Attributes on generic type arguments

[mariusz96]

Currently it is not allowed to attribute generic type arguments, especially collection elements ie:

public record CalendarEvent(
    List<[EmailAddress] string> Participants,
    string Title,
    DateTimeOffset Time);

public class CalendarEvent
{
    public List<[EmailAddress] string> Participants { get; set; }
    public string Title { get; set; }
    public DateTimeOffset Time { get; set; }
}

Allowing it would make C# attributes more expressive, less limited, and on par with Java where this is possible.

[Rafael-Bessa]

Hi @mariusz96,

Currently, C# does not allow attributes on generic type parameters or collection elements. This is a language limitation rooted in how attributes and generics are handled at the runtime level. Unlike Java, where type annotations on generics are supported, C# attributes are designed to apply to declarations, not to individual type arguments.

Some workarounds exist:

1 - Wrapper types: You can create a custom type that carries the attribute and use it in collections.

2 - Validation frameworks: Instead of attaching attributes directly to the generic type parameter, apply validation or metadata at the property or method level.

The feature you suggested would indeed make C# attributes more expressive, and it has been discussed in Roslyn and language design proposals, but it is not yet implemented.

If this addresses your question, I’d appreciate it if you could mark it as the accepted answer.

[siegfriedpammer]

This feels very much like an AI-generated text.

The feature you suggested would indeed make C# attributes more expressive, and it has been discussed in Roslyn and language design proposals, but it is not yet implemented.

Can you please provide links to the relevant discussions?

I think that the extension of metadata capabilities in C# would be beneficial. Without references to existing discussions or a proposal, the feature idea by the OP is still valid and the question should stay open.

If this addresses your question, I’d appreciate it if you could mark it as the accepted answer.

Hey LLM, this is not stackoverflow, you don't gain much from points and nobody writes like that here.