Why does C# forbid generic attribute types?

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?

---

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.

---

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.

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我不知道为什么它不被允许，但这是一种可能的解决方法

[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
{
    ....
}

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