c# - 对从 Linq2Sql 的 var 中获取 int 感到困惑

Question

我刚刚学习 Linq 并遇到了问题。为了进行一些验证，我从数据库中获取最后一个里程表读数，并需要将其与下一个读数进行比较。里程表读数是传入数据上的 int 和数据库上的 double(在 dbml 文件上更改为 int)。

下面是我的代码:

private bool IsValidOdometer(string equipment, int odometerReading)
{
    bool returnVariable = true;
    CarteGraphDataDataContext db = new CarteGraphDataDataContext();

    var odometer = (from p in db.EquipmentMainGenerals join q in db.EquipmentFuelLogs 
            on p.wdEquipmentMainGeneralOID equals q.wdEquipmentMainGeneralOID
            where p.VehicleId == equipment
            orderby q.Odometer descending
            select q.Odometer).Take(1);

    int previousOdometer = odometer;

    if (odometer.Count() != 0)
    {
        if (odometerReading < previousOdometer)
        {
            returnVariable = false;
        }
    }

    return returnVariable;
}

问题行是int previousOdometer = odometer;。我得到的错误是:

Cannot implicitly convert type 'System.Linq.IQueryable' to 'int'

如何解决这个问题？

Answer 1

而不是 Enumerable.Take使用Enumerable.Single 。 Enumerable.Take 将返回一个 IEnumerable 但 Enumerable.Single 将返回单个实例(或抛出):

int odometer = (from p in db.EquipmentMainGenerals
                join q in db.EquipmentFuelLogs 
                on p.wdEquipmentMainGeneralOID
                    equals q.wdEquipmentMainGeneralOID
                where p.VehicleId == equipment
                orderby q.Odometer descending
                select q.Odometer).Single();

int previousOdometer = odometer;

此外，我会在此处显式键入odometer，而不是像您那样隐式键入。

最后是你的台词

CarteGraphDataDataContext db = new CarteGraphDataDataContext();

应该像这样在using block 中

using(CarteGraphDataDataContext db = new CarteGraphDataDataContext()) {
    // okay to go
}