Block generic host Startup.ConfigureServices that return IServiceProvider #5149 (#6997)

Author: Tratcher

src/Hosting/Hosting/src/GenericHost/GenericWebHostBuilder.cs

@@ -232,6 +232,10 @@ private void UseStartup(Type startupType, HostBuilderContext context, IServiceCo
                 {
                     throw new NotSupportedException($"{typeof(IStartup)} isn't supported");
                 }
+                if (StartupLoader.HasConfigureServicesIServiceProviderDelegate(startupType, context.HostingEnvironment.EnvironmentName))
+                {
+                    throw new NotSupportedException($"ConfigureServices returning an {typeof(IServiceProvider)} isn't supported.");
+                }
 
                 instance = ActivatorUtilities.CreateInstance(new HostServiceProvider(webHostBuilderContext), startupType);
                 context.Properties[_startupKey] = instance;

src/Hosting/Hosting/src/Internal/StartupLoader.cs

@@ -279,6 +279,11 @@ internal static ConfigureContainerBuilder FindConfigureContainerDelegate(Type st
             return new ConfigureContainerBuilder(configureMethod);
         }
 
+        internal static bool HasConfigureServicesIServiceProviderDelegate(Type startupType, string environmentName)
+        {
+            return null != FindMethod(startupType, "Configure{0}Services", environmentName, typeof(IServiceProvider), required: false);
+        }
+
         internal static ConfigureServicesBuilder FindConfigureServicesDelegate(Type startupType, string environmentName)
         {
             var servicesMethod = FindMethod(startupType, "Configure{0}Services", environmentName, typeof(IServiceProvider), required: false)
@@ -333,4 +338,4 @@ private static MethodInfo FindMethod(Type startupType, string methodName, string
             return methodInfo;
         }
     }
-}
+}

src/Hosting/Hosting/test/Fakes/StartupWithNullConfigureServices.cs renamed to src/Hosting/Hosting/test/Fakes/StartupWithBuiltConfigureServices.cs

@@ -4,7 +4,7 @@
 
 namespace Microsoft.AspNetCore.Hosting.Fakes
 {
-    public class StartupWithNullConfigureServices
+    public class StartupWithBuiltConfigureServices
     {
         public IServiceProvider ConfigureServices(IServiceCollection services)
         {
@@ -13,4 +13,4 @@ public IServiceProvider ConfigureServices(IServiceCollection services)
 
         public void Configure(IApplicationBuilder app) { }
     }
-}
+}

src/Hosting/Hosting/test/WebHostBuilderTests.cs

@@ -889,6 +889,22 @@ public void GenericWebHostThrowsOnBuild()
             Assert.Equal("Building this implementation of IWebHostBuilder is not supported.", exception.Message);
         }
 
+        [Fact]
+        public void GenericWebHostDoesNotSupportBuildingInConfigureServices()
+        {
+            var hostBuilder = new HostBuilder()
+                   .ConfigureWebHost(builder =>
+                   {
+                       builder.UseStartup<StartupWithBuiltConfigureServices>();
+                   });
+            var exception = Assert.Throws<NotSupportedException>(() =>
+            {
+                hostBuilder.Build();
+            });
+
+            Assert.Equal($"ConfigureServices returning an {typeof(IServiceProvider)} isn't supported.", exception.Message);
+        }
+
         [Theory]
         [MemberData(nameof(DefaultWebHostBuildersWithConfig))]
         public void Build_HostingStartupAssemblyCanBeExcluded(IWebHostBuilder builder)