Summary

Filters are MVC construct that allow developers to. Unlike middlewares, filters run after the routing middleware and have access to additional data that middlewares don’t. While middlewares operate on the HttpContext, filters have access MVC-specific constructs (such as HttpContext, RotueData, ActionDescriptor, and ModelState) that they can operate on.

Filters, and particularly ActionFilters , are heavily tied to action and controllers concepts in MVC and there are no parallels that exist for endpoints. This design doc outlines an approach for adding support for filters on endpoints that are registered as route handlers. Some general notes about the approach outlined here:

• It more closely aligns with the filter implementation in SignalR than the one in MVC for simplicity’s sake.
• It attempts to outline the “minimum viable product for most valuable feedback” or the simplest implementation we can ship that will allow us to gather feedback and review the implementation.
• The document outlines new APIs that need to be added. The API shape is there for illustrative purposes and not meant to be taken at face value.

Goals

• Provide support for filters that give access to request and response data
• Providing an “flexible” design that allows us to go in different directions in response to user feedback
• Endpoints without filters registered should not encounter any negative side effects related to perf/etc

Non-goals

• Implementing a default set of filters in the framework as part of this work
• Providing first-class support for handler-specific filters

Risks/Unknowns

• How will this API intersect with features like route groups?
• What are the performance ramifications for this implementation?

Proposed Design Walkthrough

1. The user implements UnsupportedContentTypeFilter by extending the IEndpointFilter interface.

public class UnsupportedContentTypeFilter : IEndpointFilter
{
	public override ValueTask<object> RunAsync(IEndpointFilterContext context, Func<IEndpointFilterContext, ValueTask<object?>> next)
	{
			// Assume last argument is some Boolean flag
			if (context.Parameters.Last())
			{
				var result = await next(context);
				if (result.ContentType != "application/json")
				{
					return new UnsupportedMediaTypeResult();
				}
				return result;
			}
	} 
}

• Framework Code