CSHARP-1485: Fix ExpandoObject serialization to permit nulls. (#542)

Author: JamesKovacs

src/MongoDB.Bson/Serialization/Serializers/DynamicDocumentBaseSerializer.cs

@@ -15,7 +15,6 @@
 
 using System;
 using System.Dynamic;
-using System.IO;
 using System.Linq.Expressions;
 using MongoDB.Bson.IO;
 
@@ -25,7 +24,7 @@ namespace MongoDB.Bson.Serialization.Serializers
     /// Base serializer for dynamic types.
     /// </summary>
     /// <typeparam name="T">The dynamic type.</typeparam>
-    public abstract class DynamicDocumentBaseSerializer<T> : SerializerBase<T> where T : IDynamicMetaObjectProvider
+    public abstract class DynamicDocumentBaseSerializer<T> : SerializerBase<T> where T : class, IDynamicMetaObjectProvider
     {
         // private static fields
         private static readonly IBsonSerializer<object> _objectSerializer = BsonSerializer.LookupSerializer<object>();
@@ -65,6 +64,10 @@ public override T Deserialize(BsonDeserializationContext context, BsonDeserializ
                     bsonReader.ReadEndDocument();
                     return document;
 
+                case BsonType.Null:
+                    bsonReader.ReadNull();
+                    return null;
+
                 default:
                     message = string.Format("Cannot deserialize a '{0}' from BsonType '{1}'.", BsonUtils.GetFriendlyTypeName(typeof(T)), bsonType);
                     throw new FormatException(message);
@@ -81,6 +84,12 @@ public override void Serialize(BsonSerializationContext context, BsonSerializati
         {
             var bsonWriter = context.Writer;
 
+            if (value == null)
+            {
+                bsonWriter.WriteNull();
+                return;
+            }
+
             var metaObject = value.GetMetaObject(Expression.Constant(value));
             var memberNames = metaObject.GetDynamicMemberNames();
             var dynamicContext = context.With(ConfigureSerializationContext);

tests/MongoDB.Bson.Tests/Serialization/Serializers/ExpandoObjectSerializerTests.cs

@@ -51,6 +51,40 @@ public void TestRoundTrip()
             Assert.True(bson.SequenceEqual((rehydrated).ToBson()));
         }
 
+        [Fact]
+        public void TestNestedExpandoRoundTrip()
+        {
+            var document = new NestedExpando {Id = ObjectId.GenerateNewId(), ExtraData = new ExpandoObject()};
+
+            var json = document.ToJson();
+            var expected = $"{{ \"_id\" : ObjectId(\"{document.Id}\"), \"ExtraData\" : {{ }} }}";
+            Assert.Equal(expected, json);
+
+            var bson = document.ToBson();
+            var rehydrated = BsonSerializer.Deserialize<NestedExpando>(bson);
+            Assert.True(bson.SequenceEqual(rehydrated.ToBson()));
+        }
+
+        [Fact]
+        public void TestNullNestedExpandoRoundTrip()
+        {
+            var document = new NestedExpando {Id = ObjectId.GenerateNewId(), ExtraData = null};
+
+            var json = document.ToJson();
+            var expected = $"{{ \"_id\" : ObjectId(\"{document.Id}\"), \"ExtraData\" : null }}";
+            Assert.Equal(expected, json);
+
+            var bson = document.ToBson();
+            var rehydrated = BsonSerializer.Deserialize<NestedExpando>(bson);
+            Assert.True(bson.SequenceEqual(rehydrated.ToBson()));
+        }
+
+        class NestedExpando
+        {
+            public ObjectId Id { get; set; }
+            public ExpandoObject ExtraData { get; set; }
+        }
+
 #if NET452
         [Fact]
         public void TestDeserializingDiscriminatedVersion()