LINQ to SQL加入FirstOrDefault（）的问题

Question

I posted a question earlier, but I've narrowed down the problems now and it's something pretty different. Basically, I have a rather long query which aims to select values from a database into a custom structure (which I'll show below), but I'm having issues with the joining - paticulary trying to only select the first row of a join. Here is my query:

IENumerable<VehicleDetails> list = (
    from s in dc.Vehicles
    join veh in dc.VehicleTypes 
        on s.VehicleType equals veh.ID into vg
    from v in vg.DefaultIfEmpty()
    join consignments in dc.Consignments
        .FirstOrDefault(x => x.TripDate > dateFrom && 
            x.TripDate < dateTo && 
            x.DeliveryDepot == depot.Letter && 
            (x.DeliveryStatus == 2 || x.DeliveryStatus == 3)) 
        on new
        {
            Reg = s.VehicleReg, 
            Depot = s.VehicleDepot 
        } equals new
        {
            Reg = consignments.VehicleReg, 
            Depot = consignments.DeliveryDepot
        } into con
    from c in con.DefaultIfEmpty()
    select new 
    {
        VehicleType = (
            s.VehicleType == null ? "?":
            v.VehicleType1.ToString()),
        TotalRate = c.Rate + c.AddCharges,
        VehicleReg = (string.IsNullOrEmpty(c.VehicleReg) ? 
            c.Subcontractor: c.VehicleReg),
        VehicleTypeName = (v.VehicleTypeDescription ==  null ? 
            "SubContractor": v.VehicleTypeDescription)
    }); 

My struct:

public struct VehicleDetails
{
     internal string VehicleType;
     internal decimal TotalRate;
     internal string VehicleReg;
     internal string VehicleTypeName;
}

With the FirstOrDefault() in the second join, I get:

The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to group join.

Without it (and replacing the FirstOrDefault with a Where instead), I get an error about implicity converting an anonymoustype into a 'vehicledetials' type. Both errors occur on the from c in con.DefaultIfEmpty() call.

Any ideas would be much, much appreciated

Answer 1

FirstOrDefault() will eagerly return a single element, but what you need is a collection (IQueryable) of elements.

So

dc.Consignments
    .Where(x => x.TripDate > dateFrom && 
                x.TripDate < dateTo && 
                x.DeliveryDepot == depot.Letter && 
                (x.DeliveryStatus == 2 || x.DeliveryStatus == 3))
   .Take(1)

will return a deferred collection that will have only one element.

Answer 2

Why don't you make your query more simpler!!

1. I highly recommend to use FirstOrDefault() at the end of the query (last method of your query).
2. Put your condition on Where better instead of FirstOrDefault since you're going to put it at the end

I think your mistake here is FirstOrDefault with join , when u're using join you should join with a collection not a single item (single is the result of FirstOrDefault)