Summary

I'm using Builders<T>.Filter.Ne("fieldName", BsonNull.Value) wishing to filter objects where 'fieldName' is null. When I use this filter

col.AsQueryable().Where(x => managerIdEmptyFilter.Inject()).ToList();

I'm getting a TargetInvocationException, with an Inner Exception message

_

._

So it seems for some reason, the driver tries to convert 

BsonNull.Value

 to an ObjectId.

I even think I know why - the field ManagerId is a reference and since I can't use MongoDb dependencies on my data model, I'm using a ClassMap and declare the ManagerId string to be serialized as an ObjectId:

cm.MapProperty(x => x.ManagerId).SetSerializer(new StringSerializer(BsonType.ObjectId));

Please provide the version of the driver. If applicable, please provide the MongoDB server version and topology (standalone, replica set, or sharded cluster).

I'm using the C# driver 2.28 on an M0 dev cluster.

How to Reproduce

I've created a ready-to-run repro sample that I published here: ssteiner/MongoDbSearch (github.com).

Just put your connection string into MongoDbSearchTester.cs's 

connectionString

 variable, and run it.

You'll get the exception when running this line:

var contactsWithEmptyManagerFromDb = col.AsQueryable().Where(x => managerIdEmptyFilter.Inject()).ToList();

Additional Background

I'm writing a generic data access layer that works on any of my entities (I have about 80 of them). and I have a generic search in my UI, where users can search any field on any entity.. so the search API takes a field by name, hence I cannot use expression syntax

var managerIdEmptyFilter = Builders<PhoneBookContact>.Filter.Ne(x => x.ManagerId, null); 

And if I don't store ManagerId as an ObjectId, then the search goes through, but it returns all contacts, except returning just 3 of them (contact1 has a non empty ManagerId.. so it should be filtered out)