Introduce ProblemDescription

Author: pranavkm

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

@@ -1468,6 +1468,49 @@ public virtual BadRequestObjectResult BadRequest(ModelStateDictionary modelState
             return new BadRequestObjectResult(modelState);
         }
 
+        /// <summary>
+        /// Creates an <see cref="BadRequestObjectResult"/> that produces a <see cref="StatusCodes.Status400BadRequest"/> response.
+        /// </summary>
+        /// <returns>The created <see cref="BadRequestObjectResult"/> for the response.</returns>
+        [NonAction]
+        public virtual BadRequestObjectResult ValidationProblem(ValidationProblemDescription descriptor)
+        {
+            if (descriptor == null)
+            {
+                throw new ArgumentNullException(nameof(descriptor));
+            }
+
+            return new BadRequestObjectResult(descriptor);
+        }
+
+        /// <summary>
+        /// Creates an <see cref="BadRequestObjectResult"/> that produces a <see cref="StatusCodes.Status400BadRequest"/> response.
+        /// </summary>
+        /// <returns>The created <see cref="BadRequestObjectResult"/> for the response.</returns>
+        [NonAction]
+        public virtual BadRequestObjectResult ValidationProblem(ModelStateDictionary modelStateDictionary)
+        {
+            if (modelStateDictionary == null)
+            {
+                throw new ArgumentNullException(nameof(modelStateDictionary));
+            }
+
+            var validationProblem = new ValidationProblemDescription(modelStateDictionary);
+            return new BadRequestObjectResult(validationProblem);
+        }
+
+        /// <summary>
+        /// Creates an <see cref="BadRequestObjectResult"/> that produces a <see cref="StatusCodes.Status400BadRequest"/> response
+        /// with validation errors from <see cref="ModelState"/>.
+        /// </summary>
+        /// <returns>The created <see cref="BadRequestObjectResult"/> for the response.</returns>
+        [NonAction]
+        public virtual BadRequestObjectResult ValidationProblem()
+        {
+            var validationProblem = new ValidationProblemDescription(ModelState);
+            return new BadRequestObjectResult(validationProblem);
+        }
+
         /// <summary>
         /// Creates a <see cref="CreatedResult"/> object that produces a <see cref="StatusCodes.Status201Created"/> response.
         /// </summary>

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

@@ -0,0 +1,44 @@
+// 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 ProblemDescription
+    {
+        /// <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; set; }
+
+        /// <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; set; }
+
+        /// <summary>
+        /// The HTTP status code([RFC7231], Section 6) generated by the origin server for this occurrence of the problem.
+        /// </summary>
+        public int? Status { get; set; }
+
+        /// <summary>
+        /// A human-readable explanation specific to this occurrence of the problem.
+        /// </summary>
+        public string Detail { get; set; }
+
+        /// <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; set; }
+    }
+}

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

@@ -412,4 +412,7 @@
   <data name="UrlHelper_RelativePagePathIsNotSupported" xml:space="preserve">
     <value>The relative page path '{0}' can only be used while executing a Razor Page. Specify a root relative path with a leading '/' to generate a URL outside of a Razor Page.</value>
   </data>
+  <data name="ValidationProblemDescription_Title" xml:space="preserve">
+    <value>One or more validation errors occured.</value>
+  </data>
 </root>

src/Microsoft.AspNetCore.Mvc.Core/Resources.resx

@@ -0,0 +1,69 @@
+// 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 Microsoft.AspNetCore.Mvc.Core;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+
+namespace Microsoft.AspNetCore.Mvc
+{
+    /// <summary>
+    /// A <see cref="ProblemDescription"/> for validation errors.
+    /// </summary>
+    public class ValidationProblemDescription : ProblemDescription
+    {
+        /// <summary>
+        /// Intializes a new instance of <see cref="ValidationProblemDescription"/>.
+        /// </summary>
+        public ValidationProblemDescription()
+        {
+            Title = Resources.ValidationProblemDescription_Title;
+        }
+
+        public ValidationProblemDescription(ModelStateDictionary modelState)
+            : this()
+        {
+            if (modelState == null)
+            {
+                throw new ArgumentNullException(nameof(modelState));
+            }
+
+            foreach (var keyModelStatePair in modelState)
+            {
+                var key = keyModelStatePair.Key;
+                var errors = keyModelStatePair.Value.Errors;
+                if (errors != null && errors.Count > 0)
+                {
+                    if (errors.Count == 1)
+                    {
+                        var errorMessage = GetErrorMessage(errors[0]);
+                        Errors.Add(key, new[] { errorMessage });
+                    }
+                    else
+                    {
+                        var errorMessages = new string[errors.Count];
+                        for (var i = 0; i < errors.Count; i++)
+                        {
+                            errorMessages[i] = GetErrorMessage(errors[i]);
+                        }
+
+                        Errors.Add(key, errorMessages);
+                    }
+                }
+            }
+
+            string GetErrorMessage(ModelError error)
+            {
+                return string.IsNullOrEmpty(error.ErrorMessage) ?
+                    Resources.SerializableError_DefaultError :
+                    error.ErrorMessage;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the validation errors associated with this instance of <see cref="ValidationProblemDescription"/>.
+        /// </summary>
+        public IDictionary<string, string[]> Errors { get; } = new Dictionary<string, string[]>(StringComparer.Ordinal);
+    }
+}

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

@@ -0,0 +1,51 @@
+// 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;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Mvc
+{
+    public class ValidationProblemDescriptionTest
+    {
+        [Fact]
+        public void Constructor_SetsTitle()
+        {
+            // Arrange & Act
+            var problemDescription = new ValidationProblemDescription();
+
+            // Assert
+            Assert.Equal("One or more validation errors occured.", problemDescription.Title);
+            Assert.Empty(problemDescription.Errors);
+        }
+
+        [Fact]
+        public void Constructor_SerializesErrorsFromModelStateDictionary()
+        {
+            // Arrange
+            var modelStateDictionary = new ModelStateDictionary();
+            modelStateDictionary.AddModelError("key1", "error1");
+            modelStateDictionary.SetModelValue("key2", new object(), "value");
+            modelStateDictionary.AddModelError("key3", "error2");
+            modelStateDictionary.AddModelError("key3", "error3");
+
+            // Act
+            var problemDescription = new ValidationProblemDescription(modelStateDictionary);
+
+            // Assert
+            Assert.Equal("One or more validation errors occured.", problemDescription.Title);
+            Assert.Collection(
+                problemDescription.Errors,
+                item =>
+                {
+                    Assert.Equal("key1", item.Key);
+                    Assert.Equal(new[] { "error1" }, item.Value);
+                },
+                item =>
+                {
+                    Assert.Equal("key3", item.Key);
+                    Assert.Equal(new[] { "error2", "error3" }, item.Value);
+                });
+        }
+    }
+}