CSHARP-4466: MongoQueryable.AppendStage uses incorrect result serializer type.

Author: rstam

src/MongoDB.Driver/Linq/MongoQueryable.cs

@@ -96,7 +96,7 @@ public static IMongoQueryable<TResult> AppendStage<TSource, TResult>(
                     GetMethodInfo(AppendStage, source, stage, resultSerializer),
                     Expression.Convert(source.Expression, typeof(IMongoQueryable<TSource>)),
                     Expression.Constant(stage),
-                    Expression.Constant(resultSerializer, typeof(IBsonSerializer<TSource>))));
+                    Expression.Constant(resultSerializer, typeof(IBsonSerializer<TResult>))));
         }
 
         /// <summary>

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4466Tests.cs

@@ -0,0 +1,85 @@
+/* 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.Linq;
+using FluentAssertions;
+using MongoDB.Bson.Serialization;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4466Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void AppendStage_with_null_resultSerializer_should_work()
+        {
+            var collection = CreateCollection();
+
+            var queryable =
+                collection.AsQueryable()
+                .AppendStage<C, D>("{ $project : { X : '$_id', _id : 0 } }", resultSerializer: null);
+
+            var stages = Translate(collection, queryable, out var outputSerializer);
+
+            AssertStages(stages, "{ $project : { X : '$_id', _id : 0 } }");
+            outputSerializer.ValueType.Should().Be(typeof(D));
+
+            var results = queryable.ToList();
+            results.Select(x => x.X).Should().Equal(1, 2);
+        }
+
+        [Fact]
+        public void AppendStage_with_resultSerializer_should_work()
+        {
+            var collection = CreateCollection();
+            var resultSerializer = BsonSerializer.LookupSerializer<D>();
+
+            var queryable =
+                collection.AsQueryable()
+                .AppendStage<C, D>("{ $project : { X : '$_id', _id : 0 } }", resultSerializer);
+
+            var stages = Translate(collection, queryable, out var outputSerializer);
+
+            AssertStages(stages, "{ $project : { X : '$_id', _id : 0 } }");
+            outputSerializer.Should().BeSameAs(resultSerializer);
+
+            var results = queryable.ToList();
+            results.Select(x => x.X).Should().Equal(1, 2);
+        }
+
+        private IMongoCollection<C> CreateCollection()
+        {
+            var collection = GetCollection<C>("C");
+
+            CreateCollection(
+                collection,
+                new C { Id = 1 },
+                new C { Id = 2 });
+
+            return collection;
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+        }
+
+        private class D
+        {
+            public int X { get; set; }
+        }
+    }
+}