Access Violation in Marshal.PtrToStructure

Question

I am trying to deserialize a structure, and I'm getting an AV exception in PtrToStructure. The only wrinkle is that this is a variable length structure, so I need to adjust the length before deserializing. Here's my code, is there anything obviously wrong? The structure contains only integer/short/byte arrays, nothing fancy.

the incoming data is 374 bytes, and I need to adjust it to match the 576 bytes data structure. Basically the incoming packet has a shorter last field than the max possible, which is normal.

public static ... FromByteArray(byte[] receivedData)
    {
        int rawsize = Marshal.SizeOf(typeof(MyPacket));
    // allocate a new buffer of the maximum size, to help deserialization
    byte[] newBuffer = new byte[rawsize]; 
    Array.Copy(receivedData, newBuffer, receivedData.Length);

    IntPtr buffer = Marshal.AllocHGlobal(rawsize);
    Marshal.Copy(newBuffer, 0, buffer, rawsize);

/// CRASHES ON NEXT LINE
    MyPacketDefinition def = (MyPacketDefinition ) Marshal.PtrToStructure(buffer, typeof(MyPacketDefinition ));   
    Marshal.FreeHGlobal(buffer);

    //...
}

My structure looks something like this:

 [StructLayout (LayoutKind.Explicit, Pack=1, Size=576, CharSet=CharSet.Ansi)]
 public struct MyPacket
    {

    [FieldOffset(0)]
    public System.Byte Type;

    .
    . // a few more INT/SHORT fields
    .

        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
        [FieldOffset(28)]  public System.Byte[] Address;


        [MarshalAs(UnmanagedType.LPStr, SizeConst=64)]
        [FieldOffset(44)] public System.String Name;


        [MarshalAs(UnmanagedType.LPStr, SizeConst = 128)]
        [FieldOffset(108)] public System.String SystemData;


        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 340)]
        [FieldOffset(236)] public System.Byte[] Options;

}

With the last parameter (Option) being a variable length field of maximum 340 bytes (normally shorter)

Answer 1

I was able to get it to work using this method, as suggested by Franci in the comments (although I'm not sure if this is what he meant).

I do not know why the other way was raising an AV, I've used that method before with no issues.

    byte[] mem = new byte[sizeof(typeof(MyPacketDefinition))];
    Array.Copy(mem, receivedData, receivedData.Length);
    using (MemoryStream ms = new MemoryStream(mem))
    {
        using (BinaryReader br = new BinaryReader(ms))
        {
            byte[] buff = br.ReadBytes(Marshal.SizeOf(typeof(MyPacketDefinition )));

            GCHandle handle = GCHandle.Alloc(buff, GCHandleType.Pinned);
            try
            {
                MyPacketDefinition s = (MyPacketDefinition )Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(MyPacketDefinition ));
            }
            finally
            {
                handle.Free();
            }

        }
   }