WebServer.cs

using System;
using System.Net;
using System.Net.Http;
using System.Net.Sockets;
using System.Threading;
using System.Threading.Tasks;
using System.Web.Http;
using System.Web.Http.Routing;
using Microsoft.Owin.Hosting;
using Owin;

namespace WireMockFake
{
    public class WebServer
    {
        private readonly HttpConfiguration _config;
        public int Port { get; }
        public HttpClient Client { get; }
        private IDisposable _server;

        public WebServer()
        {
            Port = FreeTcpPort();
            Client = new HttpClient { BaseAddress = new Uri($"http://localhost:{Port}") };
            _config = new HttpConfiguration();
        }

        public WebServer StartServer()
        {
            _server = WebApp.Start(new StartOptions {Port = Port, Urls = {$"http://localhost:{Port}"}}, appBuilder => appBuilder.UseWebApi(_config));
            return this;
        }

        public void StopServer()
        {
            _server?.Dispose();
        }

        public WebServer RespondTo(string url, Func<HttpRequestMessage, HttpRouteData, HttpResponseMessage> withThis = null)
        {
            withThis = withThis ?? ((message, data) => new HttpResponseMessage(HttpStatusCode.NotFound));

            _config.Routes.Add(Guid.NewGuid().ToString(),
                new HttpRoute(url, new HttpRouteValueDictionary(), new HttpRouteValueDictionary(),
                    new HttpRouteValueDictionary(), new CustomRouteHandler(withThis)));

            StopServer();
            StartServer();

            return this;
        }

        public void Dispose() => StopServer();

        private static int FreeTcpPort()
        {
            var l = new TcpListener(IPAddress.Loopback, 0);
            l.Start();
            var port = ((IPEndPoint)l.LocalEndpoint).Port;
            l.Stop();
            return port;
        }

        public class CustomRouteHandler : HttpMessageHandler
        {
            private readonly Func<HttpRequestMessage, HttpRouteData, HttpResponseMessage> _responder;

            public CustomRouteHandler(Func<HttpRequestMessage, HttpRouteData, HttpResponseMessage> responder)
            {
                _responder = responder;
            }

            protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
            {
                var routeData = request.Properties["MS_HttpRouteData"] as HttpRouteData;
                return Task.FromResult(_responder(request, routeData));
            }
        }
    }
}