public void WriteInt64(long i)
{
Contract.Requires(WriteSpaceLeft >= sizeof(long));
var pos = _writePosition;
_buf[pos++] = (byte)(i >> 56);
_buf[pos++] = (byte)(i >> 48);
_buf[pos++] = (byte)(i >> 40);
_buf[pos++] = (byte)(i >> 32);
_buf[pos++] = (byte)(i >> 24);
_buf[pos++] = (byte)(i >> 16);
_buf[pos++] = (byte)(i >> 8);
_buf[pos++] = (byte)i;
_writePosition = pos;
}