LINQ to SQL加入FirstOrDefault()的问题

Question

我之前发过一个问题,但我现在已经把问题缩小了,这是一个非常不同的问题.基本上,我有一个相当长的查询,旨在从数据库中选择值到一个自定义结构(我将在下面显示),但我遇到了加入问题 - paiculary试图只选择连接的第一行.这是我的查询:

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)
    }); 

我的结构:

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

随着FirstOrDefault()在第二个连接,我得到:

join子句中某个表达式的类型不正确.在对组加入的调用中类型推断失败.

如果没有它(并将其替换FirstOrDefault为Where),我会收到一个错误,即将匿名类型转换为"vehicledetials"类型.这两个错误都发生在from c in con.DefaultIfEmpty()通话中.

任何想法都会非常受欢迎

Answer 1

FirstOrDefault()会急切地返回一个元素,但你需要的是一个元素集合(IQueryable).

所以

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

将返回一个只有一个元素的延迟集合.