如何从接口访问派生类成员？

Question

我有三门课； Stamp、Letter 和 Parcel 实现了 IProduct 接口，并且它们也有一些自己的功能。

public interface IProduct
{
    string Name { get; }
    int Quantity { get; set; }
    float Amount { get; }
}

public class Stamp : IProduct
{
    public string Name { get { return "Stamp"; } }
    public int Quantity { get; set; }
    public float Amount { get; set; }
    public float UnitPrice { get; set; }
}

public class Letter : IProduct
{
    public string Name { get { return "Letter"; } }
    public int Quantity { get; set; }        
    public float Amount { get; set; }
    public float Weight { get; set; }
    public string Destination { get; set; }
}

public class Parcel : IProduct
{
    public string Name { get { return "Parcel"; } }
    public int Quantity { get; set; }        
    public float Amount { get; set; }
    public float Weight { get; set; }
    public string Destination { get; set; }
    public int Size { get; set; }
}

public static class ShoppingCart
{
    private static List<IProduct> products = new List<IProduct>();
    public static List<IProduct> Items { get { return products; } }
}

为什么我无法从 List 访问派生类的其他成员？

ShoppingCart.Items.Add(new Stamp { Quantity = 5, UnitPrice = 10, Amount = 50 });
ShoppingCart.Items.Add(new Letter { Destination = "US", Quantity = 1, Weight = 3.5f });
ShoppingCart.Items.Add(new Parcel { Destination = "UK", Quantity = 3, Weight = 4.2f, Size = 5 });

foreach (IProduct product in ShoppingCart.Items)
{
    Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}", product.Name, product.Quantity, product.Amount);
}

我想过使用泛型，但在这种情况下，我将不得不编写单独的代码对于每种特定类型的产品。

public static class ShoppingCart<T> where T : IProduct
{
    private static List<T> items = new List<T>();
    public static List<T> Items { get { return items; } }
}


ShoppingCart<Stamp>.Items.Add(new Stamp { Quantity = 5, Amount = 10, UnitPrice = 50 });
ShoppingCart<Letter>.Items.Add(new Letter { Destination = "US", Quantity = 1, Weight = 3.5f });

foreach (Stamp s in ShoppingCart<Stamp>.Items)
{
    Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}", s.Name, s.Quantity, s.Amount); 
}

foreach (Letter l in ShoppingCart<Letter>.Items)
{
    Console.WriteLine("Name: {0}, Destination: {1}, Weight: {2}", l.Name, l.Destination, l.Weight);      
}

难道就没有一种设计模式可以解决这种问题吗？工厂模式？

Answer 1

您无法访问实现接口的类的其他成员，因为您仅在项目列表中公开 IProduct。我将购物车中每个项目的特定列表类型添加到 ShoppingCart 类，然后您可以为仅需要使用 IProduct 接口的任何内容公开购物车中所有产品的序列：

public class ShoppingCart
{
    public IList<Stamp> Stamps { get; }
    public IList<Letter> Letters { get; }
    public IList<Parcel> Parcels { get; }

    public IEnumerable<IProduct> Products
    {
        get
        {
            return this.Stamps.Cast<IProduct>()
                .Concat(this.Letters.Cast<IProduct>())
                .Concat(this.Parcels.Cast<IProduct>());
        }
    }
}

Answer 2

这是因为您在 foreach 循环中将购物车中的每个项目转换为 IProduct。您需要做的是这样的：

foreach(IProduct product in ShoppingCart.Items)
{
    if (product is Stamp)
    {
        var stamp = product as Stamp;
        Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}, UnitPrice: {3}", stamp.Name, stamp.Quantity, stamp.Amount, stamp.UnitPrice);
    }
    else if (product is Letter)
    {
        var letter = product as Letter;
        Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}, Weight: {3}, Destination: {4}", letter.Name, letter.Quantity, letter.Amount, letter.Weight, letter.Destination);
    }
    else if (product is Parcel)
    {
        var parcel = product as Parcel;
        Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}, Weight: {3}, Destination: {4}, Size: {5}", parcel.Name, parcel.Quantity, parcel.Amount, parcel.Weight, parcel.Destination, parcel.Size);
    }
}

或者，C# 中现在提供了这种更现代的语法，它将 is 运算符与变量声明相结合：

foreach(IProduct product in ShoppingCart.Items)
{
    if (product is Stamp stamp)
    {
        Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}, UnitPrice: {3}", stamp.Name, stamp.Quantity, stamp.Amount, stamp.UnitPrice);
    }
    else if (product is Letter letter)
    {
        Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}, Weight: {3}, Destination: {4}", letter.Name, letter.Quantity, letter.Amount, letter.Weight, letter.Destination);
    }
    else if (product is Parcel parcel)
    {
        Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}, Weight: {3}, Destination: {4}, Size: {5}", parcel.Name, parcel.Quantity, parcel.Amount, parcel.Weight, parcel.Destination, parcel.Size);
    }
}

此外，您还重复了不必要的属性 Name、Quantity和金额。您应该从 Product 派生每个类：

public class Stamp: Product, IProduct
{
    public double UnitPrice { get; set; }
}

public class TransitProduct: Product, IProduct
{
    public double Weight { get; set; }
    public string Destination { get; set; }   
}

public class Letter: TransitProduct, IProduct
{
}

public class Parcel: TransitProduct, IProduct
{
    public double Size { get; set; }
}

Answer 3

为什么我无法访问额外的
派生类的成员
列出？

这是因为，IProduct 接口不知道派生类属性的 UnitPrice、Destination 等。

您是否正在尝试添加智能来计算每个派生类对象 Stamp、Letter、Parcel 的 Amount ？

然后，我想说你需要重新设计一下并使用 装饰器设计模式。

DerivedClass::Amount()
{
  Base::Amount() + 
  //Amount logic based on derived class
}

Answer 4

无法从派生类访问其他成员的原因是您正在使用 List<> 中的接口。 - 因此您只能访问该界面上的属性。

可能对您有帮助的模式是双调度模式。

下面的示例：

public interface IHandler
{
    void Handle(Stamp stamp);
    void Handle(Letter letter);
    ...
}

public class Handler : IHandler
{
    public void Handle(Stamp stamp)
    {
       // do some specific thing here...
    }
    public void Handle(Letter letter)
    {
       // do some specific thing here...
    }
    ...
}

public interface IProduct
{
    string Name { get; }
    int Quantity { get; set; }
    float Amount { get; }
    void Handle(IHandler handler);
}

public class Stamp : IProduct
{
    public string Name { get { return "Stamp"; } }
    public int Quantity { get; set; }
    public float Amount { get; set; }
    public float UnitPrice { get; set; }
    public void Handle(IHandler handler)
    {
         handler.Handle(this);
    }
}

您现在可以在处理程序中编写一些特定的功能 - 我猜您想计算某种总价，例如数量 * 单价或重量和重量。目的地查找表...