Complex ef core 2.1 query produces null reference exception how we find what is causing the problem - ef-core-2.1

Question

I have a complex query that is give a null reference exception:
"Object reference not set to an instance of an object".
An example of the complex query would be a car manufacturer, that must deliver cars to a dealership on a particular date.
So we have a car:
public class Car
{
public int CarId { get; set; }
public string Model { get; set; }
public string Color { get; set; }
public int? CarActionDefId { get; set; }
public CarActionDef CarActionDef { get; set; }
}
public class ActionDef
{
public int ActionDefId { get; set; }
public string Name { get; set; }
public List<ActionSchedules> ActionSchedules { get; set; }
}
public class CarActionDef
{
public int CarActionDefId { get; set; }
public int CarId { get; set; }
public int ActionDefId { get; set; }
public Car Car { get; set; }
public ActionDef ActionDef { get; set; }
}
public class ActionSchedule
{
public int ActionScheduleId { get; set; }
public DateTime? NextDue { get; set }
public int ActionDef_RID { get; set }
public virtual ActionDef ActionDef { get; set }
}
Understand this is pseudo code to give you an idea of what is occurring.
If we want to get the cars that are scheduled for delivery between a start and end date we use a query like this:
res =
context.Cars.
Include ( g => CarActionDef ).
Include ( g => g.CarActionDef.ActionDef ).
Include ( g => g.CarActionDef.ActionDef.ActionSchedules ).
Where ( g => g.CarActionDef != null && g.CarActionDef.ActionDef != null &&
g.CarActionDef.ActionDef.ActionSchedules != null &&
g.CarActionDef.ActionDef.ActionSchedules.Count > 0 ).
Where ( g => g.CarActionDef.ActionDef.ActionSchedules [ 0 ].NextDue != null &&
g.CarActionDef.ActionDef.ActionSchedules [ 0 ].NextDue >= startDate &&
g.CarActionDef.ActionDef.ActionSchedules [ 0 ].NextDue <= endDate ).
OrderByDescending ( g => g.CarActionDef.ActionSchedules [ 0 ].NextDue ).
Select ( g => g ).
ToList ();
However when we execute this statement we get a null reference exception. We have tested the method to get any CarActionDef for a car that is null
and we get an empty collection. Likewise we tried CarActionDef.ActionDef and CarActionDef.ActionDef.ActionSchedules and both collection returned were
empty. So debugging to find a problem navigation property or list has produced nothing and there is no inner exception. Is there some way we can find what
is causing the null reference exception?

Answer 1

You can use then include
`res = context.Cars.Include ( g => CarActionDef )
.ThenInclude ( f => f.ActionDef )
.ThenInclude ( t => t.ActionSchedules )`