CS9328 states to use RuntimeAsyncMethodGenerationAttribute which does not exist in .NET 11

[Kaliumhexacyanoferrat]

Description

I am currently trying the runtime-async feature on my code base. One of my methods is not supported, so the compilers yields

...\MethodHandler.cs(255,13): error CS9328: Method 'MethodHandler.UnwrapAsync(object?)' uses a feature that is not supported by runtime async currently. Opt the method out of runtime async by attributing it with 'System.Runtime.CompilerServices.RuntimeAsyncMethodGenerationAttribute(false)'.

Yet when attributing the method with given attribute, the compiler yields

... net11.0 failed with 2 error(s) (0.4s)
...\MethodHandler.cs(240,38): error CS0234: The type or namespace name 'RuntimeAsyncMethodGenerationAttribute' does not exist in the namespace 'System.Runtime.CompilerServices' (are you missing an assembly reference?)

This is on 11.0.100-preview.4.26230.115

I suppose the documentation somehow drifted?

Reproduction Steps

Create a method that is not supported by runtime-async, for example:

    private static async ValueTask<object?> UnwrapAsync(object? result)
    {
        if (result == null)
        {
            return null;
        }

        var type = result.GetType();

        if (type == typeof(ValueTask) || type == typeof(Task))
        {
            dynamic task = result;

            await task;

            return null;
        }

        return result;
    }

Compile and get error CS9328.

Add the attribute to opt out:

    [System.Runtime.CompilerServices.RuntimeAsyncMethodGeneration(false)]
    private static async ValueTask<object?> UnwrapAsync(object? result)

Expected behavior

Code does compile, the method opted out of runtime-async.

Actual behavior

Compiler shows error CS0234.

Regression?

No response

Known Workarounds

No response

Configuration

.NET 11.0.100-preview.4.26230.115
Microsoft Windows 25H2 26200.8457 (x64)

Other information

No response

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[teo-tsirpanis]

Have you tried defining this attribute yourself? I'm not sure whether it is intentional that the attribute is missing from the BCL.

[Kaliumhexacyanoferrat]

Well defining it myself does not make too much sense because the boolean parameter is a contract used by the compiler. I doubt it is intended that we have to create the attribute.

[Kaliumhexacyanoferrat]

Okay, you are right, we can define this attribute ourselves to make this work.

namespace System.Runtime.CompilerServices;

[AttributeUsage(AttributeTargets.Method)]
internal class RuntimeAsyncMethodGenerationAttribute(bool runtimeAsync) : Attribute;

This is actually documented in the Runtime Async Design.md of Roslyn.

In my opinion the error message is missleading, but this will probably be solved until release, so I am closing this one.