Use content-type specified by ProducesAttribute if no formatter supports it

This allows users to use `ProducesAttribute` to specify the content-type
for action results such as FileStreamResult where the result determines the content type
and the specified value is informational.

Fixes https://github.com/aspnet/Mvc/issues/5701

src/Microsoft.AspNetCore.Mvc.ApiExplorer/ApiResponseTypeProvider.cs

@@ -135,12 +135,33 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
 
             if (contentTypes.Count == 0)
             {
+                // None of the IApiResponseMetadataProvider specified a content type. This is common for actions that
+                // specify one or more ProducesResponseType but no ProducesAttribute. In this case, formatters will participate in conneg
+                // and respond to the incoming request.
+                // Querying IApiResponseTypeMetadataProvider.GetSupportedContentTypes with "null" should retrieve all supported
+                // content types that each formatter may respond in.
                 contentTypes.Add((string)null);
             }
 
+            var responseTypes = results.Values;
+            CalculateResponseFormats(responseTypes, contentTypes);
+            return responseTypes;
+        }
+
+        private void CalculateResponseFormats(ICollection<ApiResponseType> responseTypes, MediaTypeCollection declaredContentTypes)
+        {
             var responseTypeMetadataProviders = _mvcOptions.OutputFormatters.OfType<IApiResponseTypeMetadataProvider>();
 
-            foreach (var apiResponse in results.Values)
+            // Given the content-types that were declared for this action, determine the formatters that support the content-type for the given
+            // response type.
+            // 1. Responses that do not specify an type do not have any associated content-type. This usually is meant for status-code only responses such
+            // as return NotFound();
+            // 2. When a type is specified, use GetSupportedContentTypes to expand wildcards and get the range of content-types formatters support.
+            // 3. When no formatter supports the specified content-type, use the user specified value as is. This is useful in actions where the user
+            // dictates the content-type.
+            // e.g. [Produces("application/pdf")] Action() => FileStream("somefile.pdf", "applicaiton/pdf");
+
+            foreach (var apiResponse in responseTypes)
             {
                 var responseType = apiResponse.Type;
                 if (responseType == null || responseType == typeof(void))
@@ -150,8 +171,10 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
 
                 apiResponse.ModelMetadata = _modelMetadataProvider.GetMetadataForType(responseType);
 
-                foreach (var contentType in contentTypes)
+                foreach (var contentType in declaredContentTypes)
                 {
+                    var isSupportedContentType = false;
+
                     foreach (var responseTypeMetadataProvider in responseTypeMetadataProviders)
                     {
                         var formatterSupportedContentTypes = responseTypeMetadataProvider.GetSupportedContentTypes(
@@ -163,6 +186,8 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                             continue;
                         }
 
+                        isSupportedContentType = true;
+
                         foreach (var formatterSupportedContentType in formatterSupportedContentTypes)
                         {
                             apiResponse.ApiResponseFormats.Add(new ApiResponseFormat
@@ -172,10 +197,17 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                             });
                         }
                     }
+
+                    if (!isSupportedContentType && contentType != null)
+                    {
+                        // No output formatter was found that supports this content type. Add the user specified content type as-is to the result.
+                        apiResponse.ApiResponseFormats.Add(new ApiResponseFormat
+                        {
+                            MediaType = contentType,
+                        });
+                    }
                 }
             }
-
-            return results.Values;
         }
 
         private Type GetDeclaredReturnType(ControllerActionDescriptor action)

test/Microsoft.AspNetCore.Mvc.ApiExplorer.Test/ApiResponseTypeProviderTest.cs

@@ -45,7 +45,11 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                     Assert.False(responseType.IsDefaultResponse);
                     Assert.Collection(
                         responseType.ApiResponseFormats,
-                        format => Assert.Equal("application/json", format.MediaType));
+                        format =>
+                        {
+                            Assert.Equal("application/json", format.MediaType);
+                            Assert.IsType<TestOutputFormatter>(format.Formatter);
+                        });
                 },
                 responseType =>
                 {
@@ -106,7 +110,11 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                     Assert.False(responseType.IsDefaultResponse);
                     Assert.Collection(
                         responseType.ApiResponseFormats,
-                        format => Assert.Equal("application/json", format.MediaType));
+                        format =>
+                        {
+                            Assert.Equal("application/json", format.MediaType);
+                            Assert.IsType<TestOutputFormatter>(format.Formatter);
+                        });
                 },
                 responseType =>
                 {
@@ -115,7 +123,11 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                     Assert.False(responseType.IsDefaultResponse);
                     Assert.Collection(
                         responseType.ApiResponseFormats,
-                        format => Assert.Equal("application/json", format.MediaType));
+                        format =>
+                        {
+                            Assert.Equal("application/json", format.MediaType);
+                            Assert.IsType<TestOutputFormatter>(format.Formatter);
+                        });
                 },
                 responseType =>
                 {
@@ -156,7 +168,11 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                    Assert.False(responseType.IsDefaultResponse);
                    Assert.Collection(
                         responseType.ApiResponseFormats,
-                        format => Assert.Equal("application/json", format.MediaType));
+                        format =>
+                        {
+                            Assert.Equal("application/json", format.MediaType);
+                            Assert.IsType<TestOutputFormatter>(format.Formatter);
+                        });
                },
                responseType =>
                {
@@ -663,6 +679,36 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                 });
         }
 
+        [Fact]
+        public void GetApiResponseTypes_UsesContentTypeWithoutWildCard_WhenNoFormatterSupportsIt()
+        {
+            // Arrange
+            var actionDescriptor = GetControllerActionDescriptor(typeof(TestController), nameof(TestController.GetUser));
+            actionDescriptor.FilterDescriptors.Add(new FilterDescriptor(new ProducesAttribute("application/pdf"), FilterScope.Action));
+
+            var provider = GetProvider();
+
+            // Act
+            var result = provider.GetApiResponseTypes(actionDescriptor);
+
+            // Assert
+            Assert.Collection(
+                result.OrderBy(r => r.StatusCode),
+                responseType =>
+                {
+                    Assert.Equal(200, responseType.StatusCode);
+                    Assert.Equal(typeof(DerivedModel), responseType.Type);
+                    Assert.False(responseType.IsDefaultResponse);
+                    Assert.Collection(
+                        responseType.ApiResponseFormats,
+                        format =>
+                        {
+                            Assert.Equal("application/pdf", format.MediaType);
+                            Assert.Null(format.Formatter);
+                        });
+                });
+        }
+
         private static ApiResponseTypeProvider GetProvider()
         {
             var mvcOptions = new MvcOptions

test/Microsoft.AspNetCore.Mvc.FunctionalTests/ApiExplorerTest.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Net;
 using System.Net.Http;
@@ -829,17 +830,27 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             // Assert
             var description = Assert.Single(result);
             var responseType = Assert.Single(description.SupportedResponseTypes);
-            Assert.Equal(1, responseType.ResponseFormats.Count);
+            Assert.Collection(
-
+                responseType.ResponseFormats,
-            var responseFormat = responseType.ResponseFormats[0];
+                responseFormat =>
-            Assert.Equal("application/hal+json", responseFormat.MediaType);
+                {
-            Assert.Equal(typeof(JsonOutputFormatter).FullName, responseFormat.FormatterType);
+                    Assert.Equal("application/hal+custom", responseFormat.MediaType);
+                    Assert.Null(responseFormat.FormatterType);
+                },
+                responseFormat =>
+                {
+                    Assert.Equal("application/hal+json", responseFormat.MediaType);
+                    Assert.Equal(typeof(JsonOutputFormatter).FullName, responseFormat.FormatterType);
+                });
         }
 
         [Fact]
         public async Task ApiExplorer_ResponseContentType_NoMatch()
         {
-            // Arrange & Act
+            // Arrange
+            var expectedMediaTypes = new[] { "application/custom", "text/hal+bson" };
+
+            // Act
             var response = await Client.GetAsync("http://localhost/ApiExplorerResponseContentType/NoMatch");
 
             var body = await response.Content.ReadAsStringAsync();
@@ -848,7 +859,11 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             // Assert
             var description = Assert.Single(result);
             var responseType = Assert.Single(description.SupportedResponseTypes);
-            Assert.Empty(responseType.ResponseFormats);
+
+
+            Assert.Equal(typeof(Product).FullName, responseType.ResponseType);
+            Assert.Equal(200, responseType.StatusCode);
+            Assert.Equal(expectedMediaTypes, GetSortedMediaTypes(responseType));
         }
 
         [ConditionalTheory]
@@ -1147,6 +1162,28 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Equal("multipart/form-data", requestFormat.MediaType);
         }
 
+        [Fact]
+        public async Task ApiBehavior_UsesContentTypeFromProducesAttribute_WhenNoFormatterSupportsIt()
+        {
+            // Arrange
+            var expectedMediaTypes = new[] { "application/pdf" };
+
+            // Act
+            var body = await Client.GetStringAsync("ApiExplorerApiController/ProducesWithUnsupportedContentType");
+            var result = JsonConvert.DeserializeObject<List<ApiExplorerData>>(body);
+
+            // Assert
+            var description = Assert.Single(result);
+            Assert.Collection(
+                description.SupportedResponseTypes.OrderBy(r => r.StatusCode),
+                responseType =>
+                {
+                    Assert.Equal(typeof(Stream).FullName, responseType.ResponseType);
+                    Assert.Equal(200, responseType.StatusCode);
+                    Assert.Equal(expectedMediaTypes, GetSortedMediaTypes(responseType));
+                });
+        }
+
         [Fact]
         public Task ApiConvention_ForGetMethod_ReturningModel() => ApiConvention_ForGetMethod("GetProduct");
 

test/WebSites/ApiExplorerWebSite/Controllers/ApiExplorerApiController.cs

@@ -1,8 +1,9 @@
 // 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 Microsoft.AspNetCore.Mvc;
+using System.IO;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
 
 namespace ApiExplorerWebSite
 {
@@ -27,5 +28,8 @@ namespace ApiExplorerWebSite
         public void ActionWithFormFileCollectionParameter(IFormFileCollection formFile)
         {
         }
+
+        [Produces("application/pdf", Type = typeof(Stream))]
+        public IActionResult ProducesWithUnsupportedContentType() => null;
     }
 }