Pinvoke problem with the Windows helper API

Question

This is my first serious attempt at P/invoke. This is a console app I'm using to work out the code for a web service that will hopefully return a mac address from an ip address or fully qualified system name. The code compiles. However when run without args, I get an unhandled exception-IndexOutOfRangeException. If I run it with args, I get a different unhandled exception, AccessViolationException: Attempted to read or write protected memory. And, I'm self-taught...Please be kind if I've done something stupid. Here's my code:

using System;
using System.Diagnostics;
using System.Net;
using System.Runtime.InteropServices;
using System.Text;

namespace GetMac
{
    class Program
    {
        const int NET_STRING_IPV4_ADDRESS = 1;   // 192.168.100.10
        const int NET_STRING_IPV4_SERVICE = 2;   // 192.168.100.10:80
        const int NET_STRING_IPV4_NETWORK = 4;   // 192.168.100/24
        const int NET_STRING_IPV6_ADDRESS = 8;   // 21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A%2
        const int NET_STRING_IPV6_ADDRESS_NO_SCOPE = 8;  // 21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A
        const int NET_STRING_IPV6_SERVICE = 32;   // [21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A%2]:8080
        const int NET_STRING_IPV6_SERVICE_NO_SCOPE = 64;  // 21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A:8080
        const int NET_STRING_IPV6_NETWORK = 128;   // 21DA:D3::/48
        const int NET_STRING_NAMED_ADDRESS = 256;  // www.microsoft.com
        const int NET_STRING_NAMED_SERVICE = 512;  // www.microsoft.com:80

        internal static class NativeMethods
        {
            [DllImport("IpHlpApi.dll", CharSet = CharSet.Unicode)]
            //public static extern uint ParseNetworkString(ref string networkString, int types, out NetAddressInfo addressInfo, out ushort portNumber, out byte prefixLength);
            public static extern uint ParseNetworkString(string networkString, int types);
        }

        public static string GetMacAddress(IPAddress ipAddress)
        {
            string strMacAddress = string.Empty;

            try
            {
                char delimChar = '=';
                string strTempMacAddress = string.Empty;
                ProcessStartInfo objProcessStartInfo = new ProcessStartInfo();
                Process objProcess = new Process();
                objProcessStartInfo.FileName = "nbtstat";
                objProcessStartInfo.RedirectStandardInput = false;
                objProcessStartInfo.RedirectStandardOutput = true;
                objProcessStartInfo.Arguments = "-A " + ipAddress;
                objProcessStartInfo.UseShellExecute = false;
                objProcess = Process.Start(objProcessStartInfo);

                int Counter = -1;
                while (Counter <= -1)
                {
                    Counter = strTempMacAddress.Trim().ToLower().IndexOf("mac address", 0);
                    if (Counter > -1)
                    {
                        break;
                    }
                    strTempMacAddress = objProcess.StandardOutput.ReadLine();
                }
                objProcess.WaitForExit();
                string[] words = strTempMacAddress.Split(delimChar);
                strTempMacAddress = words[1];
                strMacAddress = strTempMacAddress.Trim();
                strMacAddress = strMacAddress.Replace('-', ':');
            }
            catch (Exception Ex)
            {
                Console.WriteLine(Ex.ToString());
                Console.ReadLine();
            }
            return strMacAddress;
        }

        static void Main(string[] args)
        {
            string macAddress = null;
            IPAddress ipAddress = null;
            string strHostOrIp;

            if (args[0] == null)
            {
                Console.WriteLine("Please enter an IP address or fully qualified host name");
                return;
            }
            else
            {
                strHostOrIp = args[0];
            }
            if (NativeMethods.ParseNetworkString(strHostOrIp, NET_STRING_IPV4_ADDRESS) == 0) //true if it's an IP address
            {
                IPAddress address = IPAddress.Parse(strHostOrIp);
                ipAddress = address;
            }
            else if (NativeMethods.ParseNetworkString(strHostOrIp, NET_STRING_NAMED_SERVICE) == 0) //true if it's a fully qualified host name
            {
                IPHostEntry hostEntry = null;
                try
                {
                    hostEntry = Dns.GetHostEntry(strHostOrIp);
                }
                catch
                {
                    return;
                }
                if (hostEntry.AddressList.Length == 0)
                {
                    return;
                }
                foreach (IPAddress ip in hostEntry.AddressList)
                {
                    if (ip.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork) // true if we have IPv4
                    {
                        ipAddress = ip;
                        break;
                    }
                }
            }
            else
            {
                ipAddress = null;
            }
            macAddress = GetMacAddress(ipAddress);
            Console.WriteLine(macAddress);
        }
    }
}
// DWORD WINAPI ParseNetworkString( -in const WCHAR *NetworkString, 
//                                  -in DWORD Types, 
//                                  -out_opt  PNET_ADDRESS_INFO AddressInfo, 
//                                  -out_opt  USHORT *PortNumber, 
//                                  -out_opt  BYTE *PrefixLength);

// ParseNetworkString function parses the input network string and checks whether it is a legal 
// representation of the specified IP network string type. If the string matches a type and its 
// specification, the function can optionally return the parsed result

// Return Value
// If the function succeeds, the return value is ERROR_SUCCESS. // I think this means it returns 0.
// If the function fails, the return value is one of the following error codes.
//  ERROR_INSUFFICIENT_BUFFER - The buffer passed to the function is too small. This error is returned if the buffer pointed to by the AddressInfo parameter is too small to hold the parsed network address.
//  ERROR_INVALID_PARAMETER - An invalid parameter was passed to the function. This error is returned if a NULL pointer is passed in the NetworkString parameter

// http://msdn.microsoft.com/en-us/library/bb408412(VS.85).aspx

Thanx in advance.

Answer 1

The three "optional" parameters to ParseNetworkString are not optional in the sense that you can omit them. You must provide them, but you can pass null (or zero) to say that you don't want that particular value returned.

Answer 2

This is a "more" correct DllImport:

[DllImport("iphlpapi.dll", CharSet = CharSet.Unicode)]
public static extern uint ParseNetworkString(string networkString, uint types, IntPtr addressInfo, IntPtr portNumber, IntPtr prefixLength);

Example Usage:

Console.WriteLine(ParseNetworkString("192.168.100.10:20", NET_STRING_IPV4_SERVICE, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero));