List.Reverse calls Array.Reverse, which may or may not box

You'd expect List<T>.Reverse to not box (ie, allocate memory on the managed heap) when you're using structs, right? Well, you would be wrong. And you'd expect List<T>.Reverse to not box when using basic builtin types like short, byte, right? Sometimes, you may be right.

This is the Array.Reverse implementation in Unity's Mono (some branch of 2.x). It won't box values when the array (List<T> uses an array internally) is an int[] or double[], but will on everything else (see line 1261).

This is the Array.Reverse implementation in the latest version of Mono. It won't box arrays of builtin types, but your enums and custom structs will still result in boxed values.

This is the Array.Reverse implementation in MS.NET's reference source. At the time of this writing, it will try to call the externally defined TrySZReverse function before going on to just box all teh things. Apparently, according to this API issue, that function is a fast-path for reversing primitive types (but your structs, and I'd imagine your enums, don't fall into that category). That API issue is up for review, so it could be that MS.NET will have sane generic Reverse'ing in the future with no boxing!

So yes, you'd expect that your .NET runtimes wouldn't box when performing List<T>.Reverse or Array.Reverse<T> when T is a value type, but you'd mostly be wrong most of the time.

Leave the boxing to Mike Tyson and Rocky!