Runtime: [API Proposal] API for interpretting Span<byte> as struct without copying

[jkotas]

MemoryMarshal contains several APIs that allow reinterpreting Span<byte>, but none of them are a good fit for interpreting Span as struct without copying:

• MemoryMarshal.Read allows interpreting Span<byte> as arbitrary struct. It creates a local copy that is fine and beneficial for small structs, but becomes expensive for large structs.
• MemoryMarshal.Cast allows interpreting Span<byte> as sequence of arbitrary structs. It does not create a local copy, but comes with extra overhead required to compute the length of the resulting span.
• MemoryMarshal.GetReference + Unsafe.As does not create a local copy, but also skips any bounds checks that is not good for security defense in depth.

Proposal

Add methods that allow interpreting Span<byte> as arbitrary large struct without copying to MemoryMarshal.

static class MemoryMarshal
{
    public static ref readonly T AsRef<T>(System.ReadOnlySpan<byte> span) where T : struct { throw null; }
    public static ref T AsRef(System.Span<byte> span) where T : struct { throw null; }
}

Implementation of these methods is going to be faster functional equivalent of ref MemoryMarshal.Cast<byte, T>(span)[0].

These APIs come with the same set of portability caveats as Memory.Cast. They can be only used when the memory block is sufficiently aligned by other means; or when the target platform does not require strong memory alignment.

[jkotas]

Related proposal for other direction dotnet/runtime#24656

dotnet/corefx#30614 has multiple examples of how these methods can be used in real-world code.

[benaadams]

A bit like Marshal.PtrToStructure<T> Method (IntPtr) but generally safer, including a >= sizeof(T) check; and ref returning rather than copying?

[jkotas]

Right.

[morganbr]

Could this use the unmanaged constraint instead of the struct constraint? GC references wouldn't be a good thing with this API.

[jkotas]

The implementation will enforce no-GC references at runtime just like MemoryMarshal.Cast and other similar APIs. unmanaged constrain is enforcing more than no-GC references. It would make this API less useful and inconsistent with the rest of the MemoryMarshal and Span methods.

[terrajobst]

Video

This API and scenarios make sense but we shouldn't use Struct in the name. It seems to make more sense to align this with Unsafe.AsRef(), thus:

static class MemoryMarshal
{
    public static ref readonly T AsRef<T>(ReadOnlySpan<byte> span) where T : struct;
    public static ref T AsRef(Span<byte> span) where T : struct;
}

[mgravell]

@terrajobst where T : unmanaged ?

[jkotas]

@mgravell https://github.com/dotnet/corefx/issues/30613#issuecomment-400143239

[mgravell]

@jkotas apparently I fail at reading today, thx

[virzak]

Is it possible to use AsRef in .NET Standard 2.1? I was only able to use it targeting netcoreapp3.0

[danmosemsft]

@virzak best to ask this in the dotnet/standard repo