Hi
Span<Message> valSpan stackalloc Message[1] ;
valSpan[0] =Message {Header = 123, Data = 456 } ;
Span<byte> bytes = MemoryMarshal.Cast<Message, byte>(valSpan); // has a length of 8
Note that I am using Span<T> here. You can do much the same with Memory<T> if necessary - but you need a support array or similar, which will usually require allocation - if not the array, then a custom MemoryManager<T> :
var arr = new message[1] ;
arr[0] = new message {Header = 123, Data = 456 } ;
Memory<byte> bytes = MemoryMarshal.Cast<Message, byte>(arr); // has a length of 8
Essentially, you are very close here:
MemoryMarshal.CreateReadOnlySpan(ref message, 1)
The trick, however, is to use MemoryMarshal.Cast<TFrom
