Check BackgroundService.ExecuteTask for null

src/libraries/Microsoft.Extensions.Hosting/src/Internal/Host.cs

@@ -79,15 +79,22 @@ public async Task StartAsync(CancellationToken cancellationToken = default)
 
         private async Task TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)
         {
+            // backgroundService.ExecuteTask may not be set (e.g. if the derived class doesn't call base.StartAsync)
+            Task backgroundTask = backgroundService.ExecuteTask;
+            if (backgroundTask == null)
+            {
+                return;
+            }
+
             try
             {
-                await backgroundService.ExecuteTask.ConfigureAwait(false);
+                await backgroundTask.ConfigureAwait(false);
             }
             catch (Exception ex)
             {
                 // When the host is being stopped, it cancels the background services.
                 // This isn't an error condition, so don't log it as an error.
-                if (_stopCalled && backgroundService.ExecuteTask.IsCanceled && ex is OperationCanceledException)
+                if (_stopCalled && backgroundTask.IsCanceled && ex is OperationCanceledException)
                 {
                     return;
                 }

src/libraries/Microsoft.Extensions.Hosting/tests/UnitTests/Internal/HostTests.cs

@@ -1394,6 +1394,36 @@ public async Task HostNoErrorWhenServiceIsCanceledAsPartOfStop()
             }
         }
 
+        /// <summary>
+        /// Tests that when a BackgroundService does not call base, the Host still starts and stops successfully.
+        /// </summary>
+        [Fact]
+        public async Task StartOnBackgroundServiceThatDoesNotCallBase()
+        {
+            TestLoggerProvider logger = new TestLoggerProvider();
+
+            using IHost host = CreateBuilder()
+                .ConfigureLogging(logging =>
+                {
+                    logging.AddProvider(logger);
+                })
+                .ConfigureServices(services =>
+                {
+                    services.AddHostedService<BackgroundServiceDoesNotCallBase>();
+                })
+                .Build();
+
+            host.Start();
+            await host.StopAsync();
+
+            foreach (LogEvent logEvent in logger.GetEvents())
+            {
+                Assert.True(logEvent.LogLevel <= LogLevel.Information, "All logged events should be less than or equal to Information. No Warnings or Errors.");
+
+                Assert.NotEqual("BackgroundServiceFaulted", logEvent.EventId.Name);
+            }
+        }
+
         private IHostBuilder CreateBuilder(IConfiguration config = null)
         {
             return new HostBuilder().ConfigureHostConfiguration(builder => builder.AddConfiguration(config ?? new ConfigurationBuilder().Build()));
@@ -1562,5 +1592,14 @@ protected override async Task ExecuteAsync(CancellationToken stoppingToken)
                 }
             }
         }
+
+        private class BackgroundServiceDoesNotCallBase : BackgroundService
+        {
+            public override Task StartAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+
+            protected override Task ExecuteAsync(CancellationToken stoppingToken) => Task.CompletedTask;
+
+            public override Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+        }
     }
 }