Why does C# forbid generic attribute types?

Question

This causes a compile-time exception:

public sealed class ValidatesAttribute<T> : Attribute
{

}

[Validates<string>]
public static class StringValidation
{

}

I realize C# does not support generic attributes. However, after much Googling, I can't seem to find the reason.

Does anyone know why generic types cannot derive from Attribute? Any theories?

Answer 1

An attribute decorates a class at compile-time, but a generic class does not receive its final type information until runtime. Since the attribute can affect compilation, it has to be "complete" at compile time.

See this MSDN article for more information.

Answer 2

This is a very good question. In my experience with attributes, I think the constraint is in place because when reflecting on an attribute it would create a condition in which you would have to check for all possible type permutations: typeof(Validates<string>), typeof(Validates<SomeCustomType>), etc...

In my opinion, if a custom validation is required depending on the type, an attribute may not be the best approach.

Perhaps a validation class that takes in a SomeCustomValidationDelegate or an ISomeCustomValidator as a parameter would be a better approach.

Answer 3

Well, I can't answer why it's not available, but I can confirm that it's not a CLI issue. The CLI spec doesn't mention it (as far as I can see) and if you use IL directly you can create a generic attribute. The part of the C# 3 spec that bans it - section 10.1.4 "Class base specification" doesn't give any justification.

The annotated ECMA C# 2 spec doesn't give any helpful information either, although it does provide an example of what's not allowed.

My copy of the annotated C# 3 spec should arrive tomorrow... I'll see if that gives any more information. Anyway, it's definitely a language decision rather than a runtime one.

EDIT: Answer from Eric Lippert (paraphrased): no particular reason, except to avoid complexity in both the language and compiler for a use case which doesn't add much value.

Answer 4

I don't know why it's not allowed, but this is one possible workaround

[AttributeUsage(AttributeTargets.Class)]
public class ClassDescriptionAttribute : Attribute
{
    public ClassDescriptionAttribute(Type KeyDataType)
    {
        _KeyDataType = KeyDataType;
    }

    public Type KeyDataType
    {
        get { return _KeyDataType; }
    }
    private Type _KeyDataType;
}


[ClassDescriptionAttribute(typeof(string))]
class Program
{
    ....
}