在EF查询语法中使用let时发生NullReferenceException

Question

I have a query where if I use a let I get a NullReferenceException however if I use the same statement in the select it works fine.

var q = db.WorkOrders;

The following query throws an exception:

(from wo in q.OrderByDescending(x => x.WorkOrderNumber)
let cost = wo.Costs.Select(x => x.BaseCost + x.AdminOverhead + x.LabourOverhead).DefaultIfEmpty(0).Sum()
select new ManageWorkOrderData
{
    Id = wo.Id,
    TotalCost = cost
}).ToListAsync();

However the following does not:

(from wo in q
select new ManageWorkOrderData
{
    Id = wo.Id,
    TotalCost = wo.Costs.Select(x => x.BaseCost + x.AdminOverhead + x.LabourOverhead).DefaultIfEmpty(0).Sum()
}).ToListAsync();

Notice that in the second the subquery is the same, just not as a let .

Update This question has been marked as a duplicate, however I don't agree as the code works fine when not used in the let part of the linq query. None of the types are nullable, and it's Entity Framework interpreting the code so I can't check for nulls.

There is no way to debug the code in VS as the code is all executed by Entity Framework/SQL. Interestingly the query isnt executed on the server, which leads me to believe its an EF issue. I'm interested to see if anyone else has any insight as to why EF throws an exception with the let and not when used in the select.

Classes

public class WorkOrder
{
    public int Id { get; set; } 
    public int WorkOrderNumber { get; set; } 
    public virtual ICollection<Cost> Costs { get; set; }

    public WorkOrder()
    {
        Costs = new HashSet<Cost>();
    }
}

public class Cost
{
    public int Id { get; set; }
    public int WorkOrderId { get; set; }
    public decimal BaseCost { get; set; }
    public decimal AdminOverhead { get; set; }
    public decimal LabourOverhead { get; set; }

    public virtual WorkOrder WorkOrder { get; set; }
}

public class ManageWorkOrderData
{
    public int Id { get; set; }
    public decimal TotalCost { get; set; }
}

Stack Trace Available from http://pastebin.com/uXkdM30A

Answer 1

This is an Entity Framework bug. This can be seen from the stack trace. This is an uncontrolled crash deep inside of EF. Microsoft classes never intentionally throw meaningless programmer error exceptions like NRE, ArrayIndexEx, DivByZeroEx, ...

Create a small, executable repro and report the bug on GitHub. The team is responsive.