Extend WebApplicationFactory<T> (#7414)

* Support host-agnostic services access

* Support host agnostic access to the IServiceCollection associated with
      WebApplicationFactory<T>.

src/Mvc/Mvc.Testing/ref/Microsoft.AspNetCore.Mvc.Testing.netcoreapp3.0.cs

@@ -26,6 +26,7 @@ namespace Microsoft.AspNetCore.Mvc.Testing
         public Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactoryClientOptions ClientOptions { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public System.Collections.Generic.IReadOnlyList<Microsoft.AspNetCore.Mvc.Testing.WebApplicationFactory<TEntryPoint>> Factories { get { throw null; } }
         public Microsoft.AspNetCore.TestHost.TestServer Server { get { throw null; } }
+        public virtual System.IServiceProvider Services { get { throw null; } }
         protected virtual void ConfigureClient(System.Net.Http.HttpClient client) { }
         protected virtual void ConfigureWebHost(Microsoft.AspNetCore.Hosting.IWebHostBuilder builder) { }
         public System.Net.Http.HttpClient CreateClient() { throw null; }

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

@@ -79,6 +79,18 @@ namespace Microsoft.AspNetCore.Mvc.Testing
             }
         }
 
+        /// <summary>
+        /// Gets the <see cref="IServiceProvider"/> created by the server associated with this <see cref="WebApplicationFactory{TEntryPoint}"/>.
+        /// </summary>
+        public virtual IServiceProvider Services
+        {
+            get
+            {
+                EnsureServer();
+                return _host?.Services ?? _server.Host.Services;
+            }
+        }
+
         /// <summary>
         /// Gets the <see cref="IReadOnlyList{WebApplicationFactory}"/> of factories created from this factory
         /// by further customizing the <see cref="IWebHostBuilder"/> when calling 

src/Mvc/test/Mvc.FunctionalTests/TestingInfrastructureInheritanceTests.cs

@@ -7,6 +7,7 @@ using System.Reflection;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Mvc.Testing;
 using Microsoft.AspNetCore.TestHost;
+using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.Hosting;
 using Xunit;
 
@@ -52,6 +53,17 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.False(factory.CreateWebHostBuilderCalled);
         }
 
+        [Fact]
+        public void TestingInfrastructure_GenericHost_WithWithHostBuilderHasServices()
+        {
+            // Act
+            var factory = new CustomizedFactory<GenericHostWebSite.Startup>();
+
+            // Assert
+            Assert.NotNull(factory.Services);
+            Assert.NotNull(factory.Services.GetService(typeof(IConfiguration)));
+        }
+
         private class CustomizedFactory<TEntryPoint> : WebApplicationFactory<TEntryPoint> where TEntryPoint : class
         {
             public bool GetTestAssembliesCalled { get; private set; }

src/Mvc/test/Mvc.FunctionalTests/TestingInfrastructureTests.cs

@@ -14,6 +14,7 @@ using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Mvc.Testing;
 using Microsoft.AspNetCore.Mvc.Testing.Handlers;
 using Microsoft.AspNetCore.TestHost;
+using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using RazorPagesClassLibrary;
 using Xunit;
@@ -131,6 +132,42 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Equal("GenericTest", response);
         }
 
+        [Fact]
+        public void TestingInfrastructure_HasServicesUsingWebHostProgram()
+        {
+            var factory = new WebApplicationFactory<BasicWebSite.Program>();
+
+            Assert.NotNull(factory.Services);
+            Assert.NotNull(factory.Services.GetService(typeof(IConfiguration)));
+        }
+
+        [Fact]
+        public void TestingInfrastructure_HasServicesUsingWebHostStartup()
+        {
+            var factory = new WebApplicationFactory<BasicWebSite.Startup>();
+
+            Assert.NotNull(factory.Services);
+            Assert.NotNull(factory.Services.GetService(typeof(IConfiguration)));
+        }
+
+        [Fact]
+        public void TestingInfrastructure_HasServicesUsingGenericHostProgram()
+        {
+            var factory = new WebApplicationFactory<GenericHostWebSite.Program>();
+
+            Assert.NotNull(factory.Services);
+            Assert.NotNull(factory.Services.GetService(typeof(IConfiguration)));
+        }
+
+        [Fact]
+        public void TestingInfrastructure_HasServicesUsingGenericHostStartup()
+        {
+            var factory = new WebApplicationFactory<GenericHostWebSite.Startup>();
+
+            Assert.NotNull(factory.Services);
+            Assert.NotNull(factory.Services.GetService(typeof(IConfiguration)));
+        }
+
         private class OverridenService : TestService
         {
             public OverridenService()