Implementing and registering a message handler in ASP.NET Web API 2

RateLimitingHandler.cs

using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

namespace MyApi.Infrastructure
{
	public class RateLimitingHandler : DelegatingHandler
	{
		private const string RateLimitHeaderName = "X-MyApi-Rate-Limit-Remaining";

		// The service that handles tracking rate limits for API consumers.
		RateLimitingService _service = new RateLimitingService();

		protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
		{
			HttpResponseMessage response;

			// Get the token that is making this request.
			string consumerToken = request.Headers.Authorization.Parameter;

			// Get the number of calls that this token is authorized to make.
			int remainingCalls = _service.GetRemainingCalls(consumerToken);

			// If the number of calls < 1 then the consumer has hit their rate limit.
			// Return an appropriate response.
			if(remainingCalls < 1)
			{
				response = request.CreateResponse(
					(HttpStatusCode)429, new { Message = "Rate limit exceeded!" });
			}
			else // Otherwise return the response from the controller.
			{
				response = await base.SendAsync(request, cancellationToken);
				// Append a custom header so the API consumer can see the remaining request limit.
				response.Headers.Add(RateLimitHeaderName, (--remainingCalls).ToString());
			}

			return response;
		}
	}
}

WebApiConfig.cs

using MyApi.Infrastructure;
using System.Web.Http;

namespace MyApi
{
    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            ...

            // Register as a global handler - all Web API requests will go through 
            // RateLimitingHandler.
            config.MessageHandlers.Add(new RateLimitingHandler());
            
            // -- OR --
            
            // Register the handler for a specific route - only Web API requests
            // that match this route will go through RateLimitingHandler.
            config.Routes.MapHttpRoute(
                name: "TasksApi",
                routeTemplate: "api/Tasks/{id}",
                defaults: new { id = RouteParameter.Optional },
				constraints: null,
				handler: new RateLimitingHandler()
            );
        }
    }
}