Marshalling a variable-length array from unmanaged code in C#

marshal-variable-length-array.cs

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace DnsMarshallingTest
{
    #region Data structures

    [Flags]
    internal enum DnsRecordType : ushort
    {
        TEXT = 0x0010,
    }

    [Flags]
    internal enum DnsQueryType : uint
    {
        STANDARD = 0x00000000,
    }

    internal enum DnsFreeType : uint
    {
        FreeFlat = 0,
        FreeRecordList
    }

    [StructLayout(LayoutKind.Sequential)]
    internal struct DnsRecord
    {
        public IntPtr Next; // 4 bytes
        public string Name; // 4 bytes (ptr)
        [MarshalAs(UnmanagedType.U2)] public DnsRecordType RecordType; // 2 bytes
        [MarshalAs(UnmanagedType.U2)] public ushort DataLength; // 2 bytes

        [StructLayout(LayoutKind.Explicit)] // 4 bytes
        internal struct DnsRecordFlags
        {
            [FieldOffset(0)] [MarshalAs(UnmanagedType.U4)] public uint DW;
            [FieldOffset(0)] [MarshalAs(UnmanagedType.U4)] public uint S;
        }

        public DnsRecordFlags Flags;
        [MarshalAs(UnmanagedType.U4)] public uint Ttl; // 4 bytes
        [MarshalAs(UnmanagedType.U4)] public uint Reserved; // 4 bytes
    }

    [StructLayout(LayoutKind.Sequential)]
    internal struct DNS_TXT_DATA
    {
        public uint dwStringCount;
        [MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 1, ArraySubType = UnmanagedType.SysUInt)] public IntPtr[] pStringArray;
    }

    #endregion

    internal class Program
    {
        #region Imports

        [DllImport("dnsapi", EntryPoint = "DnsQuery_A")]
        private static extern uint DnsQuery(
            [MarshalAs(UnmanagedType.LPStr)] string Name,
            [MarshalAs(UnmanagedType.U2)] DnsRecordType Type,
            [MarshalAs(UnmanagedType.U4)] DnsQueryType Options,
            IntPtr Servers,
            [In, Out] ref IntPtr QueryResultsSet,
            IntPtr Reserved
            );

        [DllImport("dnsapi", EntryPoint = "DnsRecordListFree")]
        private static extern void DnsRecordListFree(IntPtr RecordList, DnsFreeType FreeType);

        #endregion

        private static void Main(string[] args)
        {
            var domain = "enter-your-own-value-here";
            var pServers = IntPtr.Zero;
            var ppQueryResultsSet = IntPtr.Zero;

            var ret = DnsQuery(domain, DnsRecordType.TEXT, DnsQueryType.STANDARD, pServers, ref ppQueryResultsSet,
                               IntPtr.Zero);
            if (ret != 0) throw new ApplicationException("DnsQuery failed: " + ret);

            // Marshal DnsRecord and advance pointer to start of DNS_TXT_DATA record
            var dnsRecord = (DnsRecord) Marshal.PtrToStructure(ppQueryResultsSet, typeof (DnsRecord));
            var ptr = new IntPtr(ppQueryResultsSet.ToInt32() + Marshal.SizeOf(dnsRecord));

            // Marshal DNS_TXT_DATA and any pointers beyond pStringArray
            var txtData = (DNS_TXT_DATA) Marshal.PtrToStructure(ptr, typeof (DNS_TXT_DATA));
            ptr = new IntPtr(ptr.ToInt32() + sizeof (uint));
            var ptrs = new IntPtr[txtData.dwStringCount];
            Marshal.Copy(ptr, ptrs, 0, ptrs.Length);

            // Each pointer in ptrs points to a string. Marshal those too.
            var strings = new List<string>();
            for (var i = 0; i < ptrs.Length; ++i)
            {
                strings.Add(Marshal.PtrToStringAnsi(ptrs[i]));
            }

            DnsRecordListFree(ppQueryResultsSet, DnsFreeType.FreeRecordList);
        }
    }
}