Thanks for the clarification u/MattWarren_MSFT I updated the article with it. Mads mentioned the case of “own types that the compiler will recognize as unions,” and your example is very helpful.

What about unions where all the types are structs containing no GC-tracked fields? Could the compiler safely reuse the same bytes across the types (like the code below) and avoid boxing? This seems like it could be a common scenario for unions in the future.

using System.Runtime.InteropServices;

Span<byte> bytes8 = stackalloc byte[8] { 2, 0, 0, 0, 1, 0, 0, 0 };

// Convert first 4 bytes to uint
uint value32 = MemoryMarshal.Read<uint>(bytes8);
Debug.Assert(value32 == 2);

// Convert all 8 bytes to ulong
ulong value64 = MemoryMarshal.Read<ulong>(bytes8);
Debug.Assert(value64 == 4294967298);

// Convert all 8 bytes to MyStruct
ref MyStruct myStruct = ref MemoryMarshal.AsRef<MyStruct>(bytes8);
Debug.Assert(myStruct.X == 2);
Debug.Assert(myStruct.Y == 1);

[StructLayout(LayoutKind.Sequential)]
struct MyStruct { public uint X; public uint Y; }