If I have an object with a null on it for a property that is marked "[BsonDefaultValue(null, SerializeDefaultValue = false)]", I'd expect that no value would get written to the database. But what happens is that the test code to decide whether null == null blows up because of the way it is written.

Looking at the code I can see that I should have used [BsonIgnoreIfNull] but why isn't [BsonIgnoreIfNull] equivalent to [BsonDefaultValue(null, SerializeDefaultValue = false)]?

Suggest that with a small tweak this code could be made to work whichever way a user chooses to specify that null values are the default and are to be ignored.

private void SerializeMember(
BsonWriter bsonWriter,
object obj,
BsonMemberMap memberMap
) {
var value = memberMap.Getter(obj);
if (value == null && memberMap.IgnoreIfNull)

{ /// SUGGEST: Make this work if memberMap.DefaultValue == null too ... return; // don't serialize null value }

if (memberMap.HasDefaultValue && !memberMap.SerializeDefaultValue && value.Equals(memberMap.DefaultValue))

{ return; // don't serialize default value }