Add support for API error response type based on RFC 7807

Author: pranavkm

samples/MvcSandbox/Controllers/HomeController.cs

@@ -1,6 +1,7 @@
 // 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.ComponentModel.DataAnnotations;
 using Microsoft.AspNetCore.Mvc;
 
 namespace MvcSandbox.Controllers
@@ -14,5 +15,58 @@ public IActionResult Index()
         {
             return View();
         }
+
+        [ProblemErrorPolicy]
+        [HttpGet("/test1")]
+        public ActionResult<Person> PostTest([FromQuery] int id)
+        {
+            if (id <= 0)
+            {
+                return NotFound();
+            }
+
+            return new Person { Id = id };
+        }
+
+        [ProblemErrorPolicy]
+        [HttpPost("/test2")]
+        public ActionResult<Person> ModelStateTest([FromBody] Person person)
+        {
+            if (!ModelState.IsValid)
+            {
+                return BadRequest(ModelState);
+            }
+
+            return person;
+        }
+
+        [HttpPost("/problem")]
+        public ActionResult<Person> Problem(int cooks)
+        {
+            if (cooks > 10)
+            {
+                return new Problem
+                {
+                    Title = "Too many cooks",
+                    Status = Microsoft.AspNetCore.Http.StatusCodes.Status429TooManyRequests,
+                    ["items-being-cooked"] = new[]
+                    {
+                        "Pancakes",
+                        "Donuts",
+                        "Waffles"
+                    }
+                };
+            }
+
+            return new Person { Id = 1 };
+        }
+
+        public class Person
+        {
+            public int Id { get; set; }
+
+            [Required]
+            public string Name { get; set; }
+        }
     }
 }

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

@@ -0,0 +1,28 @@
+namespace Microsoft.AspNetCore.Mvc.ApiExplorer
+{
+    public class ActionResultApiDescriptionProvider : IApiDescriptionProvider
+    {
+        public int Order => -1000 + 10;
+
+        public void OnProvidersExecuted(ApiDescriptionProviderContext context)
+        {
+
+        }
+
+        public void OnProvidersExecuting(ApiDescriptionProviderContext context)
+        {
+            for (var i = 0; i < context.Results.Count; i++)
+            {
+                var apiDescription = context.Results[i];
+                foreach (var responseType in apiDescription.SupportedResponseTypes)
+                {
+                    if (responseType.Type.IsGenericType && typeof(ActionResult<>).IsAssignableFrom(responseType.Type.GetGenericTypeDefinition()))
+                    {
+                        responseType.Type = responseType.Type.GetGenericArguments()[0];
+                    }
+                }
+            }
+        }
+    }
+}
+

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

@@ -193,8 +193,7 @@ private IList<ApiParameterDescription> GetParameters(ApiParameterContext context
                     parameter.Source == BindingSource.ModelBinding ||
                     parameter.Source == BindingSource.Custom)
                 {
-                    ApiParameterRouteInfo routeInfo;
-                    if (routeParameters.TryGetValue(parameter.Name, out routeInfo))
+                    if (routeParameters.TryGetValue(parameter.Name, out var routeInfo))
                     {
                         parameter.RouteInfo = routeInfo;
                         routeParameters.Remove(parameter.Name);
@@ -322,8 +321,7 @@ private IReadOnlyList<ApiRequestFormat> GetRequestFormats(
             {
                 foreach (var formatter in _inputFormatters)
                 {
-                    var requestFormatMetadataProvider = formatter as IApiRequestFormatMetadataProvider;
-                    if (requestFormatMetadataProvider != null)
+                    if (formatter is IApiRequestFormatMetadataProvider requestFormatMetadataProvider)
                     {
                         var supportedTypes = requestFormatMetadataProvider.GetSupportedContentTypes(contentType, type);
 

src/Microsoft.AspNetCore.Mvc.ApiExplorer/DependencyInjection/MvcApiExplorerMvcCoreBuilderExtensions.cs

@@ -26,6 +26,10 @@ internal static void AddApiExplorerServices(IServiceCollection services)
             services.TryAddSingleton<IApiDescriptionGroupCollectionProvider, ApiDescriptionGroupCollectionProvider>();
             services.TryAddEnumerable(
                 ServiceDescriptor.Transient<IApiDescriptionProvider, DefaultApiDescriptionProvider>());
+            services.TryAddEnumerable(
+                ServiceDescriptor.Transient<IApiDescriptionProvider, ActionResultApiDescriptionProvider>());
+            services.TryAddEnumerable(
+                ServiceDescriptor.Transient<IApiDescriptionProvider, ErrorPolicyApiDescriptorProvider>());
         }
     }
 }

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

@@ -0,0 +1,55 @@
+// 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 Microsoft.AspNetCore.Mvc.Abstractions;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+
+namespace Microsoft.AspNetCore.Mvc.ApiExplorer
+{
+    public class ErrorPolicyApiDescriptorProvider : IApiDescriptionProvider
+    {
+        private readonly IModelMetadataProvider _metadataProvider;
+
+        public ErrorPolicyApiDescriptorProvider(IModelMetadataProvider modelMetadataProvider)
+        {
+            _metadataProvider = modelMetadataProvider;
+        }
+
+        public int Order => -1000 + 10;
+
+        public void OnProvidersExecuted(ApiDescriptionProviderContext context)
+        {
+            
+        }
+
+        public void OnProvidersExecuting(ApiDescriptionProviderContext context)
+        {
+            for (var i = 0; i < context.Results.Count; i++)
+            {
+                var apiDescription = context.Results[i];
+                var policy = FindErrorPolicy(apiDescription.ActionDescriptor);
+                if (policy != null)
+                {
+                    var errorPolicyContext = new ErrorPolicyContext(_metadataProvider) { Description = apiDescription };
+                    policy.Apply(errorPolicyContext);
+
+                    context.Results[i] = errorPolicyContext.Description;
+                }
+            }
+        }
+
+        private IErrorPolicy FindErrorPolicy(ActionDescriptor action)
+        {
+            for (var i = action.FilterDescriptors.Count - 1; i >= 0; i--)
+            {
+                if (action.FilterDescriptors[i].Filter is IErrorPolicy policy)
+                {
+                    return policy;
+                }
+            }
+
+            return null;
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/ActionResultOfT.cs

@@ -50,6 +50,17 @@ public static implicit operator ActionResult<TValue>(ActionResult result)
             return new ActionResult<TValue>(result);
         }
 
+        public static implicit operator ActionResult<TValue>(Problem problem)
+        {
+            var badRequestResultObject = new BadRequestObjectResult(problem);
+            if (problem.Status != null)
+            {
+                badRequestResultObject.StatusCode = problem.Status;
+            }
+
+            return new ActionResult<TValue>(badRequestResultObject);
+        }
+
         IActionResult IConvertToActionResult.Convert()
         {
             return Result ?? new ObjectResult(Value)

src/Microsoft.AspNetCore.Mvc.Core/ApiExplorer/ErrorPolicyContext.cs

@@ -0,0 +1,19 @@
+// 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.ModelBinding;
+
+namespace Microsoft.AspNetCore.Mvc.ApiExplorer
+{
+    public class ErrorPolicyContext
+    {
+        public ErrorPolicyContext(IModelMetadataProvider metadataProvider)
+        {
+            MetadataProvider = metadataProvider;
+        }
+
+        public ApiDescription Description { get; set; }
+
+        public IModelMetadataProvider MetadataProvider { get; }
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/ApiExplorer/IErrorPolicy.cs

@@ -0,0 +1,10 @@
+// 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.
+
+namespace Microsoft.AspNetCore.Mvc.ApiExplorer
+{
+    public interface IErrorPolicy
+    {
+        void Apply(ErrorPolicyContext context);
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/ErrorPolicy.cs

@@ -0,0 +1,10 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Microsoft.AspNetCore.Mvc.Core
+{
+    class ErrorPolicy
+    {
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/Problem.cs

@@ -0,0 +1,82 @@
+// 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;
+
+namespace Microsoft.AspNetCore.Mvc
+{
+    /// <summary>
+    /// A machine-readable format for specifying errors in HTTP API responses based on https://tools.ietf.org/html/rfc7807.
+    /// </summary>
+    public class Problem : Dictionary<string, object>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SerializableError"/> class.
+        /// </summary>
+        public Problem()
+            : base(StringComparer.OrdinalIgnoreCase)
+        {
+        }
+
+        /// <summary>
+        /// A URI reference [RFC3986] that identifies the problem type. This specification encourages that, when
+        /// dereferenced, it provide human-readable documentation for the problem type
+        /// (e.g., using HTML [W3C.REC-html5-20141028]).  When this member is not present, its value is assumed to be
+        /// "about:blank".
+        /// </summary>
+        public string Type
+        {
+            get => GetValue<string>(nameof(Type));
+            set => Add(nameof(Type), value);
+        }
+
+        /// <summary>
+        /// A short, human-readable summary of the problem type.It SHOULD NOT change from occurrence to occurrence
+        /// of the problem, except for purposes of localization(e.g., using proactive content negotiation; 
+        /// see[RFC7231], Section 3.4).
+        /// </summary>
+        public string Title
+        {
+            get => GetValue<string>(nameof(Title));
+            set => Add(nameof(Title), value);
+        }
+
+        /// <summary>
+        /// The HTTP status code([RFC7231], Section 6) generated by the origin server for this occurrence of the problem. 
+        /// </summary>
+        public int? Status
+        {
+            get => GetValue<int?>(nameof(Status));
+            set => Add(nameof(Status), value);
+        }
+
+        /// <summary>
+        /// A human-readable explanation specific to this occurrence of the problem. 
+        /// </summary>
+        public string Detail
+        {
+            get => GetValue<string>(nameof(Detail));
+            set => Add(nameof(Detail), value);
+        }
+
+        /// <summary>
+        /// A URI reference that identifies the specific occurrence of the problem.It may or may not yield further information if dereferenced.
+        /// </summary>
+        public string Instance
+        {
+            get => GetValue<string>(nameof(Instance));
+            set => Add(nameof(Instance), value);
+        }
+
+        private TValue GetValue<TValue>(string key)
+        {
+            if (TryGetValue(key, out var value))
+            {
+                return (TValue)value;
+            }
+
+            return default(TValue);
+        }
+    }
+}