Show helpful error when ActionResult returns unsupported type

src/Examples/JsonApiDotNetCoreExample/Controllers/TodoItemsTestController.cs

@@ -61,7 +61,9 @@ public override async Task<IActionResult> PostAsync(TodoItem entity)
         [HttpPatch("{id}")]
         public override async Task<IActionResult> PatchAsync(int id, [FromBody] TodoItem entity)
         {
-            return await base.PatchAsync(id, entity);
+            await Task.Yield();
+
+            return Conflict("Something went wrong");
         }
 
         [HttpPatch("{id}/relationships/{relationshipName}")]

src/JsonApiDotNetCore/Serialization/Client/IRequestSerializer.cs

@@ -22,7 +22,7 @@ public interface IRequestSerializer
         /// </summary>
         /// <param name="entities">Entities to serialize</param>
         /// <returns>The serialized content</returns>
-        string Serialize(IEnumerable entities);
+        string Serialize(IEnumerable<IIdentifiable> entities);
         /// <summary>
         /// Sets the attributes that will be included in the serialized payload.
         /// You can use <see cref="IResourceGraph.GetAttributes{TResource}"/>

src/JsonApiDotNetCore/Serialization/Client/RequestSerializer.cs

@@ -41,7 +41,7 @@ public string Serialize(IIdentifiable entity)
         }
 
         /// <inheritdoc/>
-        public string Serialize(IEnumerable entities)
+        public string Serialize(IEnumerable<IIdentifiable> entities)
         {
             IIdentifiable entity = null;
             foreach (IIdentifiable item in entities)

src/JsonApiDotNetCore/Serialization/Common/DocumentBuilder.cs

@@ -44,7 +44,7 @@ protected Document Build(IIdentifiable entity, IReadOnlyCollection<AttrAttribute
         /// <param name="attributes">Attributes to include in the building process</param>
         /// <param name="relationships">Relationships to include in the building process</param>
         /// <returns>The resource object that was built</returns>
-        protected Document Build(IEnumerable entities, IReadOnlyCollection<AttrAttribute> attributes, IReadOnlyCollection<RelationshipAttribute> relationships)
+        protected Document Build(IEnumerable<IIdentifiable> entities, IReadOnlyCollection<AttrAttribute> attributes, IReadOnlyCollection<RelationshipAttribute> relationships)
         {
             var data = new List<ResourceObject>();
             foreach (IIdentifiable entity in entities)

src/JsonApiDotNetCore/Serialization/Server/ResponseSerializer.cs

@@ -3,6 +3,7 @@
 using System.Collections.Generic;
 using JsonApiDotNetCore.Configuration;
 using JsonApiDotNetCore.Extensions;
+using JsonApiDotNetCore.Internal;
 using JsonApiDotNetCore.Models;
 using Newtonsoft.Json;
 using JsonApiDotNetCore.Managers.Contracts;
@@ -54,11 +55,22 @@ public class ResponseSerializer<TResource> : BaseDocumentBuilder, IJsonApiSerial
         /// <inheritdoc/>
         public string Serialize(object data)
         {
+            if (data == null || data is IIdentifiable)
+            {
+                return SerializeSingle((IIdentifiable)data);
+            }
+
+            if (data is IEnumerable<IIdentifiable> collectionOfIdentifiable)
+            {
+                return SerializeMany(collectionOfIdentifiable);
+            }
+
             if (data is ErrorDocument errorDocument)
+            {
                 return SerializeErrorDocument(errorDocument);
-            if (data is IEnumerable entities)
-                return SerializeMany(entities);
-            return SerializeSingle((IIdentifiable)data);
+            }
+
+            throw new InvalidOperationException("Data being returned must be errors or resources.");
         }
 
         private string SerializeErrorDocument(ErrorDocument errorDocument)
@@ -102,11 +114,11 @@ internal string SerializeSingle(IIdentifiable entity)
         /// <remarks>
         /// This method is set internal instead of private for easier testability.
         /// </remarks>
-        internal string SerializeMany(IEnumerable entities)
+        internal string SerializeMany(IEnumerable<IIdentifiable> entities)
         {
             var (attributes, relationships) = GetFieldsToSerialize();
             var document = Build(entities, attributes, relationships);
-            foreach (ResourceObject resourceObject in (IEnumerable)document.Data)
+            foreach (ResourceObject resourceObject in document.ManyData)
             {
                 var links = _linkBuilder.GetResourceLinks(resourceObject.Type, resourceObject.Id);
                 if (links == null)

test/JsonApiDotNetCoreExampleTests/Acceptance/ActionResultTests.cs

@@ -77,5 +77,41 @@ public async Task Empty_ActionResult_Is_Converted_To_Error_Collection()
             Assert.Equal("NotFound", errorDocument.Errors[0].Title);
             Assert.Null(errorDocument.Errors[0].Detail);
         }
+
+        [Fact]
+        public async Task ActionResult_With_String_Object_Is_Converted_To_Error_Collection()
+        {
+            // Arrange
+            var route = "/abstract/123";
+            var request = new HttpRequestMessage(HttpMethod.Patch, route);
+            var content = new
+            {
+                data = new
+                {
+                    type = "todoItems",
+                    id = 123,
+                    attributes = new Dictionary<string, object>
+                    {
+                        {"ordinal", 1}
+                    }
+                }
+            };
+
+            request.Content = new StringContent(JsonConvert.SerializeObject(content));
+            request.Content.Headers.ContentType = new MediaTypeHeaderValue(HeaderConstants.MediaType);
+
+            // Act
+            var response = await _fixture.Client.SendAsync(request);
+
+            // Assert
+            var body = await response.Content.ReadAsStringAsync();
+            Assert.Equal(HttpStatusCode.InternalServerError, response.StatusCode);
+
+            var errorDocument = JsonConvert.DeserializeObject<ErrorDocument>(body);
+            Assert.Single(errorDocument.Errors);
+            Assert.Equal(HttpStatusCode.InternalServerError, errorDocument.Errors[0].StatusCode);
+            Assert.Equal("An unhandled error occurred while processing this request.", errorDocument.Errors[0].Title);
+            Assert.Equal("Data being returned must be errors or resources.", errorDocument.Errors[0].Detail);
+        }
     }
 }

test/UnitTests/Serialization/SerializerTestsSetup.cs

@@ -115,7 +115,7 @@ public new Document Build(IIdentifiable entity, IReadOnlyCollection<AttrAttribut
                 return base.Build(entity, attributes, relationships);
             }
 
-            public new Document Build(IEnumerable entities, IReadOnlyCollection<AttrAttribute> attributes = null, IReadOnlyCollection<RelationshipAttribute> relationships = null)
+            public new Document Build(IEnumerable<IIdentifiable> entities, IReadOnlyCollection<AttrAttribute> attributes = null, IReadOnlyCollection<RelationshipAttribute> relationships = null)
             {
                 return base.Build(entities, attributes, relationships);
             }