using System;
using System.Net;

//...

public static string GetFQDN()
{
    string domainName = NetworkInformation.IPGlobalProperties.GetIPGlobalProperties().DomainName;
    string hostName = Dns.GetHostName();
    string fqdn = "";
    if (!hostName.Contains(domainName))
        fqdn = hostName + "." + domainName;
    else
        fqdn = hostName;

    return fqdn;
}

public static string GetLocalhostFqdn()
    {
        var ipProperties = IPGlobalProperties.GetIPGlobalProperties();
        return string.Format("{0}.{1}", ipProperties.HostName, ipProperties.DomainName);
    }

$ipProperties = [System.Net.NetworkInformation.IPGlobalProperties]::GetIPGlobalProperties()
"{0}.{1}" -f $ipProperties.HostName, $ipProperties.DomainName

System.Net.Dns.GetHostByName("localhost").HostName

public static string GetLocalhostFQDN()
        {
            string domainName = string.Empty;
            try
            {
                domainName = NetworkInformation.IPGlobalProperties.GetIPGlobalProperties().DomainName;
            }
            catch
            {
            }
            string fqdn = "localhost";
            try
            {
                fqdn = System.Net.Dns.GetHostName();
                if (!string.IsNullOrEmpty(domainName))
                {
                    if (!fqdn.ToLowerInvariant().EndsWith("." + domainName.ToLowerInvariant()))
                    {
                        fqdn += "." + domainName;
                    }
                }
            }
            catch
            {
            }
            return fqdn;
        }