Casting a byte array to a managed structure

Question

Update: Answers to this question helped me code the open sourced project AlicanC's Modern Warfare 2 Tool on GitHub. You can see how I am reading these packets in MW2Packets.cs and the extensions I've coded to read big endian data in Extensions.cs.

I am capturing UDP packets of Call of Duty: Modern Warfare 2 using Pcap.Net in my C# application. I receive a byte[] from the library. I tried to parse it like a string, but that didn't work well.

The byte[] I have has a generic packet header, then another header specific to the packet type then info about each player in the lobby.

A helpful person inspected some packets for me and came up with these structures:

// Fields are big endian unless specified otherwise.
struct packet_header
{
    uint16_t magic;
    uint16_t packet_size;
    uint32_t unknown1;
    uint32_t unknown2;
    uint32_t unknown3;
    uint32_t unknown4;
    uint16_t unknown5;
    uint16_t unknown6;
    uint32_t unknown7;
    uint32_t unknown8;
    cstring_t packet_type; // \0 terminated string
};

// Fields are little endian unless specified otherwise.
struct header_partystate //Header for the "partystate" packet type
{
    uint32_t unknown1;
    uint8_t unknown2;
    uint8_t player_entry_count;
    uint32_t unknown4;
    uint32_t unknown5;
    uint32_t unknown6;
    uint32_t unknown7;
    uint8_t unknown8;
    uint32_t unknown9;
    uint16_t unknown10;
    uint8_t unknown11;
    uint8_t unknown12[9];
    uint32_t unknown13;
    uint32_t unknown14;
    uint16_t unknown15;
    uint16_t unknown16;
    uint32_t unknown17[10];
    uint32_t unknown18;
    uint32_t unknown19;
    uint8_t unknown20;
    uint32_t unknown21;
    uint32_t unknown22;
    uint32_t unknown23;
};

// Fields are little endian unless specified otherwise.
struct player_entry
{
    uint8_t player_id;

    // The following fields may not actually exist in the data if it's an empty entry.
    uint8_t unknown1[3];
    cstring_t player_name;
    uint32_t unknown2;
    uint64_t steam_id;
    uint32_t internal_ip;
    uint32_t external_ip;
    uint16_t unknown3;
    uint16_t unknown4;
    uint32_t unknown5;
    uint32_t unknown6;
    uint32_t unknown7;
    uint32_t unknown8;
    uint32_t unknown9;
    uint32_t unknown10;
    uint32_t unknown11;
    uint32_t unknown12;
    uint16_t unknown13;
    uint8_t unknown14[???];     // Appears to be a bit mask, sometimes the length is zero, sometimes it's one. (First entry is always zero?)
    uint8_t unknown15;
    uint32_t unknown16;
    uint16_t unknown17;
    uint8_t unknown18[???];     // Most of the time this is 4 bytes, other times it is 3 bytes.
};

I recreated the packet header structure in my C# application like this:

[StructLayout(LayoutKind.Sequential, Pack=1)]
struct PacketHeader
{
    public UInt16 magic;
    public UInt16 packetSize;
    public UInt32 unknown1;
    public UInt32 unknown2;
    public UInt32 unknown3;
    public UInt32 unknown4;
    public UInt16 unknown5;
    public UInt16 unknown6;
    public UInt32 unknown7;
    public UInt32 unknown8;
    public String packetType;
}

Then I tried to make a structure for the "partystate" header, but I got errors saying fixed keyword is unsafe:

[StructLayout(LayoutKind.Sequential, Pack=1)]
struct PartyStateHeader
{
    UInt32 unknown1;
    Byte unknown2;
    Byte playerEntryCount;
    UInt32 unknown4;
    UInt32 unknown5;
    UInt32 unknown6;
    UInt32 unknown7;
    Byte unknown8;
    UInt32 unknown9;
    UInt16 unknown10;
    Byte unknown11;
    fixed Byte unknown12[9];
    UInt32 unknown13;
    UInt32 unknown14;
    UInt16 unknown15;
    UInt16 unknown16;
    fixed UInt32 unknown17[10];
    UInt32 unknown18;
    UInt32 unknown19;
    Byte unknown20;
    UInt32 unknown21;
    UInt32 unknown22;
    UInt32 unknown23;
}

I couldn't do anything for the player entries because of the varying size of unknown14 and unknown18. (Player entries are the most important.)

Now, somehow, I have to cast the byte[] I have to these PacketHeader structures. Sadly, it's not easy as (PacketHeader)bytes. I tried this method I've found on the internet but it threw an AccessViolationException:

GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
PacketHeader packetHeader = (PacketHeader)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(PacketHeader));

How can I achieve this?

Answer 1

//I have found this at: http://code.cheesydesign.com/?p=572 (I have not tested yet, but // at first sight it will work well.)

    /// <summary>
    /// Reads in a block from a file and converts it to the struct
    /// type specified by the template parameter
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="reader"></param>
    /// <returns></returns>
    private static T FromBinaryReader<T>(BinaryReader reader)
    {

        // Read in a byte array
        byte[] bytes = reader.ReadBytes(Marshal.SizeOf(typeof(T)));

        // Pin the managed memory while, copy it out the data, then unpin it
        GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
        T theStructure = (T)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(T));
        handle.Free();

        return theStructure;
    }