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There seems to be a bug when trying to serialize properties of IReadOnlyList<T> when the property is set to an array of a derived type.
For example, running the following code:
using MongoDB.Bson;
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using MongoDB.Driver;
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var client = new MongoClient();
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var database = client.GetDatabase("test");
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var collection = database.GetCollection<Farm>("farms");
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var farm = new Farm(
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ObjectId.GenerateNewId(),
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"Farm 1",
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new[] { new Cow(1, "Bob"), new Cow(2, "Alice") }
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);
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await collection.InsertOneAsync(farm);
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// Exception thrown here
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var foundFarm = await collection.Find(x => x.Id == farm.Id).FirstAsync();
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record Farm(ObjectId Id, string Name, IReadOnlyList<Animal> Animals);
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abstract record Animal(int Id);
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record Cow(int Id, string Name) : Animal(Id);
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record Pig(int Id, string Name) : Animal(Id);
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Throws the following exception:
Unhandled exception. System.FormatException: An error occurred while deserializing the Animals property of class Farm: Cannot create an abstract class.
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---> System.MemberAccessException: Cannot create an abstract class.
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at System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject(Type type)
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at System.Runtime.Serialization.FormatterServices.GetUninitializedObject(Type type)
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at MongoDB.Bson.Serialization.BsonClassMap.<GetCreator>b__114_2()
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at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeClass(BsonDeserializationContext context)
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at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
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at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
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at MongoDB.Bson.Serialization.Serializers.EnumerableSerializerBase`2.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
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at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
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at MongoDB.Bson.Serialization.Serializers.ImpliedImplementationInterfaceSerializer`2.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
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at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
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at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize(IBsonSerializer serializer, BsonDeserializationContext context)
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at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap)
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--- End of inner exception stack trace ---
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at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap)
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at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeClass(BsonDeserializationContext context)
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at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
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at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
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at MongoDB.Driver.Core.Operations.CursorBatchDeserializationHelper.DeserializeBatch[TDocument](RawBsonArray batch, IBsonSerializer`1 documentSerializer, MessageEncoderSettings messageEncoderSettings)
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at MongoDB.Driver.Core.Operations.FindCommandOperation`1.CreateFirstCursorBatch(BsonDocument cursorDocument)
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at MongoDB.Driver.Core.Operations.FindCommandOperation`1.CreateCursor(IChannelSourceHandle channelSource, IChannelHandle channel, BsonDocument commandResult)
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at MongoDB.Driver.Core.Operations.FindCommandOperation`1.ExecuteAsync(RetryableReadContext context, CancellationToken cancellationToken)
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at MongoDB.Driver.Core.Operations.FindOperation`1.ExecuteAsync(RetryableReadContext context, CancellationToken cancellationToken)
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at MongoDB.Driver.Core.Operations.FindOperation`1.ExecuteAsync(IReadBinding binding, CancellationToken cancellationToken)
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at MongoDB.Driver.OperationExecutor.ExecuteReadOperationAsync[TResult](IReadBinding binding, IReadOperation`1 operation, CancellationToken cancellationToken)
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at MongoDB.Driver.MongoCollectionImpl`1.ExecuteReadOperationAsync[TResult](IClientSessionHandle session, IReadOperation`1 operation, ReadPreference readPreference, CancellationToken cancellationToken)
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at MongoDB.Driver.MongoCollectionImpl`1.UsingImplicitSessionAsync[TResult](Func`2 funcAsync, CancellationToken cancellationToken)
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at MongoDB.Driver.IAsyncCursorSourceExtensions.FirstAsync[TDocument](IAsyncCursorSource`1 source, CancellationToken cancellationToken)
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at Program.<Main>$(String[] args) in C:\dev\kevbite\MongoDbBug\MongoDbBug\MongoDbBug\Program.cs:line 20
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at Program.<Main>(String[] args)
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... and generates the following document in the database
{
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"_id" : ObjectId("61a9036e68774829f484e844"),
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"Name" : "Farm 1",
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"Animals" : [
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{
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"_id" : 1,
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"Name" : "Bob"
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},
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{
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"_id" : 2,
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"Name" : "Alice"
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}
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]
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}
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As you can see it's missing the discriminator field (`_t`)
I assume it's because the array is of type `Cow[]` instead of `Animal[]`. If we update the code with the following change:
var farm = new Farm(
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ObjectId.GenerateNewId(),
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"Farm 1",
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new Animal[] { new Cow(1, "Bob"), new Cow(2, "Alice") }
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);
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The following document is generated
{
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"_id" : ObjectId("61a905ea132842f1fcf18782"),
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"Name" : "Farm 1",
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"Animals" : [
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{
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"_t" : "Cow",
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"_id" : 1,
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"Name" : "Bob"
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},
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{
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"_t" : "Cow",
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"_id" : 2,
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"Name" : "Alice"
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}
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]
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}
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