问题描述
在C#中使用record
类型,看起来像建立区分工会的数据结构可能非常有用,我只是想知道我是否丢失了一些我需要了解的问题。以后会后悔的。例如:
abstract record CardType{
// Case types
public record MaleCardType(int age) : CardType{}
public record FemaleCardType : CardType{}
// Api
public static MaleCardType Male(int age) => new MaleCardType(age);
public static FemaleCardType Female => new FemaleCardType();
}
var w = CardType.Male(42);
var x = CardType.Male(42);
var y = CardType.Male(43);
var z = CardType.Female;
Assert.Equal<CardType>(w,x); //true
Assert.Equal<CardType>(x,y); //false
Assert.Equal<CardType>(y,z); //false
这似乎比用单例和相等比较器以及所有这些构建抽象类要简单得多,但是我是否缺少一些我不想这样做的原因?
解决方法
这是一个很好的方法,我一直在玩它,例如,在 https://fsharpforfunandprofit.com/posts/designing-for-correctness/ 上有一些 C# 代码示例,一些使用类型和可区分联合的 F# 代码,然后一些修改(但仍然很糟糕的 C# 代码)。所以我使用 C# 9s 记录和做 DU 的相同方式重写了 C#
示例代码,比 F# 稍微丑一点,但仍然相当简洁,并具有 F# 代码的优点。
using System;
using System.Collections.Immutable;
namespace ConsoleDU
{
record CartItem(string Value);
record Payment(decimal Amount);
abstract record Cart
{
public record Empty () : Cart
{
public new static Active Add(CartItem item) => new(ImmutableList.Create(item));
}
public record Active (ImmutableList<CartItem> UnpaidItems) : Cart
{
public new Active Add(CartItem item) => this with {UnpaidItems = UnpaidItems.Add(item)};
public new Cart Remove(CartItem item) => this with {UnpaidItems = UnpaidItems.Remove(item)} switch
{
var (items) when items.IsEmpty => new Empty(),{ } active => active
};
public new Cart Pay(decimal amount) => new PaidFor(UnpaidItems,new(amount));
}
public record PaidFor (ImmutableList<CartItem> PaidItems,Payment Payment) : Cart;
public Cart Display()
{
Console.WriteLine(this switch
{
Empty => "Cart is Empty",Active cart => $"Cart has {cart.UnpaidItems.Count} items",PaidFor(var items,var payment) => $"Cart has {items.Count} paid items. Amount paid: {payment.Amount}",_ => "Unknown"
});
return this;
}
public Cart Add(CartItem item) => this switch
{
Empty => Empty.Add(item),Active state => state.Add(item),_ => this
};
public static Cart NewCart => new Empty();
public Cart Remove(CartItem item) => this switch
{
Active state => state.Remove(item),_ => this
};
public Cart Pay(decimal amount) => this switch
{
Active cart => cart.Pay(amount),_ => this
};
}
class Program
{
static void Main(string[] args)
{
Cart.NewCart
.Display()
.Add(new("apple"))
.Add(new("orange"))
.Display()
.Remove(new("orange"))
.Display()
.Remove(new("apple"))
.Display()
.Add(new("orange"))
.Pay(23M)
.Display();
;
}
}
}