Question

Data comes from outside of the program as long array has to be converted into byte array. how to do that efficiently? As well, is there are way to select type of convertion as Little Endian or Big Endian.

Answer 1

You can do like this to convert a long array to a byte array:

bool isLittleEndian = true;
byte[] data = new byte[longData.Length * 8];
int offset = 0;
foreach (long value in longData) {
  byte[] buffer = BitConverter.GetBytes(value);
  if (BitConverter.IsLittleEndian != isLittleEndian) {
    Array.Reverse(buffer);
  }
  buffer.CopyTo(data, offset);
  offset += 8;
}

This is usually efficient enough. If you need it to be faster, you should use pointers in an unsafe code block.