Why Doesn't C# Allow Generic Attribute Types?

C#’s Restriction on Generic Attributes: An Explanation

C# offers extensive customization, yet notably excludes generic attribute types. This limitation prompts the question: why?

Attempting to define a custom attribute with a generic type parameter results in a compile-time error, such as:

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

[Validates<string>]
public static class StringValidation { }</code>

Although the Common Language Infrastructure (CLI) supports generic attributes, C# specifically disallows them. This isn't a CLI limitation, but rather an internal C# design choice.

Pinpointing the exact reason for this prohibition proves difficult. The C# language specification doesn't offer a clear explanation. However, community discussions and expert opinions suggest several possibilities.

One theory posits that generic attribute types would introduce unnecessary complexity to the language and compiler. Supporting this feature would demand additional language mechanisms and compiler optimizations to maintain stability and performance.

Another perspective is that the practical applications of generic attributes are limited, and the development costs outweigh the potential benefits. The C# designers may have prioritized other features.

Eric Lippert, a former C# team member, provided a succinct explanation: the restriction exists to avoid unnecessary complexity in both the language and compiler for a feature with limited practical value. While not entirely satisfying, this insight sheds light on the decision-making process behind C#'s design choices.

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