CSHARP-2643: Automapped serializer for anonymous classes should use default values for missing elements when deserializing.

Author: rstam

src/MongoDB.Bson/Serialization/Conventions/ImmutableTypeClassMapConvention.cs

@@ -76,7 +76,12 @@ public void Apply(BsonClassMap classMap)
                 // if any constructors were mapped by this convention then map all the properties also
                 foreach (var property in properties)
                 {
-                    classMap.MapMember(property);
+                    var memberMap = classMap.MapMember(property);
+                    if (classMap.IsAnonymous)
+                    {
+                        var defaultValue = memberMap.DefaultValue;
+                        memberMap.SetDefaultValue(defaultValue);
+                    }
                 }
             }
         }

tests/MongoDB.Bson.Tests/Jira/CSharp2643Tests.cs

@@ -0,0 +1,51 @@
+/* Copyright 2019-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 FluentAssertions;
+using MongoDB.Bson.IO;
+using MongoDB.Bson.Serialization;
+using Xunit;
+
+namespace MongoDB.Bson.Tests.Jira
+{
+    public class CSharp2643Tests
+    {
+        [Theory]
+        [InlineData("{ }", 0, 0)]
+        [InlineData("{ X : 1 }", 1, 0)]
+        [InlineData("{ Y : 2 }", 0, 2)]
+        [InlineData("{ X : 1, Y : 2 }", 1, 2)]
+        public void Deserializer_should_return_expected_result(string json, int expectedX, int expectedY)
+        {
+            var prototype = new { CSharp2643 = true, X = 1, Y = 2 }; // use CSharp2643 as a property name to guarantee a unique anonymous type
+
+            T deserialize<T>(T dummy, string value)
+            {
+                var serializer = BsonSerializer.LookupSerializer<T>();
+                using (var reader = new JsonReader(value))
+                {
+                    var context = BsonDeserializationContext.CreateRoot(reader);
+                    return serializer.Deserialize(context);
+                }
+            }
+
+            var result = deserialize(prototype, json);
+
+            result.CSharp2643.Should().BeFalse();
+            result.X.Should().Be(expectedX);
+            result.Y.Should().Be(expectedY);
+        }
+    }
+}