MongoDB.Bson.Serialization.Serializers.BsonValueCSharpNullSerializer.Deserialize C# (CSharp) Method

        public override object Deserialize(
            BsonReader bsonReader,
            Type nominalType,
            Type actualType,
            IBsonSerializationOptions options)
        {
            var bsonType = bsonReader.GetCurrentBsonType();
            if (bsonType == BsonType.Document && IsCSharpNullRepresentation(bsonReader))
            {
                // if IsCSharpNullRepresentation returns true it will have consumed the document representing C# null
                return null;
            }

            // handle BSON null for backward compatibility with existing data (new data would have _csharpnull)
            if (bsonType == BsonType.Null && (nominalType != typeof(BsonValue) && nominalType != typeof(BsonNull)))
            {
                bsonReader.ReadNull();
                return null;
            }

            var serializer = BsonSerializer.LookupSerializer(actualType);
            return serializer.Deserialize(bsonReader, nominalType, actualType, options);
        }