Eagerly read IAsyncEnumerable{object} instances before formatting (#11118)

* Eagerly read IAsyncEnumerable{object} instances before formatting

Fixes https://github.com/aspnet/AspNetCore/issues/4833

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

@@ -875,6 +875,7 @@ namespace Microsoft.AspNetCore.Mvc
         public Microsoft.AspNetCore.Mvc.Filters.FilterCollection Filters { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public Microsoft.AspNetCore.Mvc.Formatters.FormatterMappings FormatterMappings { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         public Microsoft.AspNetCore.Mvc.Formatters.FormatterCollection<Microsoft.AspNetCore.Mvc.Formatters.IInputFormatter> InputFormatters { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+        public int MaxIAsyncEnumerableBufferLimit { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
         public int MaxModelBindingCollectionSize { get { throw null; } set { } }
         public int MaxModelBindingRecursionDepth { get { throw null; } set { } }
         public int MaxModelValidationErrors { get { throw null; } set { } }
@@ -2086,7 +2087,9 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
     }
     public partial class ObjectResultExecutor : Microsoft.AspNetCore.Mvc.Infrastructure.IActionResultExecutor<Microsoft.AspNetCore.Mvc.ObjectResult>
     {
+        [System.ObsoleteAttribute("This constructor is obsolete and will be removed in a future release.")]
         public ObjectResultExecutor(Microsoft.AspNetCore.Mvc.Infrastructure.OutputFormatterSelector formatterSelector, Microsoft.AspNetCore.Mvc.Infrastructure.IHttpResponseStreamWriterFactory writerFactory, Microsoft.Extensions.Logging.ILoggerFactory loggerFactory) { }
+        public ObjectResultExecutor(Microsoft.AspNetCore.Mvc.Infrastructure.OutputFormatterSelector formatterSelector, Microsoft.AspNetCore.Mvc.Infrastructure.IHttpResponseStreamWriterFactory writerFactory, Microsoft.Extensions.Logging.ILoggerFactory loggerFactory, Microsoft.Extensions.Options.IOptions<Microsoft.AspNetCore.Mvc.MvcOptions> mvcOptions) { }
         protected Microsoft.AspNetCore.Mvc.Infrastructure.OutputFormatterSelector FormatterSelector { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         protected Microsoft.Extensions.Logging.ILogger Logger { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
         protected System.Func<System.IO.Stream, System.Text.Encoding, System.IO.TextWriter> WriterFactory { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }

src/Mvc/Mvc.Core/src/Infrastructure/AsyncEnumerableReader.cs

@@ -0,0 +1,102 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Reflection;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc.Core;
+using Microsoft.Extensions.Internal;
+
+#if JSONNET
+namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
+#else
+namespace Microsoft.AspNetCore.Mvc.Infrastructure
+#endif
+{
+    using ReaderFunc = Func<IAsyncEnumerable<object>, Task<ICollection>>;
+
+    /// <summary>
+    /// Type that reads an <see cref="IAsyncEnumerable{T}"/> instance into a
+    /// generic collection instance.
+    /// </summary>
+    /// <remarks>
+    /// This type is used to create a strongly typed synchronous <see cref="ICollection{T}"/> instance from
+    /// an <see cref="IAsyncEnumerable{T}"/>. An accurate <see cref="ICollection{T}"/> is required for XML formatters to
+    /// correctly serialize.
+    /// </remarks>
+    internal sealed class AsyncEnumerableReader
+    {
+        private readonly MethodInfo Converter = typeof(AsyncEnumerableReader).GetMethod(
+            nameof(ReadInternal),
+            BindingFlags.NonPublic | BindingFlags.Instance);
+
+        private readonly ConcurrentDictionary<Type, ReaderFunc> _asyncEnumerableConverters =
+            new ConcurrentDictionary<Type, ReaderFunc>();
+        private readonly MvcOptions _mvcOptions;
+
+        /// <summary>
+        /// Initializes a new instance of <see cref="AsyncEnumerableReader"/>.
+        /// </summary>
+        /// <param name="mvcOptions">Accessor to <see cref="MvcOptions"/>.</param>
+        public AsyncEnumerableReader(MvcOptions mvcOptions)
+        {
+            _mvcOptions = mvcOptions;
+        }
+
+        /// <summary>
+        /// Reads a <see cref="IAsyncEnumerable{T}"/> into an <see cref="ICollection{T}"/>.
+        /// </summary>
+        /// <param name="value">The <see cref="IAsyncEnumerable{T}"/> to read.</param>
+        /// <returns>The <see cref="ICollection"/>.</returns>
+        public Task<ICollection> ReadAsync(IAsyncEnumerable<object> value)
+        {
+            if (value == null)
+            {
+                throw new ArgumentNullException(nameof(value));
+            }
+
+            var type = value.GetType();
+            if (!_asyncEnumerableConverters.TryGetValue(type, out var result))
+            {
+                var enumerableType = ClosedGenericMatcher.ExtractGenericInterface(type, typeof(IAsyncEnumerable<>));
+                Debug.Assert(enumerableType != null);
+
+                var enumeratedObjectType = enumerableType.GetGenericArguments()[0];
+
+                var converter = (ReaderFunc)Converter
+                    .MakeGenericMethod(enumeratedObjectType)
+                    .CreateDelegate(typeof(ReaderFunc), this);
+
+                _asyncEnumerableConverters.TryAdd(type, converter);
+                result = converter;
+            }
+
+            return result(value);
+        }
+
+        private async Task<ICollection> ReadInternal<T>(IAsyncEnumerable<object> value)
+        {
+            var asyncEnumerable = (IAsyncEnumerable<T>)value;
+            var result = new List<T>();
+            var count = 0;
+
+            await foreach (var item in asyncEnumerable)
+            {
+                if (count++ >= _mvcOptions.MaxIAsyncEnumerableBufferLimit)
+                {
+                    throw new InvalidOperationException(Resources.FormatObjectResultExecutor_MaxEnumerationExceeded(
+                        nameof(AsyncEnumerableReader),
+                        value.GetType()));
+                }
+
+                result.Add(item);
+            }
+
+            return result;
+        }
+    }
+}

src/Mvc/Mvc.Core/src/Infrastructure/ObjectResultExecutor.cs

@@ -10,6 +10,7 @@ using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc.Formatters;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
 
 namespace Microsoft.AspNetCore.Mvc.Infrastructure
 {
@@ -18,16 +19,35 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
     /// </summary>
     public class ObjectResultExecutor : IActionResultExecutor<ObjectResult>
     {
+        private readonly AsyncEnumerableReader _asyncEnumerableReader;
+
         /// <summary>
         /// Creates a new <see cref="ObjectResultExecutor"/>.
         /// </summary>
         /// <param name="formatterSelector">The <see cref="OutputFormatterSelector"/>.</param>
         /// <param name="writerFactory">The <see cref="IHttpResponseStreamWriterFactory"/>.</param>
         /// <param name="loggerFactory">The <see cref="ILoggerFactory"/>.</param>
+        [Obsolete("This constructor is obsolete and will be removed in a future release.")]
         public ObjectResultExecutor(
             OutputFormatterSelector formatterSelector,
             IHttpResponseStreamWriterFactory writerFactory,
             ILoggerFactory loggerFactory)
+            : this(formatterSelector, writerFactory, loggerFactory, mvcOptions: null)
+        {
+        }
+
+        /// <summary>
+        /// Creates a new <see cref="ObjectResultExecutor"/>.
+        /// </summary>
+        /// <param name="formatterSelector">The <see cref="OutputFormatterSelector"/>.</param>
+        /// <param name="writerFactory">The <see cref="IHttpResponseStreamWriterFactory"/>.</param>
+        /// <param name="loggerFactory">The <see cref="ILoggerFactory"/>.</param>
+        /// <param name="mvcOptions">Accessor to <see cref="MvcOptions"/>.</param>
+        public ObjectResultExecutor(
+            OutputFormatterSelector formatterSelector,
+            IHttpResponseStreamWriterFactory writerFactory,
+            ILoggerFactory loggerFactory,
+            IOptions<MvcOptions> mvcOptions)
         {
             if (formatterSelector == null)
             {
@@ -47,6 +67,8 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             FormatterSelector = formatterSelector;
             WriterFactory = writerFactory.CreateWriter;
             Logger = loggerFactory.CreateLogger<ObjectResultExecutor>();
+            var options = mvcOptions?.Value ?? throw new ArgumentNullException(nameof(mvcOptions));
+            _asyncEnumerableReader = new AsyncEnumerableReader(options);
         }
 
         /// <summary>
@@ -87,16 +109,37 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             InferContentTypes(context, result);
 
             var objectType = result.DeclaredType;
+
             if (objectType == null || objectType == typeof(object))
             {
                 objectType = result.Value?.GetType();
             }
 
+            var value = result.Value;
+
+            if (value is IAsyncEnumerable<object> asyncEnumerable)
+            {
+                return ExecuteAsyncEnumerable(context, result, asyncEnumerable);
+            }
+
+            return ExecuteAsyncCore(context, result, objectType, value);
+        }
+
+        private async Task ExecuteAsyncEnumerable(ActionContext context, ObjectResult result, IAsyncEnumerable<object> asyncEnumerable)
+        {
+            Log.BufferingAsyncEnumerable(Logger, asyncEnumerable);
+
+            var enumerated = await _asyncEnumerableReader.ReadAsync(asyncEnumerable);
+            await ExecuteAsyncCore(context, result, enumerated.GetType(), enumerated);
+        }
+
+        private Task ExecuteAsyncCore(ActionContext context, ObjectResult result, Type objectType, object value)
+        {
             var formatterContext = new OutputFormatterWriteContext(
                 context.HttpContext,
                 WriterFactory,
                 objectType,
-                result.Value);
+                value);
 
             var selectedFormatter = FormatterSelector.SelectFormatter(
                 formatterContext,
@@ -138,5 +181,21 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
                 result.ContentTypes.Add("application/problem+xml");
             }
         }
+
+        private static class Log
+        {
+            private static readonly Action<ILogger, string, Exception> _bufferingAsyncEnumerable;
+
+            static Log()
+            {
+                _bufferingAsyncEnumerable = LoggerMessage.Define<string>(
+                   LogLevel.Debug,
+                   new EventId(1, "BufferingAsyncEnumerable"),
+                   "Buffering IAsyncEnumerable instance of type '{Type}'.");
+            }
+
+            public static void BufferingAsyncEnumerable(ILogger logger, IAsyncEnumerable<object> asyncEnumerable)
+                => _bufferingAsyncEnumerable(logger, asyncEnumerable.GetType().FullName, null);
+        }
     }
 }

src/Mvc/Mvc.Core/src/Infrastructure/SystemTextJsonResultExecutor.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Text;
 using System.Text.Json;
@@ -25,13 +26,16 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
 
         private readonly JsonOptions _options;
         private readonly ILogger<SystemTextJsonResultExecutor> _logger;
+        private readonly AsyncEnumerableReader _asyncEnumerableReader;
 
         public SystemTextJsonResultExecutor(
             IOptions<JsonOptions> options,
-            ILogger<SystemTextJsonResultExecutor> logger)
+            ILogger<SystemTextJsonResultExecutor> logger,
+            IOptions<MvcOptions> mvcOptions)
         {
             _options = options.Value;
             _logger = logger;
+            _asyncEnumerableReader = new AsyncEnumerableReader(mvcOptions.Value);
         }
 
         public async Task ExecuteAsync(ActionContext context, JsonResult result)
@@ -70,8 +74,15 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             var writeStream = GetWriteStream(context.HttpContext, resolvedContentTypeEncoding);
             try
             {
-                var type = result.Value?.GetType() ?? typeof(object);
+                var value = result.Value;
-                await JsonSerializer.WriteAsync(writeStream, result.Value, type, jsonSerializerOptions);
+                if (value is IAsyncEnumerable<object> asyncEnumerable)
+                {
+                    Log.BufferingAsyncEnumerable(_logger, asyncEnumerable);
+                    value = await _asyncEnumerableReader.ReadAsync(asyncEnumerable);
+                }
+
+                var type = value?.GetType() ?? typeof(object);
+                await JsonSerializer.WriteAsync(writeStream, value, type, jsonSerializerOptions);
                 await writeStream.FlushAsync();
             }
             finally
@@ -123,11 +134,19 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
                 new EventId(1, "JsonResultExecuting"),
                 "Executing JsonResult, writing value of type '{Type}'.");
 
+            private static readonly Action<ILogger, string, Exception> _bufferingAsyncEnumerable = LoggerMessage.Define<string>(
+               LogLevel.Debug,
+               new EventId(2, "BufferingAsyncEnumerable"),
+               "Buffering IAsyncEnumerable instance of type '{Type}'.");
+
             public static void JsonResultExecuting(ILogger logger, object value)
             {
                 var type = value == null ? "null" : value.GetType().FullName;
                 _jsonResultExecuting(logger, type, null);
             }
+
+            public static void BufferingAsyncEnumerable(ILogger logger, IAsyncEnumerable<object> asyncEnumerable)
+                => _bufferingAsyncEnumerable(logger, asyncEnumerable.GetType().FullName, null);
         }
     }
 }

src/Mvc/Mvc.Core/src/MvcOptions.cs

@@ -359,6 +359,19 @@ namespace Microsoft.AspNetCore.Mvc
             }
         }
 
+        /// <summary>
+        /// Gets or sets the most number of entries of an <see cref="IAsyncEnumerable{T}"/> that
+        /// that <see cref="ObjectResultExecutor"/> will buffer.
+        /// <para>
+        /// When <see cref="ObjectResult.Value" /> is an instance of <see cref="IAsyncEnumerable{T}"/>,
+        /// <see cref="ObjectResultExecutor"/> will eagerly read the enumeration and add to a synchronous collection
+        /// prior to invoking the selected formatter.
+        /// This property determines the most number of entries that the executor is allowed to buffer.
+        /// </para>
+        /// </summary>
+        /// <value>Defaults to <c>8192</c>.</value>
+        public int MaxIAsyncEnumerableBufferLimit { get; set; } = 8192;
+
         IEnumerator<ICompatibilitySwitch> IEnumerable<ICompatibilitySwitch>.GetEnumerator() => _switches.GetEnumerator();
 
         IEnumerator IEnumerable.GetEnumerator() => _switches.GetEnumerator();

src/Mvc/Mvc.Core/src/Properties/Resources.Designer.cs

@@ -1746,6 +1746,20 @@ namespace Microsoft.AspNetCore.Mvc.Core
         internal static string FormatProperty_MustBeInstanceOfType(object p0, object p1, object p2)
             => string.Format(CultureInfo.CurrentCulture, GetString("Property_MustBeInstanceOfType"), p0, p1, p2);
 
+        /// <summary>
+        /// '{0}' reached the configured maximum size of the buffer when enumerating a value of type `{1}'. This limit is in place to prevent infinite streams of `IAsyncEnumerable` from continuing indefinitely. If this is not a programming mistake, consider ways to reduce the collection size, or consider manually converting '{1}' into a list rather than increasing the limit.
+        /// </summary>
+        internal static string ObjectResultExecutor_MaxEnumerationExceeded
+        {
+            get => GetString("ObjectResultExecutor_MaxEnumerationExceeded");
+        }
+
+        /// <summary>
+        /// '{0}' reached the configured maximum size of the buffer when enumerating a value of type `{1}'. This limit is in place to prevent infinite streams of `IAsyncEnumerable` from continuing indefinitely. If this is not a programming mistake, consider ways to reduce the collection size, or consider manually converting '{1}' into a list rather than increasing the limit.
+        /// </summary>
+        internal static string FormatObjectResultExecutor_MaxEnumerationExceeded(object p0, object p1)
+            => string.Format(CultureInfo.CurrentCulture, GetString("ObjectResultExecutor_MaxEnumerationExceeded"), p0, p1);
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Mvc/Mvc.Core/src/Resources.resx

@@ -503,5 +503,8 @@
   </data>
   <data name="Property_MustBeInstanceOfType" xml:space="preserve">
     <value>Property '{0}.{1}' must be an instance of type '{2}'.</value>
-  </data>  
+  </data>
+  <data name="ObjectResultExecutor_MaxEnumerationExceeded" xml:space="preserve">
+    <value>'{0}' reached the configured maximum size of the buffer when enumerating a value of type '{1}'. This limit is in place to prevent infinite streams of 'IAsyncEnumerable&lt;&gt;' from continuing indefinitely. If this is not a programming mistake, consider ways to reduce the collection size, or consider manually converting '{1}' into a list rather than increasing the limit.</value>
+  </data>
 </root>

src/Mvc/Mvc.Core/test/AcceptedAtActionResultTests.cs

@@ -275,7 +275,8 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/AcceptedAtRouteResultTests.cs

@@ -183,7 +183,8 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/AcceptedResultTests.cs

@@ -139,7 +139,8 @@ namespace Microsoft.AspNetCore.Mvc.Core.Test
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/CreatedAtActionResultTests.cs

@@ -97,7 +97,8 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/CreatedAtRouteResultTests.cs

@@ -110,7 +110,8 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/CreatedResultTests.cs

@@ -98,7 +98,8 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/HttpNotFoundObjectResultTest.cs

@@ -75,7 +75,8 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/HttpOkObjectResultTest.cs

@@ -76,7 +76,8 @@ namespace Microsoft.AspNetCore.Mvc
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             return services.BuildServiceProvider();
         }

src/Mvc/Mvc.Core/test/Infrastructure/AsyncEnumerableReaderTest.cs

@@ -0,0 +1,84 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Options;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Mvc.Infrastructure
+{
+    public class AsyncEnumerableReaderTest
+    {
+        [Fact]
+        public async Task ReadAsync_ReadsIAsyncEnumerable()
+        {
+            // Arrange
+            var options = new MvcOptions();
+            var reader = new AsyncEnumerableReader(options);
+
+            // Act
+            var result = await reader.ReadAsync(TestEnumerable());
+
+            // Assert
+            var collection = Assert.IsAssignableFrom<ICollection<string>>(result);
+            Assert.Equal(new[] { "0", "1", "2", }, collection);
+        }
+
+        [Fact]
+        public async Task ReadAsync_ReadsIAsyncEnumerable_ImplementingMultipleAsyncEnumerableInterfaces()
+        {
+            // This test ensures the reader does not fail if you have a type that implements IAsyncEnumerable for multiple Ts
+            // Arrange
+            var options = new MvcOptions();
+            var reader = new AsyncEnumerableReader(options);
+
+            // Act
+            var result = await reader.ReadAsync(new MultiAsyncEnumerable());
+
+            // Assert
+            var collection = Assert.IsAssignableFrom<ICollection<object>>(result);
+            Assert.Equal(new[] { "0", "1", "2", }, collection);
+        }
+
+       [Fact]
+        public async Task ReadAsync_ThrowsIfBufferimitIsReached()
+        {
+            // Arrange
+            var enumerable = TestEnumerable(11);
+            var expected = $"'AsyncEnumerableReader' reached the configured maximum size of the buffer when enumerating a value of type '{enumerable.GetType()}'. " +
+                "This limit is in place to prevent infinite streams of 'IAsyncEnumerable<>' from continuing indefinitely. If this is not a programming mistake, " +
+                $"consider ways to reduce the collection size, or consider manually converting '{enumerable.GetType()}' into a list rather than increasing the limit.";
+            var options = new MvcOptions { MaxIAsyncEnumerableBufferLimit = 10 };
+            var reader = new AsyncEnumerableReader(options);
+
+            // Act
+            var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => reader.ReadAsync(enumerable));
+
+            // Assert
+            Assert.Equal(expected, ex.Message);
+        }
+
+        public static async IAsyncEnumerable<string> TestEnumerable(int count = 3)
+        {
+            await Task.Yield();
+            for (var i = 0; i < count; i++)
+            {
+                yield return i.ToString();
+            }
+        }
+
+        public class MultiAsyncEnumerable : IAsyncEnumerable<object>, IAsyncEnumerable<string>
+        {
+            public IAsyncEnumerator<string> GetAsyncEnumerator(CancellationToken cancellationToken = default)
+            {
+                return TestEnumerable().GetAsyncEnumerator(cancellationToken);
+            }
+
+            IAsyncEnumerator<object> IAsyncEnumerable<object>.GetAsyncEnumerator(CancellationToken cancellationToken)
+                => GetAsyncEnumerator(cancellationToken);
+        }
+    }
+}

src/Mvc/Mvc.Core/test/Infrastructure/ControllerActionInvokerTest.cs

@@ -1567,7 +1567,8 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
                 new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
 
             httpContext.Response.Body = new MemoryStream();
             httpContext.RequestServices = services.BuildServiceProvider();

src/Mvc/Mvc.Core/test/Infrastructure/JsonResultExecutorTestBase.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -310,6 +311,24 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             Assert.StartsWith("Property 'JsonResult.SerializerSettings' must be an instance of type", ex.Message);
         }
 
+        [Fact]
+        public async Task ExecuteAsync_SerializesAsyncEnumerables()
+        {
+            // Arrange
+            var expected = Encoding.UTF8.GetBytes(JsonSerializer.ToString(new[] { "Hello", "world" }));
+
+            var context = GetActionContext();
+            var result = new JsonResult(TestAsyncEnumerable());
+            var executor = CreateExecutor();
+
+            // Act
+            await executor.ExecuteAsync(context, result);
+
+            // Assert
+            var written = GetWrittenBytes(context.HttpContext);
+            Assert.Equal(expected, written);
+        }
+
         protected IActionResultExecutor<JsonResult> CreateExecutor() => CreateExecutor(NullLoggerFactory.Instance);
 
         protected abstract IActionResultExecutor<JsonResult> CreateExecutor(ILoggerFactory loggerFactory);
@@ -354,5 +373,12 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
         {
             public string Property { get; set; }
         }
+
+        private async IAsyncEnumerable<string> TestAsyncEnumerable()
+        {
+            await Task.Yield();
+            yield return "Hello";
+            yield return "world";
+        }
     }
 }

src/Mvc/Mvc.Core/test/Infrastructure/ObjectResultExecutorTest.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
 using System.Text;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
@@ -209,8 +210,8 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
         public async Task ExecuteAsync_FallsBackOnFormattersInOptions()
         {
             // Arrange
-            var options = Options.Create(new MvcOptions());
+            var options = new MvcOptions();
-            options.Value.OutputFormatters.Add(new TestJsonOutputFormatter());
+            options.OutputFormatters.Add(new TestJsonOutputFormatter());
 
             var executor = CreateExecutor(options: options);
 
@@ -300,8 +301,8 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             string expectedContentType)
         {
             // Arrange
-            var options = Options.Create(new MvcOptions());
+            var options = new MvcOptions();
-            options.Value.RespectBrowserAcceptHeader = false;
+            options.RespectBrowserAcceptHeader = false;
 
             var executor = CreateExecutor(options: options);
 
@@ -337,8 +338,8 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             string expectedContentType)
         {
             // Arrange
-            var options = Options.Create(new MvcOptions());
+            var options = new MvcOptions();
-            options.Value.RespectBrowserAcceptHeader = true;
+            options.RespectBrowserAcceptHeader = true;
 
             var executor = CreateExecutor(options: options);
 
@@ -360,6 +361,106 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             MediaTypeAssert.Equal(expectedContentType, responseContentType);
         }
 
+        [Fact]
+        public async Task ObjectResult_ReadsAsyncEnumerables()
+        {
+            // Arrange
+            var executor = CreateExecutor();
+            var result = new ObjectResult(AsyncEnumerable());
+            var formatter = new TestJsonOutputFormatter();
+            result.Formatters.Add(formatter);
+
+            var actionContext = new ActionContext()
+            {
+                HttpContext = GetHttpContext(),
+            };
+
+            // Act
+            await executor.ExecuteAsync(actionContext, result);
+
+            // Assert
+            var formatterContext = formatter.LastOutputFormatterContext;
+            Assert.Equal(typeof(List<string>), formatterContext.ObjectType);
+            var value = Assert.IsType<List<string>>(formatterContext.Object);
+            Assert.Equal(new[] { "Hello 0", "Hello 1", "Hello 2", "Hello 3", }, value);
+        }
+
+        [Fact]
+        public async Task ObjectResult_Throws_IfEnumerableThrows()
+        {
+            // Arrange
+            var executor = CreateExecutor();
+            var result = new ObjectResult(AsyncEnumerable(throwError: true));
+            var formatter = new TestJsonOutputFormatter();
+            result.Formatters.Add(formatter);
+
+            var actionContext = new ActionContext()
+            {
+                HttpContext = GetHttpContext(),
+            };
+
+            // Act & Assert
+            await Assert.ThrowsAsync<TimeZoneNotFoundException>(() => executor.ExecuteAsync(actionContext, result));
+        }
+
+        [Fact]
+        public async Task ObjectResult_AsyncEnumeration_AtLimit()
+        {
+            // Arrange
+            var count = 24;
+            var executor = CreateExecutor(options: new MvcOptions { MaxIAsyncEnumerableBufferLimit = count });
+            var result = new ObjectResult(AsyncEnumerable(count: count));
+            var formatter = new TestJsonOutputFormatter();
+            result.Formatters.Add(formatter);
+
+            var actionContext = new ActionContext()
+            {
+                HttpContext = GetHttpContext(),
+            };
+
+            // Act
+            await executor.ExecuteAsync(actionContext, result);
+
+            // Assert
+            var formatterContext = formatter.LastOutputFormatterContext;
+            var value = Assert.IsType<List<string>>(formatterContext.Object);
+            Assert.Equal(24, value.Count);
+        }
+
+        [Theory]
+        [InlineData(25)]
+        [InlineData(1024)]
+        public async Task ObjectResult_Throws_IfEnumerationExceedsLimit(int count)
+        {
+            // Arrange
+            var executor = CreateExecutor(options: new MvcOptions { MaxIAsyncEnumerableBufferLimit = 24 });
+            var result = new ObjectResult(AsyncEnumerable(count: count));
+            var formatter = new TestJsonOutputFormatter();
+            result.Formatters.Add(formatter);
+
+            var actionContext = new ActionContext()
+            {
+                HttpContext = GetHttpContext(),
+            };
+
+            // Act & Assert
+            var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => executor.ExecuteAsync(actionContext, result));
+        }
+
+        private static async IAsyncEnumerable<string> AsyncEnumerable(int count = 4, bool throwError = false)
+        {
+            await Task.Yield();
+            for (var i = 0; i < count; i++)
+            {
+                yield return $"Hello {i}";
+            }
+
+            if (throwError)
+            {
+                throw new TimeZoneNotFoundException();
+            }
+        }
+
         private static IServiceCollection CreateServices()
         {
             var services = new ServiceCollection();
@@ -379,10 +480,12 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
             return httpContext;
         }
 
-        private static ObjectResultExecutor CreateExecutor(IOptions<MvcOptions> options = null)
+        private static ObjectResultExecutor CreateExecutor(MvcOptions options = null)
         {
-            var selector = new DefaultOutputFormatterSelector(options ?? Options.Create<MvcOptions>(new MvcOptions()), NullLoggerFactory.Instance);
+            options ??= new MvcOptions();
-            return new ObjectResultExecutor(selector, new TestHttpResponseStreamWriterFactory(), NullLoggerFactory.Instance);
+            var optionsAccessor = Options.Create(options);
+            var selector = new DefaultOutputFormatterSelector(optionsAccessor, NullLoggerFactory.Instance);
+            return new ObjectResultExecutor(selector, new TestHttpResponseStreamWriterFactory(), NullLoggerFactory.Instance, optionsAccessor);
         }
 
         private class CannotWriteFormatter : IOutputFormatter
@@ -409,8 +512,11 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
                 SupportedEncodings.Add(Encoding.UTF8);
             }
 
+            public OutputFormatterWriteContext LastOutputFormatterContext { get; private set; }
+
             public override Task WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
             {
+                LastOutputFormatterContext = context;
                 return Task.FromResult(0);
             }
         }

src/Mvc/Mvc.Core/test/Infrastructure/SystemTextJsonResultExecutorTest.cs

@@ -11,7 +11,10 @@ namespace Microsoft.AspNetCore.Mvc.Infrastructure
     {
         protected override IActionResultExecutor<JsonResult> CreateExecutor(ILoggerFactory loggerFactory)
         {
-            return new SystemTextJsonResultExecutor(Options.Create(new JsonOptions()), loggerFactory.CreateLogger<SystemTextJsonResultExecutor>());
+            return new SystemTextJsonResultExecutor(
+                Options.Create(new JsonOptions()), 
+                loggerFactory.CreateLogger<SystemTextJsonResultExecutor>(),
+                Options.Create(new MvcOptions()));
         }
 
         protected override object GetIndentedSettings()

src/Mvc/Mvc.Core/test/ObjectResultTests.cs

@@ -97,10 +97,12 @@ namespace Microsoft.AspNetCore.Mvc
         private static IServiceProvider CreateServices()
         {
             var services = new ServiceCollection();
+            var options = Options.Create(new MvcOptions());
             services.AddSingleton<IActionResultExecutor<ObjectResult>>(new ObjectResultExecutor(
-                new DefaultOutputFormatterSelector(Options.Create(new MvcOptions()), NullLoggerFactory.Instance),
+                new DefaultOutputFormatterSelector(options, NullLoggerFactory.Instance),
                 new TestHttpResponseStreamWriterFactory(),
-                NullLoggerFactory.Instance));
+                NullLoggerFactory.Instance,
+                options));
             services.AddSingleton<ILoggerFactory>(NullLoggerFactory.Instance);
 
             return services.BuildServiceProvider();

src/Mvc/Mvc.NewtonsoftJson/src/Microsoft.AspNetCore.Mvc.NewtonsoftJson.csproj

@@ -6,6 +6,7 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;aspnetcoremvc;json</PackageTags>
     <IsShippingPackage>true</IsShippingPackage>
+    <DefineConstants>$(DefineConstants);JSONNET</DefineConstants>
   </PropertyGroup>
 
   <ItemGroup>
@@ -21,5 +22,6 @@
 
   <ItemGroup>
     <Compile Include="..\..\Mvc.Core\src\Formatters\ResponseContentTypeHelper.cs" />
+    <Compile Include="..\..\Mvc.Core\src\Infrastructure\AsyncEnumerableReader.cs" />
   </ItemGroup>
 </Project>

src/Mvc/Mvc.NewtonsoftJson/src/NewtonsoftJsonLoggerExtensions.cs

@@ -10,7 +10,6 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
     {
         private static readonly Action<ILogger, Exception> _jsonInputFormatterException;
 
-        private static readonly Action<ILogger, string, Exception> _jsonResultExecuting;
 
         static NewtonsoftJsonLoggerExtensions()
         {
@@ -18,22 +17,11 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
                 LogLevel.Debug,
                 new EventId(1, "JsonInputException"),
                 "JSON input formatter threw an exception.");
-
-            _jsonResultExecuting = LoggerMessage.Define<string>(
-                LogLevel.Information,
-                new EventId(1, "JsonResultExecuting"),
-                "Executing JsonResult, writing value of type '{Type}'.");
         }
 
         public static void JsonInputException(this ILogger logger, Exception exception)
         {
             _jsonInputFormatterException(logger, exception);
         }
-
-        public static void JsonResultExecuting(this ILogger logger, object value)
-        {
-            var type = value == null ? "null" : value.GetType().FullName;
-            _jsonResultExecuting(logger, type, null);
-        }
     }
 }

src/Mvc/Mvc.NewtonsoftJson/src/NewtonsoftJsonResultExecutor.cs

@@ -3,10 +3,9 @@
 
 using System;
 using System.Buffers;
-using System.IO;
+using System.Collections.Generic;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc.Formatters;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Microsoft.AspNetCore.WebUtilities;
@@ -32,6 +31,7 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
         private readonly MvcOptions _mvcOptions;
         private readonly MvcNewtonsoftJsonOptions _jsonOptions;
         private readonly IArrayPool<char> _charPool;
+        private readonly AsyncEnumerableReader _asyncEnumerableReader;
 
         /// <summary>
         /// Creates a new <see cref="NewtonsoftJsonResultExecutor"/>.
@@ -73,6 +73,7 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
             _mvcOptions = mvcOptions?.Value ?? throw new ArgumentNullException(nameof(mvcOptions));
             _jsonOptions = jsonOptions.Value;
             _charPool = new JsonArrayPool<char>(charPool);
+            _asyncEnumerableReader = new AsyncEnumerableReader(_mvcOptions);
         }
 
         /// <summary>
@@ -111,7 +112,7 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
                 response.StatusCode = result.StatusCode.Value;
             }
 
-            _logger.JsonResultExecuting(result.Value);
+            Log.JsonResultExecuting(_logger, result.Value);
 
             var responseStream = response.Body;
             FileBufferingWriteStream fileBufferingWriteStream = null;
@@ -131,7 +132,14 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
                     jsonWriter.AutoCompleteOnClose = false;
 
                     var jsonSerializer = JsonSerializer.Create(jsonSerializerSettings);
-                    jsonSerializer.Serialize(jsonWriter, result.Value);
+                    var value = result.Value;
+                    if (result.Value is IAsyncEnumerable<object> asyncEnumerable)
+                    {
+                        Log.BufferingAsyncEnumerable(_logger, asyncEnumerable);
+                        value = await _asyncEnumerableReader.ReadAsync(asyncEnumerable);
+                    }
+
+                    jsonSerializer.Serialize(jsonWriter, value);
                 }
 
                 if (fileBufferingWriteStream != null)
@@ -168,5 +176,33 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
                 return settingsFromResult;
             }
         }
+
+        private static class Log
+        {
+            private static readonly Action<ILogger, string, Exception> _jsonResultExecuting;
+            private static readonly Action<ILogger, string, Exception> _bufferingAsyncEnumerable;
+
+            static Log()
+            {
+                _jsonResultExecuting = LoggerMessage.Define<string>(
+                    LogLevel.Information,
+                    new EventId(1, "JsonResultExecuting"),
+                    "Executing JsonResult, writing value of type '{Type}'.");
+
+                _bufferingAsyncEnumerable = LoggerMessage.Define<string>(
+                   LogLevel.Debug,
+                   new EventId(1, "BufferingAsyncEnumerable"),
+                   "Buffering IAsyncEnumerable instance of type '{Type}'.");
+            }
+
+            public static void JsonResultExecuting(ILogger logger, object value)
+            {
+                var type = value == null ? "null" : value.GetType().FullName;
+                _jsonResultExecuting(logger, type, null);
+            }
+
+            public static void BufferingAsyncEnumerable(ILogger logger, IAsyncEnumerable<object> asyncEnumerable)
+                => _bufferingAsyncEnumerable(logger, asyncEnumerable.GetType().FullName, null);
+        }
     }
 }

src/Mvc/Mvc.NewtonsoftJson/src/Properties/Resources.Designer.cs

@@ -108,6 +108,20 @@ namespace Microsoft.AspNetCore.Mvc.NewtonsoftJson
         internal static string FormatTempData_CannotSerializeType(object p0, object p1)
             => string.Format(CultureInfo.CurrentCulture, GetString("TempData_CannotSerializeType"), p0, p1);
 
+        /// <summary>
+        /// '{0}' reached the configured maximum size of the buffer when enumerating a value of type `{1}'. This limit is in place to prevent infinite streams of `IAsyncEnumerable` from continuing indefinitely. If this is not a programming mistake, consider ways to reduce the collection size, or consider manually converting '{1}' into a list rather than increasing the limit.
+        /// </summary>
+        internal static string ObjectResultExecutor_MaxEnumerationExceeded
+        {
+            get => GetString("ObjectResultExecutor_MaxEnumerationExceeded");
+        }
+
+        /// <summary>
+        /// '{0}' reached the configured maximum size of the buffer when enumerating a value of type `{1}'. This limit is in place to prevent infinite streams of `IAsyncEnumerable` from continuing indefinitely. If this is not a programming mistake, consider ways to reduce the collection size, or consider manually converting '{1}' into a list rather than increasing the limit.
+        /// </summary>
+        internal static string FormatObjectResultExecutor_MaxEnumerationExceeded(object p0, object p1)
+            => string.Format(CultureInfo.CurrentCulture, GetString("ObjectResultExecutor_MaxEnumerationExceeded"), p0, p1);
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Mvc/Mvc.NewtonsoftJson/src/Resources.resx

@@ -1,17 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <root>
-  <!-- 
+  <!--
-    Microsoft ResX Schema 
+    Microsoft ResX Schema
-    
+
     Version 2.0
-    
+
-    The primary goals of this format is to allow a simple XML format 
+    The primary goals of this format is to allow a simple XML format
-    that is mostly human readable. The generation and parsing of the 
+    that is mostly human readable. The generation and parsing of the
-    various data types are done through the TypeConverter classes 
+    various data types are done through the TypeConverter classes
     associated with the data types.
-    
+
     Example:
-    
+
     ... ado.net/XML headers & schema ...
     <resheader name="resmimetype">text/microsoft-resx</resheader>
     <resheader name="version">2.0</resheader>
@@ -26,36 +26,36 @@
         <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
         <comment>This is a comment</comment>
     </data>
-                
+
-    There are any number of "resheader" rows that contain simple 
+    There are any number of "resheader" rows that contain simple
     name/value pairs.
-    
+
-    Each data row contains a name, and value. The row also contains a 
+    Each data row contains a name, and value. The row also contains a
-    type or mimetype. Type corresponds to a .NET class that support 
+    type or mimetype. Type corresponds to a .NET class that support
-    text/value conversion through the TypeConverter architecture. 
+    text/value conversion through the TypeConverter architecture.
-    Classes that don't support this are serialized and stored with the 
+    Classes that don't support this are serialized and stored with the
     mimetype set.
-    
+
-    The mimetype is used for serialized objects, and tells the 
+    The mimetype is used for serialized objects, and tells the
-    ResXResourceReader how to depersist the object. This is currently not 
+    ResXResourceReader how to depersist the object. This is currently not
     extensible. For a given mimetype the value must be set accordingly:
-    
+
-    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    Note - application/x-microsoft.net.object.binary.base64 is the format
-    that the ResXResourceWriter will generate, however the reader can 
+    that the ResXResourceWriter will generate, however the reader can
     read any of the formats listed below.
-    
+
     mimetype: application/x-microsoft.net.object.binary.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
             : and then encoded with base64 encoding.
-    
+
     mimetype: application/x-microsoft.net.object.soap.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
             : and then encoded with base64 encoding.
 
     mimetype: application/x-microsoft.net.object.bytearray.base64
-    value   : The object must be serialized into a byte array 
+    value   : The object must be serialized into a byte array
             : using a System.ComponentModel.TypeConverter
             : and then encoded with base64 encoding.
     -->
@@ -126,6 +126,9 @@
   <data name="JsonHelperMustBeAnInstanceOfNewtonsoftJson" xml:space="preserve">
     <value>Parameter '{0}' must be an instance of {1} provided by the '{2}' package. Configure the correct instance using '{3}' in your startup.</value>
   </data>
+  <data name="ObjectResultExecutor_MaxEnumerationExceeded" xml:space="preserve">
+    <value>'{0}' reached the configured maximum size of the buffer when enumerating a value of type '{1}'. This limit is in place to prevent infinite streams of 'IAsyncEnumerable&lt;&gt;' from continuing indefinitely. If this is not a programming mistake, consider ways to reduce the collection size, or consider manually converting '{1}' into a list rather than increasing the limit.</value>
+  </data>
   <data name="Property_MustBeInstanceOfType" xml:space="preserve">
     <value>Property '{0}.{1}' must be an instance of type '{2}'.</value>
   </data>
@@ -138,4 +141,4 @@
   <data name="TempData_CannotSerializeType" xml:space="preserve">
     <value>The '{0}' cannot serialize an object of type '{1}'.</value>
   </data>
-</root>
+</root>

src/Mvc/Mvc.NewtonsoftJson/test/Microsoft.AspNetCore.Mvc.NewtonsoftJson.Test.csproj

@@ -12,6 +12,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <Compile Include="..\..\Mvc.Core\test\Infrastructure\AsyncEnumerableReaderTest.cs" />
     <Compile Include="..\..\Mvc.Core\test\Formatters\JsonInputFormatterTestBase.cs" />
     <Compile Include="..\..\Mvc.Core\test\Formatters\JsonOutputFormatterTestBase.cs" />
     <Compile Include="..\..\Mvc.Core\test\Infrastructure\JsonResultExecutorTestBase.cs" />

src/Mvc/test/Mvc.FunctionalTests/AsyncEnumerableTestBase.cs

@@ -0,0 +1,97 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net;
+using System.Net.Http;
+using System.Text.Json;
+using System.Threading.Tasks;
+using System.Xml.Linq;
+using FormatterWebSite;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Mvc.FunctionalTests
+{
+    public class AsyncEnumerableTestBase : IClassFixture<MvcTestFixture<StartupWithJsonFormatter>>
+    {
+        public AsyncEnumerableTestBase(MvcTestFixture<StartupWithJsonFormatter> fixture)
+        {
+            Client = fixture.CreateDefaultClient();
+        }
+
+        public HttpClient Client { get; }
+
+        [Fact]
+        public Task AsyncEnumerableReturnedWorks() => AsyncEnumerableWorks();
+
+        [Fact]
+        public Task AsyncEnumerableWrappedInTask() => AsyncEnumerableWorks();
+
+        private async Task AsyncEnumerableWorks()
+        {
+            // Act
+            var response = await Client.GetAsync("asyncenumerable/getallprojects");
+
+            // Assert
+            await response.AssertStatusCodeAsync(HttpStatusCode.OK);
+
+            var content = await response.Content.ReadAsStringAsync();
+
+            // Some sanity tests to verify things serialized correctly.
+            var projects = JsonSerializer.Parse<List<Project>>(content, TestJsonSerializerOptionsProvider.Options);
+            Assert.Equal(10, projects.Count);
+            Assert.Equal("Project0", projects[0].Name);
+            Assert.Equal("Project9", projects[9].Name);
+        }
+
+        [Fact]
+        public async Task AsyncEnumerableExceptionsAreThrown()
+        {
+            // Act
+            var response = await Client.GetAsync("asyncenumerable/GetAllProjectsWithError");
+
+            // Assert
+            await response.AssertStatusCodeAsync(HttpStatusCode.InternalServerError);
+
+            var content = await response.Content.ReadAsStringAsync();
+
+            // Verify that the exception shows up in the callstack
+            Assert.Contains(nameof(InvalidTimeZoneException), content);
+        }
+
+        [Fact]
+        public async Task AsyncEnumerableWithXmlFormatterWorks()
+        {
+            // Arrange
+            var request = new HttpRequestMessage(HttpMethod.Get, "asyncenumerable/getallprojects");
+            request.Headers.Add("Accept", "application/xml");
+
+            // Act
+            var response = await Client.SendAsync(request);
+
+            // Assert
+            await response.AssertStatusCodeAsync(HttpStatusCode.OK);
+
+            var content = await response.Content.ReadAsStringAsync();
+
+            // Some sanity tests to verify things serialized correctly.
+            var xml = XDocument.Parse(content);
+            var @namespace = xml.Root.Name.NamespaceName;
+            var projects = xml.Root.Elements(XName.Get("Project", @namespace));
+            Assert.Equal(10, projects.Count());
+
+            Assert.Equal("Project0", GetName(projects.ElementAt(0)));
+            Assert.Equal("Project9", GetName(projects.ElementAt(9)));
+
+            string GetName(XElement element)
+            {
+                var name = element.Element(XName.Get("Name", @namespace));
+                Assert.NotNull(name);
+
+                return name.Value;
+            }
+        }
+    }
+}

src/Mvc/test/Mvc.FunctionalTests/InputObjectValidationTests.cs

@@ -261,9 +261,13 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
                 Content = new StringContent(@"{ ""Id"": ""S-1-5-21-1004336348-1177238915-682003330-512"" }", Encoding.UTF8, "application/json"),
             };
 
-            // Act & Assert
+            // Act
-            var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => Client.SendAsync(requestMessage));
+            var response = await Client.SendAsync(requestMessage);
-            Assert.Equal(expected, ex.Message);
+
+            // Assert
+            await response.AssertStatusCodeAsync(HttpStatusCode.InternalServerError);
+            var content = await response.Content.ReadAsStringAsync();
+            Assert.Contains(expected, content);
         }
 
         [Fact]
@@ -356,4 +360,4 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
                 });
         }
     }
-}
+}

src/Mvc/test/Mvc.FunctionalTests/TestJsonSerializationOptionsProvider.cs

@@ -0,0 +1,15 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Text.Json;
+
+namespace Microsoft.AspNetCore.Mvc.FunctionalTests
+{
+    internal static class TestJsonSerializerOptionsProvider
+    {
+        public static JsonSerializerOptions Options = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        };
+    }
+}

src/Mvc/test/WebSites/FormatterWebSite/Controllers/AsyncEnumerableController.cs

@@ -0,0 +1,48 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+
+namespace FormatterWebSite.Controllers
+{
+    [ApiController]
+    [Route("{controller}/{action}")]
+    public class AsyncEnumerableController : ControllerBase
+    {
+        [HttpGet]
+        public IAsyncEnumerable<Project> GetAllProjects()
+            => GetAllProjectsCore();
+
+        [HttpGet]
+        public async Task<IAsyncEnumerable<Project>> GetAllProjectsAsTask()
+        {
+            await Task.Yield();
+            return GetAllProjectsCore();
+        }
+
+        [HttpGet]
+        public IAsyncEnumerable<Project> GetAllProjectsWithError()
+            => GetAllProjectsCore(true);
+
+        public async IAsyncEnumerable<Project> GetAllProjectsCore(bool throwError = false)
+        {
+            await Task.Delay(5);
+            for (var i = 0; i < 10; i++)
+            {
+                if (throwError && i == 5)
+                {
+                    throw new InvalidTimeZoneException();
+                }
+
+                yield return new Project
+                {
+                    Id = i,
+                    Name = $"Project{i}",
+                };
+            }
+        }
+    }
+}

src/Mvc/test/WebSites/FormatterWebSite/Startup.cs

@@ -26,6 +26,8 @@ namespace FormatterWebSite
 
         public void Configure(IApplicationBuilder app)
         {
+            app.UseDeveloperExceptionPage();
+
             app.UseRouting();
             app.UseEndpoints(endpoints =>
             {