Bring back TypedReference

[jkotas]

TypedReference has not been very useful type so far, with non-trivial implementation cost, and that is why we have decided to not include it in .NET Standard.

However, the recent discussion #19484 revealed that it is a good building block for new high-performance reflection APIs that avoid boxing and address other reflection limitations. We should consider bringing it back.

The steps are:

1. Bring TypedReference back (to .NET Standard 2.0?)
2. Add methods to TypedReference that make it useful (e.g. use generic methods as a first class replacement for the undocumented C# __refvalue and similar keywords)
3. Add high-performance reflection APIs that avoid boxing (see Proposal: Add new GetValue method to System.Reflection #19484 for discussion)

This issue is tracking the first step.

[jkotas]

cc @Esidar @bartdesmet @MichalStrehovsky @davidwrighton

[karelz]

@weshaggard @danmosemsft any thoughts?

[danmoseley]

I have no objection (I imagine the cost of (2) and (3) is the main consideration)

I'm not sure why we'd put it in Standard -- it may be present in other platforms, but it's used for (1) __arglist that is not necessarily present (dotnet/standard@6e3ad1) and (2) these new functions which won't be present.

[weshaggard]

I also don't have any concerns as it is primarily a runtime/codegen/language feature then it is a library feature. If we did add it to .NET Standard 2.0 as @danmosemsft points out it can only contain the APIs that already exist in the full .NET framework.

[Thaina]

Could we also consider return ref in C# next version?

[jkotas]

@Thaina I am not sure what you mean. ref returns are part of C# 7. dotnet/roslyn#118. Check the readme in Roslyn on how to get latest builds of C# 7 to give it a try. Also, any questions about this feature would be best asked in the Roslyn repo.

[Thaina]

@jkotas I thought you was mention this thread to me at dotnet/coreclr#8877

What I mean is reflection GetValue should be generic and could return ref instead of TypedReference so, maybe we could drop TypedReference?

edit: well, not directly this thread. I just track this from #19484. But still the same. TypeReference might not really need if we could return ref directly

[terrajobst]

We can certainly expose TypedReference in .NET Standard, but the new methods mentioned in @jkotas' point (2) will have to wait until we can rev the standard again.

The .NET Standard work is discussed in dotnet/standard#20.

[jkotas]

I do not think we need to rush it to .NET Standard 2.0. It is good enough if we add it to netcoreapp first - together with the other APIs that will make it actually useful.

[terrajobst]

The API was already approved and it's already added to .NET Standard and .NET Core.