Check if Field Equals Null in MongoDb C# Driver 2.0

This actually works as expected with a little modification, which depends on:

BsonNull.Value

See this question for details: How to create Bson Document with Null value using C# official driver?

So the query is:

var fieldValueIsNullFilter = Builders<BsonDocument>.Filter.Eq("MyFieldName", BsonNull.Value); 

Initially, I didn't realize BsonNull has a Value property.