Lookup SPF record in C#

We have retrieved already MX and SRV records. Now let's take SPF record from DNS server. SPF string is TXT record(s) and if you are looking for google.com record proceed with
nslookup -type=TXT google.com

Csharp class looks like

public class nDnsQuery
    {
        public nDnsQuery()
        {
        }

        [DllImport("dnsapi", EntryPoint = "DnsQuery_W", CharSet = CharSet.Unicode, SetLastError = true, ExactSpelling = true)]
        private static extern int DnsQuery([MarshalAs(UnmanagedType.VBByRefStr)]ref string pszName, QueryTypes wType, QueryOptions options, int aipServers, ref IntPtr ppQueryResults, int pReserved);

        [DllImport("dnsapi", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern void DnsRecordListFree(IntPtr pRecordList, int FreeType);

  

        public static string[] GetSPFRecords(string domain)
        {

            IntPtr ptr1 = IntPtr.Zero;
            IntPtr ptr2 = IntPtr.Zero;
            SPFRecord recSPF;
            if (Environment.OSVersion.Platform != PlatformID.Win32NT)
            {
                throw new NotSupportedException();
            }
            ArrayList list1 = new ArrayList();
            try
            {

                int num1 = nDnsQuery.DnsQuery(ref domain, QueryTypes.DNS_TYPE_TEXT, QueryOptions.DNS_QUERY_BYPASS_CACHE, 0, ref ptr1, 0);
                if (num1 != 0)
                {
                    if (num1 == 9003)
                    {
                        list1.Add("DNS record does not exist");
                    }
                    else
                    {
                        throw new Win32Exception(num1);
                    }
                }
                for (ptr2 = ptr1; !ptr2.Equals(IntPtr.Zero); ptr2 = recSPF.pNext)
                {
                    recSPF = (SPFRecord)Marshal.PtrToStructure(ptr2, typeof(SPFRecord));
                    if (recSPF.wType == (ushort)QueryTypes.DNS_TYPE_TEXT)
                    {
                        for (int i = 0; i < recSPF.dwStringCount; i++)
                        {
                            IntPtr pString = recSPF.pStringArray + i;
                            string text1 = Marshal.PtrToStringAuto(pString);
                            if (text1.Contains("v=spf1") || text1.Contains("spf2.0"))
                            {
                                list1.Add(text1);
                            }
                        }
                    }
                }
            }
            finally
            {
                nDnsQuery.DnsRecordListFree(ptr1, 0);
            }
            return (string[])list1.ToArray(typeof(string));
        }

        private enum QueryOptions
        {
            DNS_QUERY_ACCEPT_TRUNCATED_RESPONSE = 1,
            DNS_QUERY_BYPASS_CACHE = 8,
            DNS_QUERY_DONT_RESET_TTL_VALUES = 0x100000,
            DNS_QUERY_NO_HOSTS_FILE = 0x40,
            DNS_QUERY_NO_LOCAL_NAME = 0x20,
            DNS_QUERY_NO_NETBT = 0x80,
            DNS_QUERY_NO_RECURSION = 4,
            DNS_QUERY_NO_WIRE_QUERY = 0x10,
            DNS_QUERY_RESERVED = -16777216,
            DNS_QUERY_RETURN_MESSAGE = 0x200,
            DNS_QUERY_STANDARD = 0,
            DNS_QUERY_TREAT_AS_FQDN = 0x1000,
            DNS_QUERY_USE_TCP_ONLY = 2,
            DNS_QUERY_WIRE_ONLY = 0x100
        }

        private enum QueryTypes
        {
            DNS_TYPE_A = 0x0001,
            DNS_TYPE_MX = 0x000f,
            DNS_TYPE_TEXT = 0x0010,
            DNS_TYPE_SRV = 0x0021
        }

  

        [StructLayout(LayoutKind.Sequential)]
        private struct SPFRecord
        {
            public IntPtr pNext;
            public string pName;
            public ushort wType;
            public ushort wDataLength;
            public int flags;
            public int dwTtl;
            public int dwReserved;
            public Int32 dwStringCount;
            public IntPtr pStringArray;
        }

    }

You can take full sources here.

Enjoy!