如何检查 LINQ 中是否有 null 值？

Question

CarrierDetailViewModel.SiteOptionModel , this sometimes has a null value, then it throws the error like the below. Is there any way to check if there is no truck.SiteId then make it null otherwise assign value. How can I stop throwing the error but make it null when truck.SiteId is null.

from truck in database.Truck
   where truck.CarrierId == carrierId
         && (truck.IsDeleted == null || truck.IsDeleted == false)
   orderby truck.Code
   select new CarrierDetailViewModel.TruckModel2
   {
       Id = truck.TruckId,

       Site = new CarrierDetailViewModel.SiteOptionModel{
            Id = (int)truck.SiteId,
            Name = truck.Site.Name,
            Code = truck.Site.Code
       }
   } ;

Error

System.InvalidOperationException: 'Nullable object must have a value

Answer 1

I would create a method, eg CreateTruckModel2

from truck in database.Truck
where truck.CarrierId == carrierId
     && (truck.IsDeleted == null || truck.IsDeleted == false)
orderby truck.Code
select CreateTruckModel2(truck);

and

public TruckModel2 CreateTruckModel2(Truck truck)
{
    if(truck?.SiteId == null)
    {
        return null;
    }

    return new CarrierDetailViewModel.TruckModel2
    {
        Id = truck.TruckId,

        Site = new CarrierDetailViewModel.SiteOptionModel{
            Id = (int)truck.SiteId,
            Name = truck.Site.Name,
            Code = truck.Site.Code
        }
    };
}

Or just skip those with a where condition?

from truck in database.Truck
   where truck.CarrierId == carrierId
         && (truck.IsDeleted == null || truck.IsDeleted == false)
         && truck.SiteId != null
   orderby truck.Code
   select new CarrierDetailViewModel.TruckModel2
   {
       Id = truck.TruckId,

       Site = new CarrierDetailViewModel.SiteOptionModel{
            Id = (int)truck.SiteId,
            Name = truck.Site.Name,
            Code = truck.Site.Code
       }
   } ;