Extend the list of permitted attribute parameter types

[adejongh93]

Background

This code throws compile error

using System;

namespace CSharp_Language_Proposals.AttributeProposal
{
    class CustomAttribute : Attribute
    {
        public CustomAttribute(DateTime param)
        {
        }
    }

    class Class1
    {
        [Custom(DateTime.Now)] // CS0181 Attribute constructor parameter 'param' has type DateTime,
                               // which is not a valid attribute parameter type
        void Method()
        {
        }
    }
} 

Proposal

Modify the compiler so the attributes would accept new types and runtime evaluated expressions.

using System;

namespace CSharp_Language_Proposals.AttributeProposal
{
    class CustomAttribute : Attribute
    {
        public CustomAttribute(DateTime param)
        {
        }
    }

    class Class1
    {
        [Custom(DateTime.Now)] // Compiler builds correctly.
        void Method()
        {
        }
    }
} 

[bernd5]

What should be the value of "DateTime.Now"? Should it be the time when the binary was build? Should it re-evaluated at runtime to the current time?
Why not implementing such logic in the attribute? To configure it you could use constants, e.g. strings and enums.

[Unknown6656]

Mildly related:

• Champion "Allow Generic Attributes" #124 (see https://github.com/dotnet/roslyn/tree/features/generic-attributes)
• Allowing runtime evaluated expressions in user defined Attributes #4770
• Proposal: Allow Lambda as Argument to an Attribute #343
• [Proposal] Allow generic attributes roslyn#953

There was one other proposal, but I was unable to find it. It was about compile-time constant values for arbitrary blittable/struct types.

[HaloFour]

The TL;DR of all of the referenced issues/discussions is that the list of supported types permitted in an attribute's properties and parameters are limited by the runtime. The runtime would need to be modified to support other types and establish how those types are encoded into the metadata of the assembly.

[Unknown6656]

That is indeed true, however I am pretty sure that there was a proposal to enable compile-time constants for arbitrary blittable/struct types. This would allow System.DateTime to be used as const fields, as default parameter value, and as Attribute argument.

(though I am currently unable to find the proposal. maybe it was in dotnet/roslyn)

[HaloFour]

That depends on how it's implemented. Languages can already fake constants, which is why C# can support constants and default parameter values of System.Decimal, and why VB.NET can support constants and default parameter values of System.DateTime. But neither can be used in attributes because the runtime is not in on it. So any support for blittable constants would need to have runtime support in order to expand on the parameters allowed in attributes.

Lambdas would be a bit of a different ballgame, though. I guess technically the method token could be a constant value which could be encoded in the assembly metadata.

[acaly]

Limitation of runtime shouldn't be limitation of adding new features. There are many new features added to .NET requiring both sides to work cooperatively.

It is equally valid for the runtime maintainer to say that no language will support this feature so runtime won't add it.