C# Active Directory: Get domain name of user?

I know that this type of question has been asked before, but other methods are failing me right now.

As it stands our windows service polls AD, given an LDAP (i.e. LDAP://10.32.16.80) and a list of usergroups within that AD server to search for. It retrieves all users within those given groups, recursively searching those groups for more groups as well. Each user is then added to another applications authenticated users list.

This part of the application is running successfully. However, we're in need of each user's friendly domain name (i.e. the part of their login DOMAIN/username)

So if there is a user that is part of TEST domain, named Steve: TEST/steve is his login. I'm able to find steve in the AD, however I also need "TEST" to be stored along with his AD information.

Again, I can find 'steve' fine by using a directory searcher and the LDAP IP I'm given, but given the LDAP IP, how can I find the friendly domain name?

When I try the following code I'm given an error when attempting to access the 'defaultNamingContext':

System.Runtime.InteropServices.COMException (0x8007202A): The authentication mechanism is unknown.

Here is the code:

    private string SetCurrentDomain(string server)
    {
        string result = string.Empty;
        try
        {
            logger.Debug("'SetCurrentDomain'; Instantiating rootDSE LDAP");
            DirectoryEntry ldapRoot = new DirectoryEntry(server + "/rootDSE", username, password);
            logger.Debug("'SetCurrentDomain'; Successfully instantiated rootDSE LDAP");

            logger.Debug("Attempting to retrieve 'defaultNamingContext'...");
            string domain = (string)ldapRoot.Properties["defaultNamingContext"][0]; //THIS IS WHERE I HIT THE COMEXCEPTION
            logger.Debug("Retrieved 'defaultNamingContext': " + domain);
            if (!domain.IsEmpty())
            {

                logger.Debug("'SetCurrentDomain'; Instantiating partitions/configuration LDAP entry");
                DirectoryEntry parts = new DirectoryEntry(server + "/CN=Partitions,CN=Configuration," + domain, username, password);

                logger.Debug("'SetCurrentDomain'; Successfully instantiated partitions/configuration LDAP entry");
                foreach (DirectoryEntry part in parts.Children)
                {
                    if (part.Properties["nCName"] != null && (string)part.Properties["nCName"][0] != null)
                    {
                        logger.Debug("'SetCurrentDomain'; Found property nCName");
                        if ((string)part.Properties["nCName"][0] == domain)
                        {
                            logger.Debug("'SetCurrentDomain'; nCName matched defaultnamingcontext");
                            result = (string)part.Properties["NetBIOSName"][0];
                            logger.Debug("'SetCurrentDomain'; Found NetBIOSName (friendly domain name): " + result);
                            break;
                        }
                    }
                }
            }
            logger.Debug("finished setting current domain...");
        }
        catch (Exception ex)
        {
            logger.Error("error attempting to set domain:" + ex.ToString());
        }
        return result;
    }

edit

I added this sample method in order to attempt a suggestion but am getting an exception: "Unspecified error" when I hit the "FindAll()" call on the searcher. The string being passed in is: "CN=TEST USER,CN=Users,DC=tempe,DC=ktregression,DC=com"

        private string GetUserDomain(string dn)
    {
        string domain = string.Empty;
        string firstPart = dn.Substring(dn.IndexOf("DC="));
        string secondPart = "CN=Partitions,CN=Configuration," + firstPart;
        DirectoryEntry root = new DirectoryEntry(secondPart, textBox2.Text, textBox3.Text);
        DirectorySearcher searcher = new DirectorySearcher(root);
        searcher.SearchScope = SearchScope.Subtree;
        searcher.ReferralChasing = ReferralChasingOption.All;
        searcher.Filter = "(&(nCName=" + firstPart + ")(nETBIOSName=*))";
        try
        {
            SearchResultCollection rs = searcher.FindAll();
            if (rs != null)
            {
                domain = GetProperty(rs[0], "nETBIOSName");
            }
        }
        catch (Exception ex)
        {

        }


        return domain;

This article helped me much to understand how to work with the Active Directory.
Howto: (Almost) Everything In Active Directory via C#

From this point forward, if you require further assitance, please let me know with proper questions in comment, and I shall answer them for you to the best of my knowledge.

EDIT #1

You had better go with this example's filter instead. I have written some sample code to briefly show how to work with the System.DirectoryServices and System.DirectoryServices.ActiveDirectory namespaces. The System.DirectoryServices.ActiveDirectory namespace is used to retrieve information about the domains within your Forest.

private IEnumerable<DirectoryEntry> GetDomains() {
    ICollection<string> domains = new List<string>();

    // Querying the current Forest for the domains within.
    foreach(Domain d in Forest.GetCurrentForest().Domains)
        domains.Add(d.Name);

    return domains;
}

private string GetDomainFullName(string friendlyName) {
    DirectoryContext context = new DirectoryContext(DirectoryContextType.Domain, friendlyName);
    Domain domain = Domain.GetDomain(context);
    return domain.Name;
}

private IEnumerable<string> GetUserDomain(string userName) {
    foreach(string d in GetDomains()) 
        // From the domains obtained from the Forest, we search the domain subtree for the given userName.
        using (DirectoryEntry domain = new DirectoryEntry(GetDomainFullName(d))) {
            using (DirectorySearcher searcher = new DirectorySearcher()){
                searcher.SearchRoot = domain;
                searcher.SearchScope = SearchScope.Subtree;
                searcher.PropertiesToLoad.Add("sAMAccountName");
                // The Filter is very important, so is its query string. The 'objectClass' parameter is mandatory.
                // Once we specified the 'objectClass', we want to look for the user whose login
                // login is userName.
                searcher.Filter = string.Format("(&(objectClass=user)(sAMAccountName={0}))", userName);

                try {
                    SearchResultCollection  results = searcher.FindAll();

                    // If the user cannot be found, then let's check next domain.
                    if (results == null || results.Count = 0)
                        continue;

                     // Here, we yield return for we want all of the domain which this userName is authenticated.
                     yield return domain.Path;
                } finally {
                    searcher.Dispose();
                    domain.Dispose();
                }
            }
}

Here, I didn't test this code and might have some minor issue to fix. This sample is provided as-is for the sake of helping you. I hope this will help.

EDIT #2

I found out another way out:

1. You have first to look whether you can find the user account within your domain;
2. If found, then get the domain NetBIOS Name; and
3. concatenate it to a backslash (****) and the found login.

The example below uses a NUnit TestCase which you can test for yourself and see if it does what you are required to.

[TestCase("LDAP://fully.qualified.domain.name", "TestUser1")] 
public void GetNetBiosName(string ldapUrl, string login)
    string netBiosName = null;
    string foundLogin = null;

    using (DirectoryEntry root = new DirectoryEntry(ldapUrl))
        Using (DirectorySearcher searcher = new DirectorySearcher(root) {
            searcher.SearchScope = SearchScope.Subtree;
            searcher.PropertiesToLoad.Add("sAMAccountName");
            searcher.Filter = string.Format("(&(objectClass=user)(sAMAccountName={0}))", login);

            SearchResult result = null;

            try {
                result = searcher.FindOne();

                if (result == null) 
                    if (string.Equals(login, result.GetDirectoryEntry().Properties("sAMAccountName").Value)) 
                        foundLogin = result.GetDirectoryEntry().Properties("sAMAccountName").Value
            } finally {
                searcher.Dispose();
                root.Dispose();
                if (result != null) result = null;
            }
        }

    if (!string.IsNullOrEmpty(foundLogin)) 
        using (DirectoryEntry root = new DirectoryEntry(ldapUrl.Insert(7, "CN=Partitions,CN=Configuration,DC=").Replace(".", ",DC=")) 
            Using DirectorySearcher searcher = new DirectorySearcher(root)
                searcher.Filter = "nETBIOSName=*";
                searcher.PropertiesToLoad.Add("cn");

                SearchResultCollection results = null;

                try {
                    results = searcher.FindAll();

                    if (results != null && results.Count > 0 && results[0] != null) {
                        ResultPropertyValueCollection values = results[0].Properties("cn");
                        netBiosName = rpvc[0].ToString();
                } finally {
                    searcher.Dispose();
                    root.Dispose();

                    if (results != null) {
                        results.Dispose();
                        results = null;
                    }
                }
            }

    Assert.AreEqual("FULLY\TESTUSER1", string.Concat(netBiosName, "\", foundLogin).ToUpperInvariant())
}

The source from which I inspired myself is:
Find the NetBios Name of a domain in AD