Order enum values by DisplayAttribute.Order

Fixes #4297

src/Microsoft.AspNetCore.Mvc.DataAnnotations/Internal/DataAnnotationsMetadataProvider.cs

@@ -176,15 +176,20 @@ namespace Microsoft.AspNetCore.Mvc.DataAnnotations.Internal
 
                 // EnumDisplayNamesAndValues and EnumNamesAndValues
                 //
-                // Order EnumDisplayNamesAndValues to match Enum.GetNames(). That method orders by absolute value,
+                // Order EnumDisplayNamesAndValues by DisplayAttribute.Order, then by the order of Enum.GetNames(). 
-                // then its behavior is undefined (but hopefully stable). Add to EnumNamesAndValues in same order but
+                // That method orders by absolute value, then its behavior is undefined (but hopefully stable). 
-                // Dictionary does not guarantee order will be preserved.
+                // Add to EnumNamesAndValues in same order but Dictionary does not guarantee order will be preserved.
+
                 var groupedDisplayNamesAndValues = new List<KeyValuePair<EnumGroupAndName, string>>();
                 var namesAndValues = new Dictionary<string, string>();
                 var enumLocalizer = _stringLocalizerFactory?.Create(underlyingType);
-                foreach (var name in Enum.GetNames(underlyingType))
+
+                var enumFields = Enum.GetNames(underlyingType)
+                    .Select(name => underlyingType.GetField(name))
+                    .OrderBy(field => field.GetCustomAttribute<DisplayAttribute>(inherit: false)?.GetOrder() ?? 1000);
+
+                foreach (var field in enumFields)
                 {
-                    var field = underlyingType.GetField(name);
                     var groupName = GetDisplayGroup(field);
                     var value = ((Enum)field.GetValue(obj: null)).ToString("d");
 
@@ -193,7 +198,7 @@ namespace Microsoft.AspNetCore.Mvc.DataAnnotations.Internal
                             groupName,
                             () => GetDisplayName(field, enumLocalizer)),
                         value));
-                    namesAndValues.Add(name, value);
+                    namesAndValues.Add(field.Name, value);
                 }
 
                 displayMetadata.EnumGroupedDisplayNamesAndValues = groupedDisplayNamesAndValues;

test/Microsoft.AspNetCore.Mvc.DataAnnotations.Test/Internal/DataAnnotationsMetadataProviderTest.cs

@@ -976,6 +976,36 @@ namespace Microsoft.AspNetCore.Mvc.DataAnnotations.Internal
                 KVPEnumGroupAndNameComparer.Instance);
         }
 
+        [Fact]
+        public void CreateDisplayMetadata_EnumGroupedDisplayNamesAndValues_ReflectsDisplayAttributeOrder()
+        {
+            // Arrange
+            var expectedKeyValuePairs = new List<KeyValuePair<EnumGroupAndName, string>>
+            {
+                new KeyValuePair<EnumGroupAndName, string>(new EnumGroupAndName(string.Empty, nameof(EnumWithDisplayOrder.Three)), "2"),
+                new KeyValuePair<EnumGroupAndName, string>(new EnumGroupAndName(string.Empty, nameof(EnumWithDisplayOrder.Two)), "1"),
+                new KeyValuePair<EnumGroupAndName, string>(new EnumGroupAndName(string.Empty, nameof(EnumWithDisplayOrder.One)), "0"),
+                new KeyValuePair<EnumGroupAndName, string>(new EnumGroupAndName(string.Empty, nameof(EnumWithDisplayOrder.Null)), "3"),
+            };
+
+            var provider = new DataAnnotationsMetadataProvider(
+                new TestOptionsManager<MvcDataAnnotationsLocalizationOptions>(),
+                stringLocalizerFactory: null);
+
+            var key = ModelMetadataIdentity.ForType(typeof(EnumWithDisplayOrder));
+            var attributes = new object[0];
+            var context = new DisplayMetadataProviderContext(key, new ModelAttributes(attributes));
+
+            // Act
+            provider.CreateDisplayMetadata(context);
+
+            // Assert
+            Assert.Equal(
+                expectedKeyValuePairs,
+                context.DisplayMetadata.EnumGroupedDisplayNamesAndValues,
+                KVPEnumGroupAndNameComparer.Instance);
+        }
+
         [Fact]
         public void CreateDisplayMetadata_EnumGroupedDisplayNamesAndValues_NameWithNoIStringLocalizerAndNoResourceType()
         {
@@ -1318,6 +1348,20 @@ namespace Microsoft.AspNetCore.Mvc.DataAnnotations.Internal
             MinusTwo = -2,
         }
 
+        private enum EnumWithDisplayOrder
+        {
+            [Display(Order = 3)]
+            One,
+
+            [Display(Order = 2)]
+            Two,
+
+            [Display(Order = 1)]
+            Three,
+
+            Null,
+        }
+
         private enum EnumWithDuplicates
         {
             Zero = 0,