C# 11–Generic Attributes

C# 11 introduces a large list of new language features. One of the smaller features is the support for generic attributes.

Before C# 11, when you had to pass type information to an attribute, you had to use a type parameter.

Let’s use an example that always has annoyed me; the [ServiceFilterAttribute]. If you have ever used an ASP.NET MVC filter that needed dependencies, then you should know this attribute.

Here is an example I copied from the Microsoft documentation:

	[ServiceFilter(typeof(LoggingResponseHeaderFilterService))]
	public IActionResult WithServiceFilter() =>
	Content($"- {nameof(FilterDependenciesController)}.{nameof(WithServiceFilter)}");

view raw ServiceFilterAttribute.cs hosted with ❤ by GitHub

You see in the example above is that the type of the actual filter we want to invoke is passed as a parameter to the ServiceFilterAttribute.

The constructor looks something like this:

	/// <summary>
	/// Instantiates a new <see cref="ServiceFilterAttribute"/> instance.
	/// </summary>
	/// <param name="type">The <see cref="Type"/> of filter to find.</param>
	public ServiceFilterAttribute(Type type)
	{
	ServiceType = type ?? throw new ArgumentNullException(nameof(type));
	}

view raw ServiceFilterAttribute.cs hosted with ❤ by GitHub

By not being able to use generics, I could pass anything I want as a parameter to the ServiceFilter no matter if it is actually a filter or not. The compiler will not help me to avoid passing an incorrect argument.

A type safe ServiceFilter

With the release of C# 11 and the introduction of Generic Attributes, we can create a type safe variant of the [ServiceFilterAttribute] as an example:

	[Route("api/[controller]")]
	[ApiController]
	public class CoursesController : ControllerBase
	{
	[HttpGet]
	[TypeSafeServiceFilterAttribute<ExampleFilter>()]
	public IActionResult Get()
	{
	return Ok();
	}
	}

view raw CoursesController.cs hosted with ❤ by GitHub

	public class ExampleFilter : IActionFilter
	{
	public void OnActionExecuting(ActionExecutingContext context)
	{
	}
	public void OnActionExecuted(ActionExecutedContext context)
	{
	}
	}

view raw ExampleFilter.cs hosted with ❤ by GitHub

	public class TypeSafeServiceFilterAttribute<T> : ServiceFilterAttribute where T: IActionFilter
	{
	public TypeSafeServiceFilterAttribute():base(typeof(T))
	{
	}
	}

view raw TypeSafeServiceFilterAttribute.cs hosted with ❤ by GitHub

Now the compiler will warn me when I try to pass a type that isn’t a filter:

Remark: There are multiple interfaces that can be used to implement a filter. In the example above I only used the IActionFilter. So this is not a production ready solution for all use cases.