按基数 class 属性过滤派生 class

Question

我有两个 classes (Car/Truck) 共享一个基础 class（汽车）。我想通过属性在其基础 class 汽车上过滤 Car 和 Truck 的集合。下面的代码会导致错误：

Cannot implicitly convert type   
'System.Collections.Generic.List<Example.Program.Automobile>' to    
'System.Collections.Generic.List<Example.Program.Car>'
Program.cs    48  27  Example

是否可以按基数 class 属性进行过滤，而不必将结果转换回其适当的派生 class？

class Program
{
    public class Automobile
    {
        public string Manufacturer { get; set; }

        public static  IEnumerable<Automobile> GetByManufacturer(IEnumerable<Automobile> items, string manufacturer)
        {
            return items.Where(o => o.Manufacturer == manufacturer);
        }
    }

    public class Car : Automobile
    {
        public int TrunkSize { get; set; }
    }

    public class Truck : Automobile
    {
        public int BedSize  { get; set; }
    }

    static void Main(string[] args)
    {

        var cars = new List<Car> 
        {
            new Car { Manufacturer = "Toyota", TrunkSize = 100 },
            new Car { Manufacturer = "Kia", TrunkSize = 70 }
        };

        var trucks = new List<Truck> 
        {
            new Truck { Manufacturer = "Toyota", BedSize = 400 },
            new Truck { Manufacturer = "Dodge", BedSize = 500 }
        };

        // Problem: Get a list of Type Car and a List of Tpye Truck, 
        // where each List contains only cars manufactured by Toyota
        var mfr =  "Toyota";

        List<Car> toyotaCars = Automobile.GetByManufacturer(cars, mfr).ToList();
        List<Car> toyotaTrucks = Automobile.GetByManufacturer(trucks, mfr).ToList();

        Console.WriteLine(toyotaCars.First().GetType().Name);
        Console.WriteLine(toyotaTrucks.First().GetType().Name);
    }
}

Answer 1

您可以将定义更改为

public static IEnumerable<TAuto> GetByManufacturer(IEnumerable<TAuto> items, string manufacturer)
                    where TAuto : Automobile
{
    return items.Where(o => o.Manufacturer == manufacturer);
}

现在您正在 return 生成 IEnumerable<Automobile>，然后调用 ToList 将其转换为 List<Automobile>，然后尝试将其转换为 [=15] =] 这是不合法的，因为列表可能包含 Automobile 而不是 Car。

通过更改，您将 return 成为 IEnumerable<Car>，它可以完全转换为 List<Car>

另外，第二次调用中的return类型应该是List<Truck>，而不是List<Car>:

List<Truck> toyotaTrucks = Automobile.GetByManufacturer(trucks, mfr).ToList();

按基数 class 属性过滤派生 class

Filter derived class by base class property

.net

c#

inheritance

covariance

c#-4.0

按基数 class 属性 过滤派生 class

Filter derived class by base class property

.net

c#

inheritance

covariance

c#-4.0

按基数 class 属性过滤派生 class