Add a new UseKestel API to the WebApplicationFactory

src/Mvc/Mvc.Testing/src/Microsoft.AspNetCore.Mvc.Testing.csproj

@@ -13,6 +13,7 @@
   <ItemGroup>
     <Reference Include="Microsoft.AspNetCore.TestHost" />
     <Reference Include="Microsoft.AspNetCore.Mvc.Core" />
+    <Reference Include="Microsoft.AspNetCore.Server.Kestrel" />
     <Reference Include="Microsoft.Extensions.DependencyModel" />
     <Reference Include="Microsoft.Extensions.Hosting" />
     <Reference Include="Microsoft.Extensions.HostFactoryResolver.Sources" />

src/Mvc/Mvc.Testing/src/PublicAPI.Unshipped.txt

@@ -1 +1,5 @@
 #nullable enable
+Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory<TEntryPoint>.StartServer() -> void
+Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory<TEntryPoint>.UseKestrel() -> void
+Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory<TEntryPoint>.UseKestrel(int port) -> void
+Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory<TEntryPoint>.UseKestrel(System.Action<Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerOptions!>! configureKestrelOptions) -> void

src/Mvc/Mvc.Testing/src/Resources.resx

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <root>
   <!-- 
     Microsoft ResX Schema 
@@ -126,4 +126,13 @@
   <data name="MissingDepsFile" xml:space="preserve">
     <value>Can't find '{0}'. This file is required for functional tests to run properly. There should be a copy of the file on your source project bin folder. If that is not the case, make sure that the property PreserveCompilationContext is set to true on your project file. E.g '&lt;PreserveCompilationContext&gt;true&lt;/PreserveCompilationContext&gt;'. For functional tests to work they need to either run from the build output folder or the {1} file from your application's output directory must be copied to the folder where the tests are running on. A common cause for this error is having shadow copying enabled when the tests run.</value>
   </data>
+  <data name="ServerNotInitialized" xml:space="preserve">
+    <value>Server hasn't been initialized yet. Please initialize the server first before trying to create a client.</value>
+  </data>
+  <data name="TestServerNotSupportedWhenUsingKestrel" xml:space="preserve">
+    <value>Accessing the `Server` property isn't supported when using Kestrel server.</value>
+  </data>
+  <data name="UseKestrelCanBeCalledBeforeInitialization" xml:space="preserve">
+    <value>UseKestrel should be called before server initialization. Calling UseKestrel after the server was initialized will have no effect.</value>
+  </data>
 </root>

src/Mvc/Mvc.Testing/src/WebApplicationFactory.cs

@@ -1,14 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Net.Http;
 using System.Reflection;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.AspNetCore.Server.Kestrel.Core;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
@@ -26,9 +27,16 @@ public partial class WebApplicationFactory<TEntryPoint> : IDisposable, IAsyncDis
 {
     private bool _disposed;
     private bool _disposedAsync;
+
+    private bool _useKestrel;
+    private int? _kestrelPort;
+    private Action<KestrelServerOptions>? _configureKestrelOptions;
+
     private TestServer? _server;
     private IHost? _host;
     private Action<IWebHostBuilder> _configuration;
+    private IWebHost? _webHost;
+    private Uri? _webHostAddress;
     private readonly List<HttpClient> _clients = new();
     private readonly List<WebApplicationFactory<TEntryPoint>> _derivedFactories = new();
 
@@ -75,8 +83,13 @@ public TestServer Server
     {
         get
         {
-            EnsureServer();
-            return _server;
+            if (_useKestrel)
+            {
+                throw new NotSupportedException(Resources.TestServerNotSupportedWhenUsingKestrel);
+            }
+
+            StartServer();
+            return _server!;
         }
     }
 
@@ -87,11 +100,21 @@ public virtual IServiceProvider Services
     {
         get
         {
-            EnsureServer();
-            return _host?.Services ?? _server.Host.Services;
+            StartServer();
+            if (_useKestrel)
+            {
+                return _webHost!.Services;
+            }
+
+            return _host?.Services ?? _server!.Host.Services;
         }
     }
 
+    /// <summary>
+    /// Helps determine if the `StartServer` method has been called already.
+    /// </summary>
+    private bool ServerStarted => _webHost != null || _host != null || _server != null;
+
     /// <summary>
     /// Gets the <see cref="IReadOnlyList{WebApplicationFactory}"/> of factories created from this factory
     /// by further customizing the <see cref="IWebHostBuilder"/> when calling
@@ -136,10 +159,92 @@ internal virtual WebApplicationFactory<TEntryPoint> WithWebHostBuilderCore(Actio
         return factory;
     }
 
-    [MemberNotNull(nameof(_server))]
-    private void EnsureServer()
+    /// <summary>
+    /// Configures the factory to use Kestrel as the server.
+    /// </summary>
+    public void UseKestrel()
     {
-        if (_server != null)
+        if (ServerStarted)
+        {
+            throw new InvalidOperationException(Resources.UseKestrelCanBeCalledBeforeInitialization);
+        }
+
+        _useKestrel = true;
+    }
+
+    /// <summary>
+    /// Configures the factory to use Kestrel as the server.
+    /// </summary>
+    /// <param name="port">The port to listen to when the server starts. Use `0` to allow dynamic port selection.</param>
+    /// <exception cref="InvalidOperationException">Thrown, if this method is called after the WebHostFactory has been initialized.</exception>
+    /// <remarks>This method should be called before the factory is initialized either via one of the <see cref="CreateClient()"/> methods
+    /// or via the <see cref="StartServer"/> method.</remarks>
+    public void UseKestrel(int port)
+    {
+        UseKestrel();
+
+        this._kestrelPort = port;
+    }
+
+    /// <summary>
+    /// Configures the factory to use Kestrel as the server.
+    /// </summary>
+    /// <param name="configureKestrelOptions">A callback handler that will be used for configuring the server when it starts.</param>
+    /// <exception cref="InvalidOperationException">Thrown, if this method is called after the WebHostFactory has been initialized.</exception>
+    /// <remarks>This method should be called before the factory is initialized either via one of the <see cref="CreateClient()"/> methods
+    /// or via the <see cref="StartServer"/> method.</remarks>
+    public void UseKestrel(Action<KestrelServerOptions> configureKestrelOptions)
+    {
+        UseKestrel();
+
+        this._configureKestrelOptions = configureKestrelOptions;
+    }
+
+    private IWebHost CreateKestrelServer(IWebHostBuilder builder)
+    {
+        ConfigureBuilderToUseKestrel(builder);
+
+        var host = builder.Build();
+
+        TryConfigureServerPort(() => GetServerAddressFeature(host));
+
+        host.Start();
+        return host;
+    }
+
+    private void TryConfigureServerPort(Func<IServerAddressesFeature?> serverAddressFeatureAccessor)
+    {
+        if (_kestrelPort.HasValue)
+        {
+            var saf = serverAddressFeatureAccessor();
+            if (saf is not null)
+            {
+                saf.Addresses.Clear();
+                saf.Addresses.Add($"http://127.0.0.1:{_kestrelPort}");
+                saf.PreferHostingUrls = true;
+            }
+        }
+    }
+
+    private void ConfigureBuilderToUseKestrel(IWebHostBuilder builder)
+    {
+        if (_configureKestrelOptions is not null)
+        {
+            builder.UseKestrel(_configureKestrelOptions);
+        }
+        else
+        {
+            builder.UseKestrel();
+        }
+    }
+
+    /// <summary>
+    /// Initializes the instance by configurating the host builder.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">Thrown if the provided <typeparamref name="TEntryPoint"/> type has no factory method.</exception>
+    public void StartServer()
+    {
+        if (ServerStarted)
         {
             return;
         }
@@ -197,21 +302,53 @@ private void EnsureServer()
         {
             SetContentRoot(builder);
             _configuration(builder);
-            _server = CreateServer(builder);
+            if (_useKestrel)
+            {
+                _webHost = CreateKestrelServer(builder);
+
+                TryExtractHostAddress(GetServerAddressFeature(_webHost));
+            }
+            else
+            {
+                _server = CreateServer(builder);
+            }
+        }
+    }
+
+    private void TryExtractHostAddress(IServerAddressesFeature? serverAddressFeature)
+    {
+        if (serverAddressFeature?.Addresses.Count > 0)
+        {
+            // Store the web host address as it's going to be used every time a client is created to communicate to the server
+            _webHostAddress = new Uri(serverAddressFeature.Addresses.Last());
+            ClientOptions.BaseAddress = _webHostAddress;
         }
     }
 
-    [MemberNotNull(nameof(_server))]
     private void ConfigureHostBuilder(IHostBuilder hostBuilder)
     {
         hostBuilder.ConfigureWebHost(webHostBuilder =>
         {
             SetContentRoot(webHostBuilder);
             _configuration(webHostBuilder);
-            webHostBuilder.UseTestServer();
+            if (_useKestrel)
+            {
+                ConfigureBuilderToUseKestrel(webHostBuilder);
+            }
+            else
+            {
+                webHostBuilder.UseTestServer();
+            }
         });
         _host = CreateHost(hostBuilder);
-        _server = (TestServer)_host.Services.GetRequiredService<IServer>();
+        if (_useKestrel)
+        {
+            TryExtractHostAddress(GetServerAddressFeature(_host));
+        }
+        else
+        {
+            _server = (TestServer)_host.Services.GetRequiredService<IServer>();
+        }
     }
 
     private void SetContentRoot(IWebHostBuilder builder)
@@ -438,10 +575,15 @@ private static void EnsureDepsFile()
     protected virtual IHost CreateHost(IHostBuilder builder)
     {
         var host = builder.Build();
+        TryConfigureServerPort(() => GetServerAddressFeature(host));
         host.Start();
         return host;
     }
 
+    private static IServerAddressesFeature? GetServerAddressFeature(IHost host) => host.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>();
+
+    private static IServerAddressesFeature? GetServerAddressFeature(IWebHost webHost) => webHost.ServerFeatures.Get<IServerAddressesFeature>();
+
     /// <summary>
     /// Gives a fixture an opportunity to configure the application before it gets built.
     /// </summary>
@@ -455,8 +597,21 @@ protected virtual void ConfigureWebHost(IWebHostBuilder builder)
     /// redirects and handles cookies.
     /// </summary>
     /// <returns>The <see cref="HttpClient"/>.</returns>
-    public HttpClient CreateClient() =>
-        CreateClient(ClientOptions);
+    public HttpClient CreateClient()
+    {
+        var client = CreateClient(ClientOptions);
+
+        if (_useKestrel && object.ReferenceEquals(client.BaseAddress, WebApplicationFactoryClientOptions.DefaultBaseAddres))
+        {
+            // When using Kestrel, the server may start to listen on a pre-configured port,
+            // which can differ from the one configured via ClientOptions.
+            // Hence, if the ClientOptions haven't been set explicitly to a custom value, we will assume that
+            // the user wants the client to communicate on a port that the server listens too, and because that port may be different, we overwrite it here.
+            client.BaseAddress = ClientOptions.BaseAddress;
+        }
+
+        return client;
+    }
 
     /// <summary>
     /// Creates an instance of <see cref="HttpClient"/> that automatically follows
@@ -476,12 +631,24 @@ public HttpClient CreateClient(WebApplicationFactoryClientOptions options) =>
     /// <returns>The <see cref="HttpClient"/>.</returns>
     public HttpClient CreateDefaultClient(params DelegatingHandler[] handlers)
     {
-        EnsureServer();
+        StartServer();
 
         HttpClient client;
         if (handlers == null || handlers.Length == 0)
         {
-            client = _server.CreateClient();
+            if (_useKestrel)
+            {
+                client = new HttpClient();
+            }
+            else
+            {
+                if (_server is null)
+                {
+                    throw new InvalidOperationException(Resources.ServerNotInitialized);
+                }
+
+                client = _server.CreateClient();
+            }
         }
         else
         {
@@ -490,7 +657,7 @@ public HttpClient CreateDefaultClient(params DelegatingHandler[] handlers)
                 handlers[i - 1].InnerHandler = handlers[i];
             }
 
-            var serverHandler = _server.CreateHandler();
+            var serverHandler = CreateHandler();
             handlers[^1].InnerHandler = serverHandler;
 
             client = new HttpClient(handlers[0]);
@@ -503,6 +670,21 @@ public HttpClient CreateDefaultClient(params DelegatingHandler[] handlers)
         return client;
     }
 
+    private HttpMessageHandler CreateHandler()
+    {
+        if (_useKestrel)
+        {
+            return new HttpClientHandler();
+        }
+
+        if (_server is null)
+        {
+            throw new InvalidOperationException(Resources.ServerNotInitialized);
+        }
+
+        return _server.CreateHandler();
+    }
+
     /// <summary>
     /// Configures <see cref="HttpClient"/> instances created by this <see cref="WebApplicationFactory{TEntryPoint}"/>.
     /// </summary>
@@ -511,7 +693,19 @@ protected virtual void ConfigureClient(HttpClient client)
     {
         ArgumentNullException.ThrowIfNull(client);
 
-        client.BaseAddress = new Uri("http://localhost");
+        if (_useKestrel)
+        {
+            if (_webHost is null && _host is null)
+            {
+                throw new InvalidOperationException(Resources.ServerNotInitialized);
+            }
+
+            client.BaseAddress = _webHostAddress;
+        }
+        else
+        {
+            client.BaseAddress = new Uri("http://localhost");
+        }
     }
 
     /// <summary>