MemoryMarshal.{Try}Read/{Try}Write/AsRef should allow ref struct

[hez2010]

We are missing where T : allows ref struct on:

• MemoryMarshal.Read
• MemoryMarshal.TryRead
• MemoryMarshal.Write
• MemoryMarshal.TryWrite
• MemoryMarshal.AsRef
• MemoryMarshal.AsRef (readonly ref)

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[huoyaoyuan]

All of these are blocking IsReferenceOrContainsReferences because of GC safety. A ref struct without ref field is not so meaningful.

[hez2010]

All of these are blocking IsReferenceOrContainsReferences because of GC safety.

Yeah. But note that IsReferenceOrContainsReferences<T>() has where T: allows ref struct so it shouldn't be a problem in terms of compilation.

A ref struct without ref field is not so meaningful.

You don't have control to types coming from other libs. Some libs just define all their value types as ref struct to avoid unexpected boxing regardless of whether it has a ref field in it or not.

We can already do Unsafe.ReadUnaligned<Foo>(ref MemoryMarshal.GetReference(span)) (where Foo is a ref struct), so I don't see a reason why its short-hand MemoryMarshal.Read<Foo>(span) should be blocked.

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[huoyaoyuan]

We can already do Unsafe.ReadUnaligned<Foo>(ref MemoryMarshal.GetReference(span)) (where Foo is a ref struct), so I don't see a reason why its short-hand MemoryMarshal.Read<Foo>(span) should be blocked.

Unsafe.ReadUnaligned is not GC safe. It can also be used with class to read a GC reference from byte span. Using it with reference type or Span<T> can easily cause GC hole and access violation.

MemoryMarshal is safer than Unsafe. All its APIs don't allow breaking the boundary between value and GC ref.

You don't have control to types coming from other libs. Some libs just define all their value types as ref struct to avoid unexpected boxing regardless of whether it has a ref field in it or not.

Using pointer should be sufficient in these cases.

[hez2010]

MemoryMarshal is safer than Unsafe

That's why we need it to take a ref struct so that it can help block invalid usage because it checks gc pointer for you. Without the support, people will just copy whatever the code from its implementation and make their code even more unsafe.

Using pointer should be sufficient in these cases.

This doesn't hold as you can replace almost all Unsafe.* API usage by using pointers, so it should have been so sufficient that Unsafe.* API shouldn't even have existed in the first place.