使用多态获取相同类型的实例

Question

我有下一个场景： 有一个类负责管理应用程序的所有货币。所有货币都扩展了 Currency，因此它可以表现得像它。货币是抽象的，因此无法实例化。 在应用程序的某些部分中，我将软货币、硬货币或事件货币作为成本货币，例如，玩家按下某个菜单上的某个购买按钮。这个动作触发了一次购买，购买有货币作为参考，在这个例子中它可以是costSoftCurrency。 这个想法是，如果 PlayerCurrencies 收到货币，玩家货币会评估他的货币并返回相同类型的关联货币，然后您可以安全地减去成本。

我的问题是......如果没有这个可怕的 if，我怎么能得到相同的逻辑？我读到了double dispatch，但我不知道它是否可以在这里应用。

有谁知道这是否可以实现？

public class PlayerCurrencies
{
    public SoftCurrency softCurrency = new SoftCurrency();
    public HardCurrency hardCurrency = new HardCurrency();
    public EventCurrency eventCurrency = new EventCurrency();

    public Currency GetCurrencyFromType(Currency currency)
    {
        if (currency is SoftCurrency)
        {
            return this.softCurrency;
        }

        if (currency is HardCurrency)
        {
            return this.hardCurrency;
        }

        if (currency is EventCurrency)
        {
            return this.eventCurrency;
        }

        return null;
    }
}

public abstract class Currency
{
    public float Value
    {
        get;
        set;
    }

    public void Add(Currency currency)
    {
        this.Value += currency.Value;
    }

    public void Substract(Currency currency)
    {
        this.Value -= currency.Value;
    }
}

public class EventCurrency : Currency
{

}

public class HardCurrency : Currency
{

}

public class SoftCurrency : Currency
{

}

internal class Program
{
    private static void Main(string[] args)
    {
        PlayerCurrencies playerCurrencies = new PlayerCurrencies();

        Currency costSoftCurrency = new SoftCurrency();
        Currency costHardCurrency = new HardCurrency();

        playerCurrencies.GetCurrencyFromType(costSoftCurrency).Substract(costSoftCurrency); // there i get a SoftCurrency from Player Currencies 
        playerCurrencies.GetCurrencyFromType(costHardCurrency).Substract(costHardCurrency); // there i get a HardCurrency from Player Currencies 
    }
}

Answer 1

我会为此使用工厂方法设计模式，因为它专门用于您使用子类来确定应该创建哪些类的时候。这基本上就是你在这里所做的。我最初的回答提到了反射，因为我在链接到的网站上看到了映射配置，并认为这很酷，但在昨晚考虑之后，它也有很多开销。使用字典绕过 if 块或 switch 语句会更简单，也更快。

这是我要添加/更改您的代码的内容：

//The Factory class that creates the different types of currency based
//on a name parameter
static class CurrencyFactory
{
    private static readonly Dictionary<string, Currency> _currencyDictionary =
        new Dictionary<string, Currency>
    {
        { "HardCurrency", new HardCurrency() },
        { "SoftCurrency", new SoftCurrency() },
        { "EventCurrency", new EventCurrency() }
    };

    public static Currency Create(string currencyTypeName)
    {
        return _currencyDictionary[currencyTypeName];
    }
}

internal class Program
{
    private static void Main(string[] args)
    {
        //This class will no longer be needed as it is redundant
        PlayerCurrencies playerCurrencies = new PlayerCurrencies();

        var costSoftCurrency = CurrencyFactory.Create("softCurrency");
        var costHardCurrency = CurrencyFactory.Create("hardCurrency");

        //I wasn't sure of your goals here so I kept it as is,
        //but as I said above, playerCurrencies won't be needed.
        playerCurrencies.GetCurrencyFromType(costSoftCurrency)
            .Substract(costSoftCurrency); // there i get a SoftCurrency from Player Currencies 

        playerCurrencies.GetCurrencyFromType(costHardCurrency)
            .Substract(costHardCurrency); // there i get a HardCurrency from Player Currencies 
    }
}

您可以在此处找到更详尽的示例和对该模式的详细说明：https://dev.to/gary_woodfine/how-to-use-factory-method-design-pattern-in-c-3ia3

Answer 2

您可以使用pattern matching：

public class PlayerCurrencies
{
    public SoftCurrency Soft = new SoftCurrency();
    public HardCurrency Hard = new SoftCurrency();
    public EventCurrency Event = new EventCurrency();

    public Currency GetCurrencyFromType(Currency currency)
    {
        switch (currency)
        {
            case EventCurrency: return Event;
            case HardCurrency: return Hard;
            case SoftCurrency: return Soft;
        }
        return null;
    }
}

我将字段名称更改为Hard、Soft 和Event，以便更容易地将它们与具有相同名称的类型区分开来。

在这个特定示例中，传入currency 参数并仅将其用于其类型似乎很奇怪。可能有另一种方法可以解决您的特定问题。但至于如何在没有多个if 语句的情况下做到这一点，这就是你可以做到的。

您会经常看到这通过声明的变量来演示，如下所示：

switch (currency)
{
    case EventCurrency ec : return Event;
    case HardCurrency hc: return Hard;
    case SoftCurrency sc: return Soft;
}
return null;

例如，如果currency 的类型是EventCurrency，那么在case 的范围内，ec 将是currency 参数转换为EventCurrency。但由于在本例中您没有使用该值，因此不需要变量。