200: fixed check of element type for IEnumerable<T>

src/JsonApiDotNetCore/Builders/DocumentBuilder.cs

@@ -1,6 +1,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using JsonApiDotNetCore.Extensions;
 using JsonApiDotNetCore.Internal;
 using JsonApiDotNetCore.Models;
 using JsonApiDotNetCore.Services;
@@ -46,9 +47,7 @@ public Document Build(IIdentifiable entity)
 
         public Documents Build(IEnumerable<IIdentifiable> entities)
         {
-            var entityType = entities
-                .GetType()
-                .GenericTypeArguments[0];
+            var entityType = entities.GetElementType();
 
             var contextEntity = _contextGraph.GetContextEntity(entityType);
 
@@ -229,7 +228,7 @@ private bool RelationshipIsIncluded(string relationshipName)
 
         private List<Dictionary<string, string>> GetRelationships(IEnumerable<object> entities)
         {
-            var objType = entities.GetType().GenericTypeArguments[0];
+            var objType = entities.GetElementType();
 
             var typeName = _jsonApiContext.ContextGraph.GetContextEntity(objType);
 

src/JsonApiDotNetCore/Extensions/TypeExtensions.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace JsonApiDotNetCore.Extensions
+{
+    internal static class TypeExtensions
+    {
+        public static Type GetElementType(this IEnumerable enumerable)
+        {
+            var enumerableTypes = enumerable.GetType()
+                .GetInterfaces()
+                .Where(t => t.IsGenericType == true && t.GetGenericTypeDefinition() == typeof(IEnumerable<>))
+                .ToList();
+
+            var numberOfEnumerableTypes = enumerableTypes.Count;
+
+            if (numberOfEnumerableTypes == 0)
+            {
+                throw new ArgumentException($"{nameof(enumerable)} of type {enumerable.GetType().FullName} does not implement a generic variant of {nameof(IEnumerable)}");
+            }
+
+            if (numberOfEnumerableTypes > 1)
+            {
+                throw new ArgumentException($"{nameof(enumerable)} of type {enumerable.GetType().FullName} implements more than one generic variant of {nameof(IEnumerable)}:\n" +
+                    $"{string.Join("\n", enumerableTypes.Select(t => t.FullName))}");
+            }
+
+            var elementType = enumerableTypes[0].GenericTypeArguments[0];
+
+            return elementType;
+        }
+    }
+}

test/UnitTests/Builders/DocumentBuilder_Tests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using JsonApiDotNetCore.Builders;
 using JsonApiDotNetCore.Configuration;
@@ -118,6 +119,28 @@ public void Related_Links_Can_Be_Disabled()
             Assert.Null(document.Data.Relationships["related-model"].Links);
         }
 
+        [Fact]
+        public void Build_Can_Build_Arrays()
+        {
+            var entities = new[] { new Model() };
+            var documentBuilder = new DocumentBuilder(_jsonApiContextMock.Object);
+
+            var documents = documentBuilder.Build(entities);
+
+            Assert.Equal(1, documents.Data.Count);
+        }
+
+        [Fact]
+        public void Build_Can_Build_CustomIEnumerables()
+        {
+            var entities = new Models(new[] { new Model() });
+            var documentBuilder = new DocumentBuilder(_jsonApiContextMock.Object);
+
+            var documents = documentBuilder.Build(entities);
+
+            Assert.Equal(1, documents.Data.Count);
+        }
+
         private class Model : Identifiable
         {
             [HasOne("related-model", Link.None)]
@@ -130,5 +153,25 @@ private class RelatedModel : Identifiable
             [HasMany("models")]
             public List<Model> Models { get; set; }
         }
+
+        private class Models : IEnumerable<Model>
+        {
+            private readonly IEnumerable<Model> models;
+
+            public Models(IEnumerable<Model> models)
+            {
+                this.models = models;
+            }
+
+            public IEnumerator<Model> GetEnumerator()
+            {
+                return models.GetEnumerator();
+            }
+
+            IEnumerator IEnumerable.GetEnumerator()
+            {
+                return models.GetEnumerator();
+            }
+        }
     }
 }