using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Serializers;
using MongoDB.Driver;

var objectSerializer = new ObjectSerializer(ObjectSerializer.AllAllowedTypes); 

BsonSerializer.RegisterSerializer(objectSerializer);

var client = new MongoClient();

var collection = client.GetDatabase("test").GetCollection<D>("test_collection");

var testDocument = new D { O = new A<int> { X = 1 } };
collection.InsertOne(testDocument);

var result = collection.Find(Builders<D>.Filter.Empty).Single();
Console.WriteLine($"TestDocument X value: {((A<int>)result.O).X}");


public class A<TX>
{
    public ObjectId Id { get; set; }
    public TX X { get; set; }
}

public class D
{
    public ObjectId Id { get; set; }
    public object O { get; set; }
}

Unhandled exception. System.FormatException: An error occurred while deserializing the O property of class D: Unknown discriminator value 'A`1'.
 ---> MongoDB.Bson.BsonSerializationException: Unknown discriminator value 'A`1'.
   at MongoDB.Bson.Serialization.BsonSerializer.LookupActualType(Type nominalType, BsonValue discriminator) in mongo-csharp-driver/src/MongoDB.Bson/Serialization/BsonSerializer.cs:line 339
   at MongoDB.Bson.Serialization.Conventions.ObjectDiscriminatorConvention.GetActualType(IBsonReader bsonReader, Type nominalType) in mongo-csharp-driver/src/MongoDB.Bson/Serialization/Conventions/ObjectDiscriminatorConvention.cs:line 127
   at MongoDB.Bson.Serialization.Serializers.ObjectSerializer.DeserializeDiscriminatedValue(BsonDeserializationContext context, BsonDeserializationArgs args) in mongo-csharp-driver/src/MongoDB.Bson/Serialization/Serializers/ObjectSerializer.cs:line 380
   at MongoDB.Bson.Serialization.Serializers.ObjectSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args) in mongo-csharp-driver/src/MongoDB.Bson/Serialization/Serializers/ObjectSerializer.cs:line 223
   at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args) in mongo-csharp-driver/src/MongoDB.Bson/Serialization/Serializers/SerializerBase.cs:line 119
   at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize(IBsonSerializer serializer, BsonDeserializationContext context) in mongo-csharp-driver/src/MongoDB.Bson/Serialization/IBsonSerializerExtensions.cs:line 36
   at MongoDB.Bson.Serialization.BsonClassMapSerializer`1.DeserializeMemberValue(BsonDeserializationContext context, BsonMemberMap memberMap) in mongo-csharp-driver/src/MongoDB.Bson/Serialization/Serializers/BsonClassMapSerializer.cs:line 545