Last part of #3676

Uses the correct IEnumerable<> in validation strategies

src/Microsoft.AspNet.Mvc.Core/ModelBinding/Validation/DefaultCollectionValidationStrategy.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Reflection;
 
 namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
 {
@@ -36,6 +37,9 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
     /// </remarks>
     public class DefaultCollectionValidationStrategy : IValidationStrategy
     {
+        private static readonly MethodInfo _getEnumerator = typeof(DefaultCollectionValidationStrategy)
+            .GetMethod(nameof(GetEnumerator), BindingFlags.Static | BindingFlags.NonPublic);
+
         /// <summary>
         /// Gets an instance of <see cref="DefaultCollectionValidationStrategy"/>.
         /// </summary>
@@ -51,14 +55,26 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
             string key,
             object model)
         {
-            return new Enumerator(metadata.ElementMetadata, key, (IEnumerable)model);
+            var enumerator = GetEnumeratorForElementType(metadata, model);
+            return new Enumerator(metadata.ElementMetadata, key, enumerator);
+        }
+
+        public static IEnumerator GetEnumeratorForElementType(ModelMetadata metadata, object model)
+        {
+            var getEnumeratorMethod = _getEnumerator.MakeGenericMethod(metadata.ElementType);
+            return (IEnumerator)getEnumeratorMethod.Invoke(null, new object[] { model });
+        }
+
+        // Called via reflection.
+        private static IEnumerator GetEnumerator<T>(object model)
+        {
+            return (model as IEnumerable<T>)?.GetEnumerator() ?? ((IEnumerable)model).GetEnumerator();
         }
 
         private class Enumerator : IEnumerator<ValidationEntry>
         {
             private readonly string _key;
             private readonly ModelMetadata _metadata;
-            private readonly IEnumerable _model;
             private readonly IEnumerator _enumerator;
 
             private ValidationEntry _entry;
@@ -67,13 +83,11 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
             public Enumerator(
                 ModelMetadata metadata,
                 string key,
-                IEnumerable model)
+                IEnumerator enumerator)
             {
                 _metadata = metadata;
                 _key = key;
-                _model = model;
-
-                _enumerator = _model.GetEnumerator();
+                _enumerator = enumerator;
 
                 _index = -1;
             }
@@ -116,7 +130,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
 
             public void Reset()
             {
-                throw new NotImplementedException();
+                _enumerator.Reset();
             }
         }
     }

src/Microsoft.AspNet.Mvc.Core/ModelBinding/Validation/ExplicitIndexCollectionValidationStrategy.cs

@@ -54,7 +54,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
             string key,
             object model)
         {
-            return new Enumerator(metadata.ElementMetadata, key, ElementKeys, (IEnumerable)model);
+            var enumerator = DefaultCollectionValidationStrategy.GetEnumeratorForElementType(metadata, model);
+            return new Enumerator(metadata.ElementMetadata, key, ElementKeys, enumerator);
         }
 
         private class Enumerator : IEnumerator<ValidationEntry>
@@ -70,13 +71,13 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
                 ModelMetadata metadata,
                 string key,
                 IEnumerable<string> elementKeys,
-                IEnumerable model)
+                IEnumerator enumerator)
             {
                 _metadata = metadata;
                 _key = key;
 
                 _keyEnumerator = elementKeys.GetEnumerator();
-                _enumerator = model.GetEnumerator();
+                _enumerator = enumerator;
             }
 
             public ValidationEntry Current

test/Microsoft.AspNet.Mvc.Core.Test/ModelBinding/Validation/DefaultCollectionValidationStrategyTest.cs

@@ -1,6 +1,8 @@
 // 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;
 using System.Collections.Generic;
 using System.Linq;
 using Xunit;
@@ -84,6 +86,62 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
                 });
         }
 
+        [Fact]
+        public void EnumerateElements_TwoEnumerableImplemenations()
+        {
+            // Arrange
+            var model = new TwiceEnumerable(new int[] { 2, 3, 5 });
+
+            var metadata = TestModelMetadataProvider.CreateDefaultProvider().GetMetadataForType(typeof(TwiceEnumerable));
+            var strategy = DefaultCollectionValidationStrategy.Instance;
+
+            // Act
+            var enumerator = strategy.GetChildren(metadata, "prefix", model);
+
+            // Assert
+            Assert.Collection(
+                BufferEntries(enumerator).OrderBy(e => e.Key),
+                e =>
+                {
+                    Assert.Equal("prefix[0]", e.Key);
+                    Assert.Equal(2, e.Model);
+                    Assert.Same(metadata.ElementMetadata, e.Metadata);
+                },
+                e =>
+                {
+                    Assert.Equal("prefix[1]", e.Key);
+                    Assert.Equal(3, e.Model);
+                    Assert.Same(metadata.ElementMetadata, e.Metadata);
+                },
+                e =>
+                {
+                    Assert.Equal("prefix[2]", e.Key);
+                    Assert.Equal(5, e.Model);
+                    Assert.Same(metadata.ElementMetadata, e.Metadata);
+                });
+        }
+
+        // 'int' is chosen by validation because it's declared on the more derived type.
+        private class TwiceEnumerable : List<string>, IEnumerable<int>
+        {
+            private readonly IEnumerable<int> _enumerable;
+
+            public TwiceEnumerable(IEnumerable<int> enumerable)
+            {
+                _enumerable = enumerable;
+            }
+
+            IEnumerator<int> IEnumerable<int>.GetEnumerator()
+            {
+                return _enumerable.GetEnumerator();
+            }
+
+            IEnumerator IEnumerable.GetEnumerator()
+            {
+                throw new InvalidOperationException();
+            }
+        }
+
         private List<ValidationEntry> BufferEntries(IEnumerator<ValidationEntry> enumerator)
         {
             var entries = new List<ValidationEntry>();

test/Microsoft.AspNet.Mvc.Core.Test/ModelBinding/Validation/ExplicitIndexCollectionValidationStrategyTest.cs

@@ -1,6 +1,8 @@
 // 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;
 using System.Collections.Generic;
 using System.Linq;
 using Xunit;
@@ -85,6 +87,41 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
                 });
         }
 
+        [Fact]
+        public void EnumerateElements_TwoEnumerableImplemenations()
+        {
+            // Arrange
+            var model = new TwiceEnumerable(new int[] { 2, 3, 5 });
+
+            var metadata = TestModelMetadataProvider.CreateDefaultProvider().GetMetadataForType(typeof(TwiceEnumerable));
+            var strategy = new ExplicitIndexCollectionValidationStrategy(new string[] { "zero", "one", "two" });
+
+            // Act
+            var enumerator = strategy.GetChildren(metadata, "prefix", model);
+
+            // Assert
+            Assert.Collection(
+                BufferEntries(enumerator).OrderBy(e => e.Key),
+                e =>
+                {
+                    Assert.Equal("prefix[one]", e.Key);
+                    Assert.Equal(3, e.Model);
+                    Assert.Same(metadata.ElementMetadata, e.Metadata);
+                },
+                e =>
+                {
+                    Assert.Equal("prefix[two]", e.Key);
+                    Assert.Equal(5, e.Model);
+                    Assert.Same(metadata.ElementMetadata, e.Metadata);
+                },
+                e =>
+                {
+                    Assert.Equal("prefix[zero]", e.Key);
+                    Assert.Equal(2, e.Model);
+                    Assert.Same(metadata.ElementMetadata, e.Metadata);
+                });
+        }
+
         [Fact]
         public void EnumerateElements_RunOutOfIndices()
         {
@@ -145,6 +182,27 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Validation
                 });
         }
 
+        // 'int' is chosen by validation because it's declared on the more derived type.
+        private class TwiceEnumerable : List<string>, IEnumerable<int>
+        {
+            private readonly IEnumerable<int> _enumerable;
+
+            public TwiceEnumerable(IEnumerable<int> enumerable)
+            {
+                _enumerable = enumerable;
+            }
+
+            IEnumerator<int> IEnumerable<int>.GetEnumerator()
+            {
+                return _enumerable.GetEnumerator();
+            }
+
+            IEnumerator IEnumerable.GetEnumerator()
+            {
+                throw new InvalidOperationException();
+            }
+        }
+
         private List<ValidationEntry> BufferEntries(IEnumerator<ValidationEntry> enumerator)
         {
             var entries = new List<ValidationEntry>();