Help Pythons "struct.pack_into" in C#?

Code

    while fileSize > 0:

        chunkIndex += 1 # Increment chunk

        struct.pack_into("<H", packet, 18, chunkIndex) # Poke in chunk index

        struct.pack_into("<H", packet, 2, checksum + chunkIndex) # Checksum

        

        sock.sendto(packet, addr)

        data = sock.recvfrom(0x400)[0]

        

        if not buffer: # First chunk, extract file size

            fileSize = struct.unpack_from("<I", data, 20)[0]

            buffer = BytesIO()

        

        # Append chunk data to return buffer

        buffer.write(data[24:])

        fileSize -= 0x300

    

    # Be kind, rewind

    if buffer: buffer.seek(0)

    return buffer

Code

while (fileSize > 0)

                {

                    chunkIndex++;

                    BitConverter.GetBytes(chunkIndex).CopyTo(packet2, 18); // Poke in chunk index

                    BitConverter.GetBytes((ushort)(checksum + chunkIndex)).CopyTo(packet2, 2); // Checksum



                    //sock.Send(packet, packet.Length, addr);

                    sock.SendTo(packet2, packet2.Length, SocketFlags.None, addr);

                    Console.WriteLine("SENDING:   " + Encoding.UTF8.GetString(packet2));

                    //var data = sock.Receive(ref addr);

                    IPEndPoint sender = new IPEndPoint(IPAddress.Parse(IP), 9520);

                    EndPoint remote = (EndPoint)sender;



                    int v = sock.ReceiveFrom(data, ref remote);

                    Console.WriteLine("RECEIVING:   ");

                    if (buffer == null)

                    {

                        fileSize = BitConverter.ToInt32(data, 20);

                        buffer = new MemoryStream();

                    }



                    int lastIndex = Array.FindLastIndex(data, b => b != 0);



                    if (lastIndex >= 24) //Dirty - Removes NUL bytes from the UDP Stream.

                    {

                        buffer.Write(data, 24, lastIndex - 23);

                    } else {

                        buffer.Write(data, 24, data.Length - 24);

                    }

                    Array.Clear(data); //We clear the data array on each loop to ensure we don't write the same data twice, if the data received is smaller than the size of the array.

                    fileSize -= 0x300;

                }