C# Query - Non Static Method Requires a Target

Question

i am getting an error that says 'Non Static Method requires a target'

Here is the code that is causing me the error, could anyone possible shed some light on this?

//TODO: Error, Non static method requires a target.
var orderItem =
    _context.PurchaseOrderItems.FirstOrDefault(
        p => p.JobReference == item.JobReference && p.ItemNumber == item.ItemNumber);
return _context.DeliverySchedules.Include(d => d.PurchaseOrderItem)
           .Where(d => d.PurchaseOrderItem.Id == orderItem.Id)
               .ToList();

Answer 1

The FirstOrDefault method may return null value if no query results returned there:

var orderItem = _context.PurchaseOrderItems.FirstOrDefault(
                p => p.JobReference == item.JobReference && p.ItemNumber == item.ItemNumber);

Since orderItem.Id throws NullReferenceException when orderItem is null, it will propagate to LINQ throwing TargetException as mentioned in question (see this post and this post for more info).

Hence, you need to check presence of null value from orderItem by modifying second LINQ query to this one:

return _context.DeliverySchedules.Include(d => d.PurchaseOrderItem)
       .Where(d => (orderItem != null && d.PurchaseOrderItem.Id == orderItem.Id))
       .ToList();

NB: Null checking must takes place before retrieving property Id of orderItem to prevent NullReferenceException.

As an alternative, if condition to check against null value may be used without modifying second query:

if (orderItem != null)
{
    return _context.DeliverySchedules.Include(d => d.PurchaseOrderItem)
           .Where(d => d.PurchaseOrderItem.Id == orderItem.Id)
           .ToList();
}