ByRef-like structs can be passed via __arglist but not retrieved

[IS4Code]

Description

Calling ArgList.GetNextArg fails if the argument passed through __arglist has a ByRef-like type.

Reproduction Steps

Span<byte> s1 = stackalloc byte[1];
Span<byte> s2 = stackalloc byte[2];
Span<byte> s3 = stackalloc byte[3];
Method(__arglist(s1, s2, s3));

static void Method(__arglist)
{
    var ai = new ArgIterator(__arglist);
    while(ai.GetRemainingCount() > 0)
    {
        var tr = ai.GetNextArg();
        var val = __refvalue(tr, Span<byte>);
        Console.WriteLine(val.Length);
    }
}

Expected behavior

The value should be extracted, since C# itself allows you to pass the struct and retrieve the value without complaining, and it is not possible for the returned TypedReference to escape the method anyway.

Actual behavior

System.NotSupportedException: 'Type is not supported.' is thrown from GetNextArg.

Regression?

No response

Known Workarounds

No response

Configuration

.NET 6

Other information

No response

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[AaronRobinsonMSFT]

This is "by-design" at present. The issue here is ByRef like types are not presently supported in this part of the stack nor in any Reflection-esque APIs. Additionally the __arglist keyword and semantics are narrowly defined to support only enough to satisfy the C++/CLI semantics as they match .NET Framework on Windows and therefore no new features are likely to be supported without broader need.

As far as why this isn't naturally supported is a TypedReference is presently unable to support a ref to a ByRef-like type in a safe manner, see https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/low-level-struct-improvements.md#ref-fields-to-ref-struct. However discussion on this topic and how a TypedReference could reference a ByRef-like type are being considered for .NET 8 Reflection scenarios. See #45152 and #75358.

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Issue Details

---

Description

Calling ArgList.GetNextArg fails if the argument passed through __arglist has a ByRef-like type.

Reproduction Steps

Span<byte> s1 = stackalloc byte[1];
Span<byte> s2 = stackalloc byte[2];
Span<byte> s3 = stackalloc byte[3];
Method(__arglist(s1, s2, s3));

static void Method(__arglist)
{
    var ai = new ArgIterator(__arglist);
    while(ai.GetRemainingCount() > 0)
    {
        var tr = ai.GetNextArg();
        var val = __refvalue(tr, Span<byte>);
        Console.WriteLine(val.Length);
    }
}

Expected behavior

The value should be extracted, since C# itself allows you to pass the struct and retrieve the value without complaining, and it is not possible for the returned TypedReference to escape the method anyway.

Actual behavior

System.NotSupportedException: 'Type is not supported.' is thrown from GetNextArg.

Regression?

No response

Known Workarounds

No response

Configuration

.NET 6

Other information

No response

Author:	IllidanS4
Assignees:	-
Labels:	area-System.Reflection
Milestone:	-

[steveharter]

Moving to future. When the other higher-priority byref-like issues are resolved, this will be considered.