CSHARP-6043: NullReferenceException in IEnumerableSerializerBase.Serialize when projecting nullable array with conditional Select (#2012)

Author: papafe

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

@@ -14,6 +14,7 @@
 */
 
 using System.Collections.Generic;
+using MongoDB.Bson;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Serializers;
 using MongoDB.Driver.Core.Misc;
@@ -36,6 +37,11 @@ public IEnumerableSerializerBase(IBsonSerializer<TItem> itemSerializer)
         public override TEnumerable Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
         {
             var reader = context.Reader;
+            if (reader.GetCurrentBsonType() == BsonType.Null)
+            {
+                reader.ReadNull();
+                return default;
+            }
             reader.ReadStartArray();
             var items = new List<TItem>();
             while (reader.ReadBsonType() != 0)
@@ -64,6 +70,11 @@ obj is IEnumerableSerializerBase<TEnumerable, TItem> other &&
         public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, TEnumerable value)
         {
             var writer = context.Writer;
+            if (value == null)
+            {
+                writer.WriteNull();
+                return;
+            }
             writer.WriteStartArray();
             foreach (var item in value)
             {

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Serializers/IEnumerableSerializerBaseTests.cs

@@ -16,6 +16,7 @@
 using System.Collections.Generic;
 using FluentAssertions;
 using MongoDB.Bson;
+using MongoDB.Bson.IO;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Serializers;
 using MongoDB.Driver.Linq.Linq3Implementation.Serializers;
@@ -101,6 +102,37 @@ public void GetHashCode_should_return_zero()
             result.Should().Be(0);
         }
 
+        [Fact]
+        public void Serialize_null_should_write_null()
+        {
+            var serializer = new ConcreteIEnumerableSerializerBase<IEnumerable<int>, int>(Int32Serializer.Instance);
+            var document = new BsonDocument();
+            using var writer = new BsonDocumentWriter(document);
+            var context = BsonSerializationContext.CreateRoot(writer);
+            writer.WriteStartDocument();
+            writer.WriteName("x");
+
+            serializer.Serialize(context, new BsonSerializationArgs(), null);
+
+            writer.WriteEndDocument();
+            document["x"].Should().Be(BsonNull.Value);
+        }
+
+        [Fact]
+        public void Deserialize_null_should_return_default()
+        {
+            var serializer = new ConcreteIEnumerableSerializerBase<IEnumerable<int>, int>(Int32Serializer.Instance);
+            var document = new BsonDocument("x", BsonNull.Value);
+            using var reader = new BsonDocumentReader(document);
+            reader.ReadStartDocument();
+            reader.ReadName("x");
+            var context = BsonDeserializationContext.CreateRoot(reader);
+
+            var result = serializer.Deserialize(context, new BsonDeserializationArgs { NominalType = typeof(IEnumerable<int>) });
+
+            result.Should().BeNull();
+        }
+
         internal class ConcreteIEnumerableSerializerBase<TEnumerable, TItem> : IEnumerableSerializerBase<TEnumerable, TItem>
             where TEnumerable : IEnumerable<TItem>
         {