Web API 源码剖析之默认消息处理程序链之路由分发器（HttpRoutingDispatcher）

protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
// Lookup route data, or if not found as a request property then we look it up in the route table
IHttpRouteData routeData = request.GetRouteData();
if (routeData == null)
{
routeData = _configuration.Routes.GetRouteData(request);
if (routeData != null)
{
request.SetRouteData(routeData);
}
}

if (routeData == null || (routeData.Route != null && routeData.Route.Handler is StopRoutingHandler))
{
request.Properties.Add(HttpPropertyKeys.NoRouteMatched, true);
return Task.FromResult(request.CreateErrorResponse(
HttpStatusCode.NotFound,
Error.Format(SRResources.ResourceNotFound, request.RequestUri),
SRResources.NoRouteData));
}

routeData.RemoveOptionalRoutingParameters();

// routeData.Route could be null if user adds a custom route that derives from System.Web.Routing.Route explicitly
// and add that to the RouteCollection in the web hosted case
var invoker = (routeData.Route == null || routeData.Route.Handler == null) ?
_defaultInvoker : new HttpMessageInvoker(routeData.Route.Handler, disposeHandler: false);
return invoker.SendAsync(request, cancellationToken);
}

public HttpRoutingDispatcher(HttpConfiguration configuration)
: this(configuration, new HttpControllerDispatcher(configuration))
{
}