namespace Common.DNS {
    using System;
    using System.Collections;
    using System.ComponentModel;
    using System.Runtime.InteropServices;
    using System.Collections.Generic;
    public class DnsMx {
        public DnsMx() { }
        [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);
        private static string[] GetMXRecords(string domain) {
            IntPtr ptr1=IntPtr.Zero;
            IntPtr ptr2=IntPtr.Zero;
            MXRecord recMx;
            if(Environment.OSVersion.Platform!=PlatformID.Win32NT) {
                throw new NotSupportedException();
            }
            ArrayList list1=new ArrayList();
            int num1=DnsMx.DnsQuery(ref domain,QueryTypes.DNS_TYPE_MX,QueryOptions.DNS_QUERY_BYPASS_CACHE,0,ref ptr1,0);
            if(num1!=0)
                throw new Win32Exception(num1);
            for(ptr2=ptr1;!ptr2.Equals(IntPtr.Zero);
                ptr2=recMx.pNext) {
                recMx=(MXRecord)Marshal.PtrToStructure(ptr2,typeof(MXRecord));
                if(recMx.wType==15) {
                    string text1=Marshal.PtrToStringAuto(recMx.pNameExchange);
                    list1.Add(text1);
                }
            }
            DnsMx.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_MX=15 }
        [StructLayout(LayoutKind.Sequential)]
        private struct MXRecord {
            public IntPtr pNext;
            public string pName;
            public short wType;
            public short wDataLength;
            public int flags;
            public int dwTtl;
            public int dwReserved;
            public IntPtr pNameExchange;
            public short wPreference;
            public short Pad;
        }
        private Dictionary<string,string[]> cache=new Dictionary<string,string[]>();
        public string[] resolve(string domain) {
            if(!cache.ContainsKey(domain))
                cache.Add(domain,GetMXRecords(domain));
            return cache[domain];
        }
    }
}