CSHARP-4538: WrappedValueSerializer should handle empty documents.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Serializers/WrappedValueSerializer.cs

@@ -14,6 +14,7 @@
 */
 
 using System;
+using MongoDB.Bson;
 using MongoDB.Bson.IO;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Serializers;
@@ -50,10 +51,18 @@ public WrappedValueSerializer(string fieldName, IBsonSerializer<TValue> valueSer
         // public methods
         public override TValue Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
         {
+            TValue value;
             var reader = context.Reader;
             reader.ReadStartDocument();
-            reader.ReadName(_fieldName);
-            var value = _valueSerializer.Deserialize(context);
+            if (reader.ReadBsonType() == BsonType.EndOfDocument)
+            {
+                value = default;
+            }
+            else
+            {
+                reader.ReadName(_fieldName);
+                value = _valueSerializer.Deserialize(context);
+            }
             reader.ReadEndDocument();
             return value;
         }

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4538Tests.cs

@@ -0,0 +1,77 @@
+/* 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;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4538Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Project_with_nullable_value_should_work()
+        {
+            var collection = CreateCollection();
+
+            var find = collection
+                .Find(_ => true)
+                .Project(x => x.D);
+
+            var projection = TranslateFindProjection(collection, find);
+            projection.Should().Be("{ _v : '$D', _id : 0  }");
+
+            var results = find.ToList();
+            results.Should().Equal(1.0, null, null);
+        }
+
+        [Fact]
+        public void Project_with_anonymous_class_with_nullable_value_should_work()
+        {
+            var collection = CreateCollection();
+
+            var find = collection
+                .Find(_ => true)
+                .Project(x => new { V = x.D });
+
+            var projection = TranslateFindProjection(collection, find);
+            projection.Should().Be("{ V : '$D', _id : 0  }");
+
+            var results = find.ToList();
+            results.Select(x => x.V).Should().Equal(1.0, null, null);
+        }
+
+        private IMongoCollection<C> CreateCollection()
+        {
+            var collection = GetCollection<C>("C");
+
+            var bsonDocumentCollection = collection.Database.GetCollection<BsonDocument>(collection.CollectionNamespace.CollectionName);
+            CreateCollection(
+                bsonDocumentCollection,
+                new BsonDocument { { "_id", 1 }, { "D", 1.0 } },
+                new BsonDocument { { "_id", 2 }, { "D", BsonNull.Value } },
+                new BsonDocument { { "_id", 3 } });
+
+            return collection;
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            public double? D { get; set; }
+        }
+    }
+}

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Linq3IntegrationTest.cs

@@ -159,5 +159,18 @@ protected BsonDocument Translate<TDocument>(IMongoCollection<TDocument> collecti
             var linqProvider = collection.Database.Client.Settings.LinqProvider;
             return filterDefinition.Render(documentSerializer, serializerRegistry, linqProvider);
         }
+
+        protected BsonDocument TranslateFindProjection<TDocument, TProjection>(
+            IMongoCollection<TDocument> collection,
+            IFindFluent<TDocument, TProjection> find,
+            LinqProvider linqProvider = LinqProvider.V3)
+        {
+            var findOptions = ((FindFluent<TDocument, TProjection>)find).Options;
+            var projection = findOptions.Projection;
+            var documentSerializer = collection.DocumentSerializer;
+            var serializerRegistry = BsonSerializer.SerializerRegistry;
+            var renderedProjection = projection.Render(documentSerializer, serializerRegistry, linqProvider);
+            return renderedProjection.Document;
+        }
     }
 }