如何从列表访问属性

Question

If I have the following code and want to iterate the list of vehicles and display the Price, how can I access that property?

    public class Vehicle
    {
        public string Name { get; set; }
        public string Price { get; set; }
    }

    public class ElectricalDriven : Vehicle
    {
        public string SuperChargerCord { get; set; }
    }

    public class GasolinDriven : Vehicle
    {
        public string GasolineTank { get; set; }
    }

        var myListOfVechicles = new List<Vehicle>();
        myListOfVechicles.Add(new ElectricalDriven { Name = "Tesla", Price = "100 000", SuperChargerCord = "Yellow cord"});
        myListOfVechicles.Add(new GasolinDriven { Name = "Ford", Price = "50 000", GasolineTank = "Large VZ Tank"});


        foreach (var row in myListOfVechicles)
        {
            Console.WriteLine($"- {row.Name}");
        }

Answer 1

The problem is that Vehicle does not have a Value property, so you can't ask for it in a type-safe manner.

Some options are:

• Override ToString so that the class determines what is displayed:
• Add a virtual method to VEhivel like GetDescription that returns a string if you don't want that string to override ToString`
• Move Price to Vehicle or a subtype that ElectricalDriven and GasolinDriven both inherit from.

Answer 2

In the foreach you can check if the current 'row' is ElectricalDriven or GasolinDriven with the 'is' operator and then cast the row to the matched concrete implementation. Should be something like that:

if (row is ElectricalDriven)
{
    Console.WriteLine($"Price: {((ElectricalDriven)row).Price}");
}
else if (row is GasolinDriven)
{
    Console.WriteLine($"Price: {((GasolinDriven)row).Price}");
}

However, you can see that the more different vehicles, the more ifs. You should probably move the price property in the base class or in an interface.

EDIT: Seeing that the author moved the price property to the Vehicle class, you can now access it just like you accessed the Name property.

Answer 3

If you want to define a unique property for both Electric and Gasoline vehicles, and be able to retrieve its value from the generic vehicle class, you can declare a virtual method and override it in the inheriting classes:

 public class Vehicle
{
    public string Name { get; set; }
    public virtual string GetCustomProperty() { return string.Empty; }
}

public class ElectricalDriven : Vehicle
{
    public string ElectricalVehicleProperty { get; set; }
    public override string GetCustomProperty() { return ElectricalVehicleProperty; }
}

public class GasolinDriven : Vehicle
{
    public string GasolinVehicleProprety { get; set; }
    public override string GetCustomProperty() { return GasolinVehicleProprety; }
}
class Program
{
    static void Main(string[] args)
    {
        var myListOfVechicles = new List<Vehicle>();
        myListOfVechicles.Add(new ElectricalDriven { Name = "Tesla", ElectricalVehicleProperty = "100 000" });
        myListOfVechicles.Add(new GasolinDriven { Name = "Ford", GasolinVehicleProprety = "50 000" });

        foreach (var row in myListOfVechicles)
        {
            Console.WriteLine("Price: " + row.GetCustomProperty()); //will output 100,000 and 50,000
        }
        Console.Read();
    }
}

Answer 4

By doing this:

var myListOfVechicles = new List<Vehicle>();
myListOfVechicles.Add(new ElectricalDriven { Name = "Tesla", Price = "100 000" });
myListOfVechicles.Add(new GasolinDriven { Name = "Ford", Price = "50 000" });

The compiler will assign an object of the subclass (like ElectricalDriven) to object to the base class (Vehicle) after implicitly converting it to the base class type.

Know because of this conversion, when you are trying to access that object you're accessing an object that only have the base class members which is only Name .

To be able to access Price you need to explicitly cast the object to the subclass type before accessing its members. The simplest for example you can try this:

   string name, price;
    foreach (var item in myListOfVechicles)
    {
        name = item is ElectricalDriven ? 
                       (item as ElectricalDriven).Name : 
                       (item as GasolinDriven).Name;
        price = item is ElectricalDriven ? 
                        (item as ElectricalDriven).Price :
                        (item as GasolinDriven).Price;

        Console.WriteLine($"Name: {name} - Price: {price}");
    }