CSHARP-4976: MongoDB C# Driver no longer compatible with DevExpress ASP.NET Core Middleware

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToExecutableQueryTranslators/ExecutableQuery.cs

@@ -13,14 +13,17 @@
 * limitations under the License.
 */
 
+using System;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MongoDB.Bson;
 using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.Serializers;
 using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Optimizers;
+using MongoDB.Driver.Linq.Linq3Implementation.Misc;
 
 namespace MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators
 {
@@ -141,17 +144,42 @@ public override string ToString()
         // private methods
         private BsonDocumentStagePipelineDefinition<TDocument, TOutput> CreateCollectionPipelineDefinition(BsonDocument[] stages)
         {
-            return new BsonDocumentStagePipelineDefinition<TDocument, TOutput>(stages, (IBsonSerializer<TOutput>)_pipeline.OutputSerializer);
+            var outputSerializer = GetOutputSerializer();
+            return new BsonDocumentStagePipelineDefinition<TDocument, TOutput>(stages, outputSerializer);
         }
 
         private BsonDocumentStagePipelineDefinition<NoPipelineInput, TOutput> CreateDatabasePipelineDefinition(BsonDocument[] stages)
         {
-            return new BsonDocumentStagePipelineDefinition<NoPipelineInput, TOutput>(stages, (IBsonSerializer<TOutput>)_pipeline.OutputSerializer);
+            var outputSerializer = GetOutputSerializer();
+            return new BsonDocumentStagePipelineDefinition<NoPipelineInput, TOutput>(stages, outputSerializer);
         }
 
         private BsonDocument[] RenderPipeline()
         {
             return _pipeline.Render().AsBsonArray.Cast<BsonDocument>().ToArray();
         }
+
+        private IBsonSerializer<TOutput> GetOutputSerializer()
+        {
+            var outputSerializer = _pipeline.OutputSerializer;
+            var outputType = outputSerializer.ValueType;
+
+            if (outputType == typeof(TOutput))
+            {
+                return (IBsonSerializer<TOutput>)outputSerializer;
+            }
+
+            if (!typeof(TOutput).IsAssignableFrom(outputType))
+            {
+                throw new NotSupportedException($"The type of the pipeline output is {outputType} which is not assignable to {typeof(TOutput)}.");
+            }
+
+            if (typeof(TOutput).IsNullableOf(outputType))
+            {
+                return (IBsonSerializer<TOutput>)NullableSerializer.Create(outputSerializer);
+            }
+
+            return (IBsonSerializer<TOutput>)DowncastingSerializer.Create(typeof(TOutput), outputType, outputSerializer);
+        }
     }
 }

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Translators/ExpressionToExecutableQueryTranslators/ExecutableQueryTests.cs

@@ -0,0 +1,149 @@
+/* Copyright 2010-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System;
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators
+{
+    public class ExecutableQueryTests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Cast_to_object_should_work()
+        {
+            var collection = GetCollection();
+            var queryable1 = collection.AsQueryable();
+            var queryable2 = queryable1.Provider.CreateQuery<object>(queryable1.Expression);
+
+            var results = queryable2.ToList();
+
+            results.Should().HaveCount(5);
+        }
+
+        [Fact]
+        public void Cast_aggregation_to_object_should_work()
+        {
+            var collection = GetCollection();
+            var queryable1 = collection.AsQueryable().GroupBy(
+                p => p.Type,
+                (k, p) => new ProductAggregation {Type = k, MaxPrice = p.Select(i => i.Price).Max()});
+            var queryable2 = queryable1.Provider.CreateQuery<object>(queryable1.Expression);
+
+            var results = queryable2.ToList();
+
+            results.Should().HaveCount(2);
+        }
+
+        [Fact]
+        public void Cast_int_to_object_should_work()
+        {
+            var collection = GetCollection();
+            var queryable1 = collection.AsQueryable().Select(p => p.Id);
+            var queryable2 = queryable1.Provider.CreateQuery<object>(queryable1.Expression);
+
+            var results = queryable2.ToList();
+
+            results.Should().HaveCount(5);
+        }
+
+        [Fact]
+        public void Cast_to_nullable_should_work()
+        {
+            var collection = GetCollection();
+            var queryable1 = collection.AsQueryable().Select(p => p.Id);
+            var queryable2 = queryable1.Provider.CreateQuery<int?>(queryable1.Expression);
+
+            var results = queryable2.ToList();
+
+            results.Should().HaveCount(5);
+        }
+
+        [Fact]
+        public void Cast_to_incompatible_type_should_throw()
+        {
+            var collection = GetCollection();
+            var queryable1 = collection.AsQueryable();
+            var queryable2 = queryable1.Provider.CreateQuery<ProductAggregation>(queryable1.Expression);
+
+            var exception = Record.Exception(() => queryable2.ToList());
+
+            exception.Should().BeOfType<NotSupportedException>();
+            exception.Message.Should().Contain($"The type of the pipeline output is {typeof(DerivedProduct)} which is not assignable to {typeof(ProductAggregation)}");
+        }
+
+        [Fact]
+        public void Cast_to_interface_should_work()
+        {
+            var collection = GetCollection();
+            var queryable1 = collection.AsQueryable();
+            var queryable2 = queryable1.Provider.CreateQuery<IProduct>(queryable1.Expression);
+
+            var results = queryable2.ToList();
+
+            results.Should().HaveCount(5);
+        }
+
+        [Fact]
+        public void Cast_to_base_class_should_work()
+        {
+            var collection = GetCollection();
+            var queryable1 = collection.AsQueryable();
+            var queryable2 = queryable1.Provider.CreateQuery<ProductBase>(queryable1.Expression);
+
+            var results = queryable2.ToList();
+
+            results.Should().HaveCount(5);
+        }
+
+        private IMongoCollection<DerivedProduct> GetCollection()
+        {
+            var collection = GetCollection<DerivedProduct>("test");
+            CreateCollection(
+                collection,
+                new DerivedProduct { Id = 1, Type = "a", Price = 1 },
+                new DerivedProduct { Id = 2, Type = "a", Price = 5 },
+                new DerivedProduct { Id = 3, Type = "a", Price = 12 },
+                new DerivedProduct { Id = 4, Type = "b", Price = 2 },
+                new DerivedProduct { Id = 5, Type = "b", Price = 7 });
+            return collection;
+        }
+
+        private interface IProduct
+        {
+            string Type { get; set; }
+            decimal Price { get; set; }
+        }
+
+        private class ProductBase : IProduct
+        {
+            public int Id { get; set; }
+            public string Type { get; set; }
+            public decimal Price { get; set; }
+        }
+
+        private class DerivedProduct : ProductBase
+        {
+        }
+
+        private class ProductAggregation
+        {
+            public string Type { get; set; }
+            public decimal MaxPrice { get; set; }
+        }
+    }
+}