[API Proposal]: ParamCollectionAttribute

[AlekseyTs]

Background and motivation

As part of supporting Params Collections feature, we need a new attribute that will be applied by C# compiler to non-array params parameters.

API Proposal

This is effectively a clone of ParamArrayAttribute, different namespace, different name, different meaning.
See https://github.com/dotnet/csharplang/blob/main/proposals/params-collections.md#metadata

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Parameter, Inherited = true, AllowMultiple = false)]
    public sealed class ParamCollectionAttribute : Attribute
    {
        public ParamCollectionAttribute() { }
    }
}

API Usage

Not meant for use in C# code.

Alternative Designs

No response

Risks

No response

[AlekseyTs]

CC @stephentoub

[stephentoub]

@AlekseyTs, I assume this is on the same plan as with other similar attributes, where the compiler with synthesize it internally if it can't find it in the compilation? I'm asking because of the "blocking" label; the feature will be usable even before this is added to the core libraries, right?

[stephentoub]

Also, the "Inherited = true" part... I assume that was just copied from ParamArrayAttribute. Will that have any effect on the compiler? It's odd to me that it (both ParamArray and ParamCollection) would be inherited from a reflection perspective.

[ghost]

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Issue Details

---

Background and motivation

As part of supporting Params Collections feature, we need a new attribute that will be applied by C# compiler to non-array params parameters.

API Proposal

This is effectively a clone of ParamArrayAttribute, different namespace, different name, different meaning.
See https://github.com/dotnet/csharplang/blob/main/proposals/params-collections.md#metadata

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Parameter, Inherited = true, AllowMultiple = false)]
    public sealed class ParamCollectionAttribute : Attribute
    {
        public ParamCollectionAttribute() { }
    }
}

API Usage

Not meant for use in C# code.

Alternative Designs

No response

Risks

No response

Author:	AlekseyTs
Assignees:	-
Labels:	area-System.Runtime.CompilerServices, blocking, api-ready-for-review
Milestone:	-

[AlekseyTs]

I assume this is on the same plan as with other similar attributes, where the compiler with synthesize it internally if it can't find it in the compilation? I'm asking because of the "blocking" label; the feature will be usable even before this is added to the core libraries, right?

That is correct, but we need to make sure the API shape is approved in order to know what to synthesize.

[AlekseyTs]

Also, the "Inherited = true" part... I assume that was just copied from ParamArrayAttribute.

Yes.

Will that have any effect on the compiler? It's odd to me that it (both ParamArray and ParamCollection) would be inherited from a reflection perspective.

I do not know why that property even exists. Neither C#, nor VB compiler ever pays any attention to it.

[stephentoub]

we need to make sure the API shape is approved in order to know what to synthesize

Ok

[333fred]

I suspect that it is true because it is indeed inherited; we use the base definition to determine if a parameter is params. C# will always include ParamArrayAttribute on every override, but other languages or custom IL may not, and it's likely still desirable to have the attribute show up in reflection on overrides missing the attribute in IL.

[bartonjs]

Video

• It was asked whether there should be an "s" at the end of "Param", and the answer was "no".
• We also talked about the Inherited=true, and are sticking with "match what ParamArrayAttribute does", which is true.

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Parameter, Inherited = true, AllowMultiple = false)]
    public sealed class ParamCollectionAttribute : Attribute
    {
        public ParamCollectionAttribute() { }
    }
}