How to find FQDN of local machine in C#/.NET ?

NOTE: This solution only works when targeting the .NET 2.0 (and newer) frameworks.

using System;
using System.Net;
using System.Net.NetworkInformation;
//...

public static string GetFQDN()
{
    string domainName = IPGlobalProperties.GetIPGlobalProperties().DomainName;
    string hostName = Dns.GetHostName();

    domainName = "." + domainName;
    if(!hostName.EndsWith(domainName))  // if hostname does not already include domain name
    {
        hostName += domainName;   // add the domain name part
    }

    return hostName;                    // return the fully qualified name
}

UPDATE

Since a lot of people have commented that Sam's Answer is more concise I've decided to add some comments to the answer.

The most important thing to note is that the code I gave is not equivalent to the following code:

Dns.GetHostEntry("LocalHost").HostName

While in the general case when the machine is networked and part of a domain, both methods will generally produce the same result, in other scenarios the results will differ.

A scenario where the output will be different is when the machine is not part of a domain. In this case, the Dns.GetHostEntry("LocalHost").HostName will return localhost while the GetFQDN() method above will return the NETBIOS name of the host.

This distinction is important when the purpose of finding the machine FQDN is to log information, or generate a report. Most of the time I've used this method in logs or reports that are subsequently used to map information back to a specific machine. If the machines are not networked, the localhost identifier is useless, whereas the name gives the needed information.

So ultimately it's up to each user which method is better suited for their application, depending on what result they need. But to say that this answer is wrong for not being concise enough is superficial at best.

See an example where the output will be different: http://ideone.com/q4S4I0

A slight simplification of Miky D's code

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

We have implemented suggested result to use this way:

return System.Net.Dns.GetHostEntry(Environment.MachineName).HostName;

However, turned out that this does not work right when computer name is longer than 15 characters and using NetBios name. The Environment.MachineName returns only partial name and resolving host name returns same computer name.

After some research we found a solution to fix this problem:

System.Net.Dns.GetHostEntry(System.Net.Dns.GetHostName()).HostName

This resolved all problems including computer name.

None of the answers provided that I tested actually provided the DNS suffix I was looking for. Here's what I came up with.

public static string GetFqdn()
{
    var networkInterfaces = NetworkInterface.GetAllNetworkInterfaces();
    var ipprops = networkInterfaces.First().GetIPProperties();
    var suffix = ipprops.DnsSuffix;
    return $"{IPGlobalProperties.GetIPGlobalProperties().HostName}.{suffix}";
}

I've used this approach:

private static string GetLocalhostFQDN()
{
    var ipProperties = IPGlobalProperties.GetIPGlobalProperties();
    return $"{ipProperties.HostName}.{ipProperties.DomainName}";
}

My collection of methods to handle all cases around FQ Hostname / Hostname / NetBIOS Machinename / DomainName

    /// <summary>
    /// Get the full qualified hostname
    /// </summary>
    /// <param name="throwOnMissingDomainName"></param>
    /// <returns></returns>
    public static string GetMachineFQHostName(bool throwOnMissingDomainName = false)
    {
        string domainName = GetMachineFQDomainName();
        string hostName = GetMachineHostName();

        if (string.IsNullOrEmpty(domainName) && throwOnMissingDomainName) throw new Exception($"Missing domain name on machine: { hostName }");
        else if (string.IsNullOrEmpty(domainName)) return hostName;
        //<----------

        return $"{ hostName }.{ domainName }";
    }


    /// <summary>
    /// Get the NetBIOS name of the local machine
    /// </summary>
    /// <returns></returns>
    public static string GetMachineName()
    {
        return Environment.MachineName;
    }

    /// <summary>
    /// Get the Hostname from the local machine which differs from the NetBIOS name when 
    /// longer than 15 characters
    /// </summary>
    /// <returns></returns>
    public static string GetMachineHostName()
    {
        /// I have been told that GetHostName() may return the FQName. Never seen that, but better safe than sorry ....
        string hostNameRaw = System.Net.Dns.GetHostName();
        return hostNameRaw.Split('.')[0];
    }

    /// <summary>
    /// Check if hostname and NetBIOS name are equal
    /// </summary>
    /// <returns></returns>
    public static bool AreHostNameAndNetBIOSNameEqual()
    {
        return GetMachineHostName().Equals(GetMachineName(), StringComparison.OrdinalIgnoreCase);
    }

    /// <summary>
    /// Get the domain name without the hostname
    /// </summary>
    /// <returns></returns>
    public static string GetMachineFQDomainName()
    {
        return IPGlobalProperties.GetIPGlobalProperties().DomainName;
    }