using System;

using System.Linq;

using MongoDB.Bson.Serialization;

using MongoDB.Driver;

using MongoDB.Driver.Linq;

​

class Factory

    {

        public virtual int Id { get; set; }

    }

    class User

    {

        public virtual int Id { get; set; }

        public virtual Factory Factory { get; set; }

    }

​

    class Order

    {

        public virtual int Id { get; set; }

        public virtual int FactoryId { get; set; }

    }

public class Program

{

    public static void Main()

    {

        var client = new MongoClient("mongodb://localhost/test");

        var db = client.GetDatabase("test");

        BsonClassMap.RegisterClassMap<Order>(m =>

            {

                m.MapIdProperty(x => x.Id);

                m.MapProperty(x => x.FactoryId);

            });

        var currentUser = new User();

        //exception was throwed here

        var orders = db

            .GetCollection<Order>("orders").AsQueryable()

            .Where(x => currentUser.Factory == null || x.FactoryId == currentUser.Factory.Id)

            .ToArray();

        //same exception for followed expression

        /*

        var orders = db

            .GetCollection<Order>("orders").AsQueryable()

            .Where(x => currentUser.Factory == null || (currentUser.Factory != null && x.FactoryId == currentUser.Factory.Id))

            .ToArray();

        */

        //no exception, all work fine

        currentUser.Factory = new Factory();

        orders = db

            .GetCollection<Order>("orders").AsQueryable()

            .Where(x => currentUser.Factory == null || x.FactoryId == currentUser.Factory.Id)

            .ToArray();

    }

}