使用MongoDB C＃驱动程序查询派生类型值

I'm porting some code from the old legacy MongoDB driver to use the new driver and have hit a problem. I have a collection which contains multiple derived types from a common base class. Previously I was able to query the collection (which is declared using the base type) using derived class properties and just retrieve the derived class documents. So given these classes :I'm porting some code from the old legacy Mongo