ASP.NET - Sharing state between Action Filters and Controllers

The support for ActionFilters in ASP.NET MVC and Web Api gives you a nice execution pipeline to run code before or after specific stages in the request processing pipeline:

They are a great fit for a lot of use cases, like validation, caching, … and allow you to have an AOP like approach in your controllers. Their are 2 things I find annoying about the usage of ActionFilters:

1. They introduce an extra layer of abstraction with some hidden magic. It is not always clear what’s going on the request pipeline and what happened in a specific filter.
2. It is not easy to share state between your action filter and your controller (which can be useful sometimes).

Recently I discovered a great trick that solves the problems above.

Did you know that a controller is also a filter – as it implements both IActionFilter and IAsyncActionFilter interfaces, you can override the related methods and turn your controller into an action filter?

	[Route("api/products")]
	[ApiController]
	public class ProductController : Controller
	{
	private readonly IProductRepository _repository;
	private Product _foundProduct;
	public ProductController(IProductRepository repository)
	{
	_repository =repository;
	}
	[HttpGet("{id}")]
	public async Task<ActionResult<Product>> Get(int id)
	{
	//We'll reuse the instance fetched through the filter
	return Ok(_foundProduct);
	}
	public override async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
	{
	if (context.ActionArguments.ContainsKey("id"))
	{
	var id = (int)context.ActionArguments["id"];
	_foundProduct=await _repository.Get(id);
	if (_foundProduct == null)
	{
	context.Result = new NotFoundResult();
	return;
	}
	}
	await next();
	}
	}

view raw ProductController hosted with ❤ by GitHub