Empty)
                             .Replace(domain.Name + "/", string.Empty) + "))",
                    SearchScope = SearchScope.Subtree
               };

            directorySearcher.PropertiesToLoad.Add("nETBIOSName");

            //Display result (should only be one)
            SearchResultCollection results = directorySearcher.FindAll();
            if (results.Count == 0)
            {
                return null;
            }

            return results[0].Properties["nETBIOSName"][0].ToString();
        }
    }

Usage:

Domain.GetCurrentDomain().GetNetbiosName()

Note You can also retrieve the netbios entry using netapi32.dll http://blog.dotsmart.net/2009/03/11/getting-a-machine%E2%80%99s-netbios-domain-name-in-csharp