Reliable method to get machine's MAC address in C#

The MACAddress property of the Win32_NetworkAdapterConfiguration WMI class can provide you with an adapter's MAC address. (System.Management Namespace)

MACAddress

    Data type: string
    Access type: Read-only

    Media Access Control (MAC) address of the network adapter. A MAC address is assigned by the manufacturer to uniquely identify the network adapter.

    Example: "00:80:C7:8F:6C:96"

If you're not familiar with the WMI API (Windows Management Instrumentation), there's a good overview here for .NET apps.

WMI is available across all version of windows with the .Net runtime.

Here's a code example:

System.Management.ManagementClass mc = default(System.Management.ManagementClass);
ManagementObject mo = default(ManagementObject);
mc = new ManagementClass("Win32_NetworkAdapterConfiguration");

ManagementObjectCollection moc = mc.GetInstances();
    foreach (var mo in moc) {
        if (mo.Item("IPEnabled") == true) {
              Adapter.Items.Add("MAC " + mo.Item("MacAddress").ToString());
         }
     }

IMHO returning first mac address isn't good idea, especially when virtual machines are hosted. Therefore i check send/received bytes sum and select most used connection, that is not perfect, but should be correct 9/10 times.

public string GetDefaultMacAddress()
{
    Dictionary<string, long> macAddresses = new Dictionary<string, long>();
    foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
    {
        if (nic.OperationalStatus == OperationalStatus.Up)
            macAddresses[nic.GetPhysicalAddress().ToString()] = nic.GetIPStatistics().BytesSent + nic.GetIPStatistics().BytesReceived;
    }
    long maxValue = 0;
    string mac = "";
    foreach(KeyValuePair<string, long> pair in macAddresses)
    {
        if (pair.Value > maxValue)
        {
            mac = pair.Key;
            maxValue = pair.Value;
        }
    }
    return mac;
}

You could go for the NIC ID:

 foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces()) {
     if (nic.OperationalStatus == OperationalStatus.Up){
         if (nic.Id == "yay!")
     }
 }

It's not the MAC address, but it is a unique identifier, if that's what you're looking for.

string mac = "";
foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
            {

                if (nic.OperationalStatus == OperationalStatus.Up && (!nic.Description.Contains("Virtual") && !nic.Description.Contains("Pseudo")))
                {
                    if (nic.GetPhysicalAddress().ToString() != "")
                    {
                        mac = nic.GetPhysicalAddress().ToString();
                    }
                }
            }
MessageBox.Show(mac);

Changed blak3r his code a bit. In case you have two adapters with the same speed. Sort by MAC, so you always get the same value.

public string GetMacAddress()
{
    const int MIN_MAC_ADDR_LENGTH = 12;
    string macAddress = string.Empty;
    Dictionary<string, long> macPlusSpeed = new Dictionary<string, long>();
    try
    {
        foreach(NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
        {
            System.Diagnostics.Debug.WriteLine("Found MAC Address: " + nic.GetPhysicalAddress() + " Type: " + nic.NetworkInterfaceType);

            string tempMac = nic.GetPhysicalAddress().ToString();

            if(!string.IsNullOrEmpty(tempMac) && tempMac.Length >= MIN_MAC_ADDR_LENGTH)
                macPlusSpeed.Add(tempMac, nic.Speed);
        }

        macAddress = macPlusSpeed.OrderByDescending(row => row.Value).ThenBy(row => row.Key).FirstOrDefault().Key;
    }
    catch{}

    System.Diagnostics.Debug.WriteLine("Fastest MAC address: " + macAddress);

    return macAddress;
}

foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
{
     if (nic.OperationalStatus == OperationalStatus.Up)
     {
            PhysicalAddress Mac = nic.GetPhysicalAddress();
     }
}

ipconfig.exe is implemented using various DLLs including iphlpapi.dll ... Googling for iphlpapi reveals a corresponding Win32 API documented in MSDN.