How to get unmanaged variable-length C-array within a struct from C to C#?

Question

How do I return the MIB_IPFORWARDROW array?

struct MIB_IPFORWARDTABLE
{
    public uint Size;

    [MarshalAs(/* what goes here? */)]
    public IPFORWARDROW[] Table;
};

[DllImport("iphlpapi", CharSet = CharSet.Auto)]
private static extern int GetIpForwardTable(
    IntPtr /* MIB_IPFORWARDTABLE* */ pIpForwardTable,
    ref uint /* ULONG* */ pdwSize,
    bool bOrder);

public static MIB_IPFORWARDROW[] Temp()
{
    var fwdTable = IntPtr.Zero;
    uint size = 0;
    var result = GetIpForwardTable(fwdTable, ref size, true);
    fwdTable = Marshal.AllocHGlobal((int) size);
    result = GetIpForwardTable(fwdTable, ref size, true);

    /*
    what now ?
    I tried:

    var table = (MIB_IPFORWARDTABLE) Marshal.PtrToStructure(fwdTable, typeof(MIB_IPFORWARDTABLE));

    but table.Table is always null
    */
}

(See GetIpForwardTable)

I've seen this: How do I marshal a struct that contains a variable-sized array to C#?

But whatever I do, fwdTable.Table is always null, IntPtr.Zero or 0. That is ofcourse, if no Exception is thrown..

examine this: stackoverflow.com/questions/162897/marshal-char-in-c-sharp — Michael Kniffen
– Michael Kniffen, Commented May 22, 2014 at 14:44

Colby Cavin · Accepted Answer · 2014-05-22 15:07:12Z

This thread seems to be exactly what you are looking for: http://social.msdn.microsoft.com/Forums/vstudio/en-US/19a3ce21-e395-4151-86f6-723a64272f0d/calling-getipforwardtable-from-c?forum=csharpgeneral

Their solution employs a brute force loop to copy the raw array into a System.Array of IPFORWARDROW.

The final code would look something like this:

    [ComVisible(false), StructLayout(LayoutKind.Sequential)]
    internal struct IPForwardTable
    {
        public uint Size;

        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 1)]
        public IPFORWARDROW[] Table;
    };

    [ComVisible(false), StructLayout(LayoutKind.Sequential)]
    internal struct IPFORWARDROW
    {
        internal int /*DWORD*/ dwForwardDest;
        internal int /*DWORD*/ dwForwardMask;
        internal int /*DWORD*/ dwForwardPolicy;
        internal int /*DWORD*/ dwForwardNextHop;
        internal int /*DWORD*/ dwForwardIfIndex;
        internal int /*DWORD*/ dwForwardType;
        internal int /*DWORD*/ dwForwardProto;
        internal int /*DWORD*/ dwForwardAge;
        internal int /*DWORD*/ dwForwardNextHopAS;
        internal int /*DWORD*/ dwForwardMetric1;
        internal int /*DWORD*/ dwForwardMetric2;
        internal int /*DWORD*/ dwForwardMetric3;
        internal int /*DWORD*/ dwForwardMetric4;
        internal int /*DWORD*/ dwForwardMetric5;
    };

    [DllImport("iphlpapi", CharSet = CharSet.Auto)]
    private extern static int GetIpForwardTable(IntPtr /*PMIB_IPFORWARDTABLE*/ pIpForwardTable, ref int /*PULONG*/ pdwSize, bool bOrder);

    [DllImport("iphlpapi", CharSet = CharSet.Auto)]
    private extern static int CreateIpForwardEntry(IntPtr /*PMIB_IPFORWARDROW*/ pRoute);

    static void Main(string[] args)
    {
        var fwdTable = IntPtr.Zero;
        int size = 0;

        var result = GetIpForwardTable(fwdTable, ref size, true);
        fwdTable = Marshal.AllocHGlobal(size);

        result = GetIpForwardTable(fwdTable, ref size, true);
        var str = (IPForwardTable)Marshal.PtrToStructure(fwdTable, typeof(IPForwardTable));

        IPFORWARDROW[] table = new IPFORWARDROW[str.Size];
        IntPtr p = fwdTable + Marshal.SizeOf(str.Size);
        for (int i = 0; i < str.Size; ++i)
        {
            table[i] = (IPFORWARDROW)Marshal.PtrToStructure(p, typeof(IPFORWARDROW));
            p += Marshal.SizeOf(typeof(IPFORWARDROW));
        }


        Marshal.FreeHGlobal(fwdTable);
    }

var str = (IPForwardTable)Marshal.PtrToStructure(fwdTable, typeof(IPForwardTable)); throws NullReferenceException
oops. Cleaning up the code, I deleted a couple of important ones (like allocating the memory needed to store the result). Edited.

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