Getting FQDN for current user in c#

Question

I encountered a apparently simple problem but which is giving me some trouble.

Let's suppose I have two trusted AD domains: DEMO.intern and DEMO.trust

I need to get the FQDN for the current user in my app which uses Windows auth. Users from both domains can use it. I need to know which user comes from which domain.

Obviously,

Environment.UserDomainName 

or

System.Security.Principal.WindowsIdentity.GetCurrent().Name

fails returning DEMO in both cases.

I know that:

System.Net.NetworkInformation.IPGlobalProperties.GetIPGlobalProperties().DomainName

gives me the full domain name for the local computer but how can this be achieved for the current logged in user?

I also tried:

var host = Dns.GetHostEntry(Environment.UserDomainName);
host.HostName.Dump();

with no success giving me the error "No such host is known". So how can I obtain the full domain name for the logged in user for example DEMO.intern not just DEMO?

Answer 1

After some research, this gets the job done:

public static class GetUserNameExUtil
{
    #region Interop Definitions
    public enum EXTENDED_NAME_FORMAT 
    {
        NameUnknown = 0,
        NameFullyQualifiedDN = 1,
        NameSamCompatible = 2,
        NameDisplay = 3,
        NameUniqueId = 6,
        NameCanonical = 7,
        NameUserPrincipal = 8,
        NameCanonicalEx = 9,
        NameServicePrincipal = 10,
        NameDnsDomain = 12,
    }
    [System.Runtime.InteropServices.DllImport("secur32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
    public static extern int GetUserNameEx(int nameFormat, StringBuilder userName, ref int userNameSize);
    #endregion

    public static string GetUserName(EXTENDED_NAME_FORMAT nameFormat)
    {
        if (Environment.OSVersion.Platform != PlatformID.Win32NT)
        {
            return null;
        }

        StringBuilder userName = new StringBuilder(1024);
        int userNameSize = userName.Capacity;
        if (GetUserNameEx((int)nameFormat, userName, ref userNameSize) != 0)
        {
            string[] nameParts = userName.ToString().Split('\\');
            return nameParts[0];
        }

        return null;
    }
    public static string GetUserFullName()
    {
        return GetUserName(EXTENDED_NAME_FORMAT.NameDnsDomain);
    }
}