Why Are Generic Types Not Allowed as Attribute Base Types in C#?

Why C# Restricts Generic Types as Attribute Bases

C# doesn't allow generic types as attribute base types. Attempting this, as shown below, results in a compile-time error:

<code class="language-csharp">public sealed class ValidatesAttribute<T> : Attribute
{

}

[Validates<string>]
public static class StringValidation
{

}</code>

This limitation, despite C#'s extensive generic support, is puzzling to many.

The Reason Remains Elusive

The CLI specification doesn't explicitly forbid generic attributes. However, direct IL examination shows their creation is possible. This points to the restriction originating within the C# language itself.

Neither the C# 3 specification (section 10.1.4) nor the ECMA C# 2 specification offers a clear explanation for this prohibition, despite documenting other attribute limitations.

A Pragmatic Approach

Eric Lippert, a leading figure in the C# community, suggests the restriction simplifies both the language and its compiler. The perceived benefits of allowing generic attributes are deemed insignificant compared to the potential complexities they could introduce.

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