create client with DelegatingHandler

Author: atwayne

src/TestServer/HttpClientWrapHandler.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Net.Http;
+using System.Reflection;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Dubstep.TestUtilities
+{
+    internal class HttpClientWrapHandler : DelegatingHandler
+    {
+        private HttpClient _httpClient;
+        public HttpClientWrapHandler(HttpClient httpClient, HttpMessageHandler innerHandler)
+        {
+            _httpClient = httpClient;
+            InnerHandler = innerHandler;
+        }
+        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            var source = new CancellationTokenSource();
+            var cancelledToken = source.Token;
+            source.Cancel();
+            try
+            {
+                await base.SendAsync(request, cancelledToken);
+            }
+            catch { }
+
+            // Reset the request status and send it again with the inner httpClient
+            ResetSendStatus(request);
+            return await _httpClient.SendAsync(request, cancellationToken);
+        }
+
+        private void ResetSendStatus(HttpRequestMessage request)
+        {
+            // https://stackoverflow.com/a/43512592/553073
+            var SEND_STATUS_FIELD_NAME = "_sendStatus";
+            var requestType = request.GetType().GetTypeInfo();
+            var sendStatusField = requestType.GetField(SEND_STATUS_FIELD_NAME, BindingFlags.Instance | BindingFlags.NonPublic);
+            if (sendStatusField != null)
+                sendStatusField.SetValue(request, 0);
+            else
+                throw new Exception($"Failed to hack HttpRequestMessage, {SEND_STATUS_FIELD_NAME} doesn't exist.");
+        }
+    }
+}

src/TestServer/TestServer.cs

@@ -18,6 +18,14 @@ public TestServer()
         /// Generate a HttpClient instance
         /// </summary>
         public HttpClient CreateClient()
+        {
+            return CreateClient(null);
+        }
+
+        /// <summary>
+        /// Generate a HttpClient instance and applies additional DelegatingHandler on this client instance
+        /// </summary>
+        public HttpClient CreateClient(DelegatingHandler handler)
         {
             var builder = new WebHostBuilder()
                 .ConfigureServices(services =>
@@ -26,7 +34,19 @@ public HttpClient CreateClient()
                 })
                 .UseStartup<Startup>();
             var testServer = new InternalTestServer(builder);
-            return testServer.CreateClient();
+            var innerHttpClient = testServer.CreateClient();
+
+            if (handler == null)
+            {
+                return innerHttpClient;
+            }
+
+            var httpMessageHandler = new HttpClientWrapHandler(innerHttpClient, handler);
+            var httpClient = new HttpClient(httpMessageHandler)
+            {
+                BaseAddress = innerHttpClient.BaseAddress
+            };
+            return httpClient;
         }
     }
 }

test/TestServer.Tests/AddUserAgentHandler.cs

@@ -0,0 +1,23 @@
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Dubstep.TestUtilities.Tests
+{
+    /// <summary>
+    /// A dummy http message handler for testing purpose
+    /// </summary>
+    internal class AddUserAgentHandler : DelegatingHandler
+    {
+        public AddUserAgentHandler()
+        {
+            InnerHandler = new HttpClientHandler();
+        }
+        public static string DefaultUserAgent => nameof(AddUserAgentHandler);
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            request.Headers.Add("User-Agent", DefaultUserAgent);
+            return base.SendAsync(request, cancellationToken);
+        }
+    }
+}

test/TestServer.Tests/TestServerTests.cs

@@ -276,5 +276,26 @@ public async Task WhenMutipleRulesMatched_ShouldActionOnFirstMatch()
             // Assert
             AssertOkResponse(actual, firstResponse);
         }
+
+        [Test]
+        public async Task WhenHaveDelegatingHandler_ShouldApplyToClient()
+        {
+            // Arrange
+            var expectedUserAgent = AddUserAgentHandler.DefaultUserAgent;
+
+            _server.CurrentRuleSet
+                .AddRule()
+                .WhenPost()
+                .WhenHeaderMatch("User-Agent", expectedUserAgent)
+                .SetOkResponse(_okResponse);
+            // AddUserAgentHandler adds http header User-Agent
+            var client = _server.CreateClient(new AddUserAgentHandler());
+
+            // Act
+            var actual = await client.PostAsync("/", new StringContent("abcd"));
+
+            // Assert
+            AssertOkResponse(actual);
+        }
     }
 }