Move from ValidationErrorResult to HttpBadRequest, and support object-level errors too

samples/angular/MusicStore/Apis/AlbumsApiController.cs

@@ -1,4 +1,6 @@
-using System.Linq;
+using System.ComponentModel.DataAnnotations;
+using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Authorization;
 using Microsoft.AspNet.Mvc;
@@ -118,7 +120,7 @@ namespace MusicStore.Apis
             if (!ModelState.IsValid)
             {
                 // Return the model errors
-                return new ValidationErrorResult(ModelState);
+                return HttpBadRequest(ModelState);
             }
 
             var dbAlbum = await _storeContext.Albums.SingleOrDefaultAsync(a => a.AlbumId == albumId);
@@ -165,7 +167,7 @@ namespace MusicStore.Apis
     }
 
     [ModelMetadataType(typeof(Album))]
-    public class AlbumChangeDto
+    public class AlbumChangeDto : IValidatableObject
     {
         public int GenreId { get; set; }
 
@@ -176,6 +178,21 @@ namespace MusicStore.Apis
         public decimal Price { get; set; }
 
         public string AlbumArtUrl { get; set; }
+
+        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
+        {
+            // An example of object-level (i.e., multi-property) validation
+            if (this.GenreId == 13 /* Indie */) {
+                switch (SentimentAnalysis.GetSentiment(Title)) {
+                    case SentimentAnalysis.SentimentResult.Positive:
+                        yield return new ValidationResult("Sounds too positive. Indie music requires more ambiguity.");
+                        break;
+                    case SentimentAnalysis.SentimentResult.Negative:
+                        yield return new ValidationResult("Sounds too negative. Indie music requires more ambiguity.");
+                        break;
+                }
+            }
+        }
     }
 
     public class AlbumResultDto : AlbumChangeDto

samples/angular/MusicStore/Apis/Models/SentimentAnalysis.cs

@@ -0,0 +1,38 @@
+using System.Linq;
+using System.Text.RegularExpressions;
+
+namespace MusicStore.Models
+{
+    // Obviously this is not a serious sentiment analyser. It is only here to provide an amusing demonstration of cross-property
+    // validation in AlbumsApiController.
+    public static class SentimentAnalysis
+    {
+        private static string[] positiveSentimentWords = new[] { "happy", "fun", "joy", "love", "delight", "bunny", "bunnies", "asp.net" };
+        
+        private static string[] negativeSentimentWords = new[] { "sad", "pain", "despair", "hate", "scorn", "death", "package management" };
+        
+        public static SentimentResult GetSentiment(string text) {
+            var numPositiveWords = CountWordOccurrences(text, positiveSentimentWords);
+            var numNegativeWords = CountWordOccurrences(text, negativeSentimentWords);
+            if (numPositiveWords > numNegativeWords) {
+                return SentimentResult.Positive;
+            } else if (numNegativeWords > numPositiveWords) {
+                return SentimentResult.Negative;
+            } else {
+                return SentimentResult.Neutral;
+            }
+        }
+
+        private static int CountWordOccurrences(string text, string[] words)
+        {
+            // Very simplistic matching technique for this sample. Not scalable and not really even correct.
+            return new Regex(string.Join("|", words), RegexOptions.IgnoreCase).Matches(text).Count;
+        }
+
+        public enum SentimentResult {
+            Negative,
+            Neutral,
+            Positive,
+        }
+    }
+}