CSHARP-2918: Watch method should work even when documents contain duplicate element names.

Author: rstam

src/MongoDB.Driver.Core/ChangeStreamDocument.cs

@@ -75,7 +75,21 @@ public ChangeStreamDocument(
         /// <value>
         /// The full document.
         /// </value>
-        public TDocument FullDocument => GetValue<TDocument>(nameof(FullDocument), default(TDocument));
+        public TDocument FullDocument
+        {
+            get
+            {
+                // if TDocument is BsonDocument avoid deserializing it again to prevent possible duplicate element name errors
+                if (typeof(TDocument) == typeof(BsonDocument) && BackingDocument.TryGetValue("fullDocument", out var fullDocument))
+                {
+                    return (TDocument)(object)fullDocument.AsBsonDocument;
+                }
+                else
+                {
+                    return GetValue<TDocument>(nameof(FullDocument), default(TDocument));
+                }
+            }
+        }
 
         /// <summary>
         /// Gets the type of the operation.

src/MongoDB.Driver.Core/ChangeStreamDocumentSerializer.cs

@@ -50,6 +50,14 @@ public ChangeStreamDocumentSerializer(
             RegisterMember("UpdateDescription", "updateDescription", ChangeStreamUpdateDescriptionSerializer.Instance);
         }
 
+        // public methods
+        /// <inheritdoc />
+        public override ChangeStreamDocument<TDocument> Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
+        {
+            context = context.With(b => b.AllowDuplicateElementNames = true);
+            return base.Deserialize(context, args);
+        }
+
         // protected methods
         /// <inheritdoc />
         protected override ChangeStreamDocument<TDocument> CreateInstance(BsonDocument backingDocument)

tests/MongoDB.Driver.Core.Tests/ChangeStreamDocumentSerializerTests.cs

@@ -88,6 +88,29 @@ public void Deserialize_should_return_expected_result_when_value_is_null()
             result.Should().BeNull();
         }
 
+        [Fact]
+        public void Deserialize_should_support_duplicate_element_names_in_full_document()
+        {
+            var json = "{ fullDocument : { x : 1, x : 2 } }";
+            var subject = CreateSubject();
+
+            ChangeStreamDocument<BsonDocument> result;
+            using (var reader = new JsonReader(json))
+            {
+                var context = BsonDeserializationContext.CreateRoot(reader);
+                result = subject.Deserialize(context);
+            }
+
+            var fullDocument = result.FullDocument;
+            fullDocument.ElementCount.Should().Be(2);
+            var firstElement = fullDocument.GetElement(0);
+            firstElement.Name.Should().Be("x");
+            firstElement.Value.Should().Be(1);
+            var secondElement = fullDocument.GetElement(1);
+            secondElement.Name.Should().Be("x");
+            secondElement.Value.Should().Be(2);
+        }
+
         [Fact]
         public void Serialize_should_have_expected_result()
         {

tests/MongoDB.Driver.Core.Tests/ChangeStreamDocumentTests.cs

@@ -72,6 +72,18 @@ public void BackingDocument_should_return_expected_result()
             result.Should().BeSameAs(backingDocument);
         }
 
+        [Fact]
+        public void BackingDocument_should_allow_duplicate_elements_in_full_document()
+        {
+            var fullDocument = new BsonDocument(allowDuplicateNames: true) { { "x", 1 }, { "x", 2 } };
+            var backingDocument = new BsonDocument("fullDocument", fullDocument);
+            var subject = CreateSubject(backingDocument: backingDocument);
+
+            var result = subject.BackingDocument;
+
+            result.Should().BeSameAs(backingDocument);
+        }
+
         [Theory]
         [InlineData(1, 2)]
         [InlineData(3, 4)]
@@ -159,6 +171,24 @@ public void FullDocument_should_return_expected_result()
             result.Should().Be(value);
         }
 
+        [Fact]
+        public void FullDocument_should_allow_duplicate_elements()
+        {
+            var fullDocument = new BsonDocument(allowDuplicateNames: true) { { "x", 1 }, { "x", 2 } };
+            var backingDocument = new BsonDocument { { "other", 1 }, { "fullDocument", fullDocument } };
+            var subject = CreateSubject(backingDocument: backingDocument);
+
+            var result = subject.FullDocument;
+
+            result.ElementCount.Should().Be(2);
+            var firstElement = result.GetElement(0);
+            firstElement.Name.Should().Be("x");
+            firstElement.Value.Should().Be(1);
+            var secondElement = result.GetElement(1);
+            secondElement.Name.Should().Be("x");
+            secondElement.Value.Should().Be(2);
+        }
+
         [Fact]
         public void FullDocument_should_return_null_when_not_present()
         {

tests/MongoDB.Driver.Tests/MongoCollectionImplTests.cs

@@ -28,8 +28,10 @@
 using MongoDB.Driver.Core.Bindings;
 using MongoDB.Driver.Core.Clusters;
 using MongoDB.Driver.Core.Connections;
+using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Core.Operations;
 using MongoDB.Driver.Core.Servers;
+using MongoDB.Driver.Core.TestHelpers.XunitExtensions;
 using MongoDB.Driver.Tests;
 using Moq;
 using Xunit;
@@ -3206,6 +3208,42 @@ public void Watch_should_throw_when_pipeline_is_null(
             e.ParamName.Should().Be("pipeline");
         }
 
+        [SkippableFact]
+        public void Watch_should_support_full_document_with_duplicate_elements()
+        {
+            RequireServer.Check().Supports(Feature.ChangeStreamStage).ClusterTypes(ClusterType.ReplicaSet, ClusterType.Sharded);
+
+            var client = DriverTestConfiguration.Client;
+            var database = client.GetDatabase(DriverTestConfiguration.DatabaseNamespace.DatabaseName);
+            var collection = database.GetCollection<BsonDocument>(DriverTestConfiguration.CollectionNamespace.CollectionName);
+            database.DropCollection(collection.CollectionNamespace.CollectionName);
+
+            try
+            {
+                var cursor = collection.Watch();
+
+                ChangeStreamDocument<BsonDocument> changeStreamDocument = null;
+                var document = new BsonDocument(allowDuplicateNames: true) { { "_id", 1 }, { "x", 2 }, { "x", 3 } };
+                collection.InsertOne(document);
+                SpinWait.SpinUntil(() => cursor.MoveNext() && (changeStreamDocument = cursor.Current.FirstOrDefault()) != null, TimeSpan.FromSeconds(5)).Should().BeTrue();
+                var fullDocument = changeStreamDocument.FullDocument;
+                fullDocument.ElementCount.Should().Be(3);
+                var firstElement = fullDocument.GetElement(0);
+                firstElement.Name.Should().Be("_id");
+                firstElement.Value.Should().Be(1);
+                var secondElement = fullDocument.GetElement(1);
+                secondElement.Name.Should().Be("x");
+                secondElement.Value.Should().Be(2);
+                var thirdElement = fullDocument.GetElement(2);
+                thirdElement.Name.Should().Be("x");
+                thirdElement.Value.Should().Be(3);
+            }
+            finally
+            {
+                database.DropCollection(collection.CollectionNamespace.CollectionName);
+            }
+        }
+
         [Fact]
         public void WithReadPreference_should_return_a_new_collection_with_the_read_preference_changed()
         {