What's New in C# 9.0

前言

本來將介紹 C# 9.0 中的 record 及加強的 pattern matching 的功能。
在 C# 中，如果想要建立 Immutable Class 需要在屬性那設定 {get; init;} ，例如，

public class DrinkBase
{
    public DrinkBase(int Id, string Name, CupOptions CupOption, DrinkTypes DrinkType)
    {
        this.Id = Id;
        this.Name = Name;
        this.CupOption = CupOption;
        this.DrinkType = DrinkType;
    }
    public int Id { get; init; }
    public string Name { get; init; }
    public CupOptions CupOption { get; init; }
    public DrinkTypes DrinkType { get; init; }
}

record

record 也是 class ，只是它讓我們可以快速定義 Immutable Class，所以上面就可以改成以下的方式，

public enum CupOptions
{
    Grande,
    Tall,
    Venti
}

public enum DrinkTypes
{
    Americanos,
    BrewedCoffees,
    Cappuccinos,
    Lattes, 
    Teas
}

public record DrinkBase(int Id, string Name, CupOptions CupOption, DrinkTypes DrinkType);

因為它也是 Class ，所以也可以有繼承，如下，

public record Coffee(int Id, string Name, CupOptions CupOption, DrinkTypes DrinkType)
        : DrinkBase(Id, Name, CupOption, DrinkType)
{
    public int MilkPrice = 5;
}

public record Tea(int Id, string Name, CupOptions CupOption, DrinkTypes DrinkType, string Origin)
    : DrinkBase(Id, Name, CupOption, DrinkType)
{
    public string Origin { get; init; } = Origin;
    public void OtherMethod()
    {

    }
}

所以建立它們後，就不允許修改它們的屬性，如下，

var myLatte = new DrinkBase(1, "Oatmilk Honey Latte", CupOptions.Grande, DrinkTypes.Lattes);
myLatte.Name = "123"; //Compile Error

With-expressions

如果想要從一個現有的 record 物件 Copy 它的屬性，並改一些屬性值，就可以透過 with 的方式，例如，

var myLatte = new DrinkBase(1, "Oatmilk Honey Latte", CupOptions.Grande, DrinkTypes.Lattes);
var youLatte = myLatte with { CupOption = CupOptions.Tall };

Value-based equality

而 record 的物件比較是透過屬性 Value 來比較的，所以如果2個物件，都是相同的 record Type ，而且屬性也都相同，所以比較出來會為 true，如下，

var latte1 = new DrinkBase(1, "Oatmilk Honey Latte", CupOptions.Grande, DrinkTypes.Lattes);
DrinkBase latte2 = new (1, "Oatmilk Honey Latte", CupOptions.Grande, DrinkTypes.Lattes);
//Value-based equality
//相同 Type 而且 屬性相同 則為 true 
var eqSameType = latte1 == latte2; //true
Coffee coffee1 = new(1, "Oatmilk Honey Latte", CupOptions.Grande, DrinkTypes.Lattes);
//不同 Type
var eqNotSameType = latte2 == coffee1; //false

如果要建立 Method ，也是可以的，如下，

public record Tea(int Id, string Name, CupOptions CupOption, DrinkTypes DrinkType, string Origin)
    : DrinkBase(Id, Name, CupOption, DrinkType)
{
    public string Origin { get; init; } = Origin;
    public void OtherMethod()
    {
        //do something
    }
}

Positional records

如果想要快速地將 record 物件指定給變數的話，可以使用 positional deconstruction ，如下，

var myLatte = new DrinkBase(1, "Oatmilk Honey Latte", CupOptions.Grande, DrinkTypes.Lattes);
var (id, name, cupOpt, drinkType) = myLatte;

Improved pattern matching

針對 switch case ，也可以作一些簡化，例如一個計算價格的 Method ，預設如下，

private static int CalculateDrink(DrinkBase drink)
{
    var price = 0;
    switch (drink.CupOption)
    {
        case CupOptions.Grande:
            price += 10;
            break;
        case CupOptions.Tall:
            price += 20;
            break;
        case CupOptions.Venti:
            price += 30;
            break;
    }


    if(drink is Tea tea)
    {
        price += tea.AddPrice; 
    }else if(drink is Coffee coffee)
    {
        var capacity = 0;
        switch (coffee.DrinkType)
        {
            case DrinkTypes.BrewedCoffees:
                capacity = 1;
                break;
            case DrinkTypes.Cappuccinos:
                capacity = 2;
                break;
            case DrinkTypes.Lattes:
                capacity = 3;
                break;
        }
        price += capacity * coffee.MilkPrice;
    }

    return price;
}

可以簡化成以下的方式，

private static int CalculateDrink2(DrinkBase drink)
{
    var price = 0;
    price += drink.CupOption switch
    {
        CupOptions.Grande => 10,
        CupOptions.Tall => 20,
        CupOptions.Venti => 30,
        _ => throw new ArgumentOutOfRangeException()
    };
    price += drink switch
    {
        Tea tea => tea.AddPrice,
        Coffee coffee => coffee.DrinkType switch
        {
            DrinkTypes.BrewedCoffees => 1,
            DrinkTypes.Cappuccinos => 2,
            DrinkTypes.Lattes => 3,
            _ => throw new ArgumentOutOfRangeException()
        } * coffee.MilkPrice,
        _ => 0
    };
    return price;
}

測試如下，

Coffee latte = new(1, "Oatmilk Honey Latte", CupOptions.Grande, DrinkTypes.Lattes);
var lattePrice = CalculateDrink(latte); //25
var lattePrice2 = CalculateDrink2(latte); //25
Coffee cappuccion = new(1, "Cappu", CupOptions.Venti, DrinkTypes.Cappuccinos);
var cappuccionPrice = CalculateDrink(cappuccion); //40
var cappuccionPrice2 = CalculateDrink2(cappuccion); //40
var blackTea = new Tea(1, "FishBlack", CupOptions.Tall, DrinkTypes.Teas, "Taiwan");
var teaPrice = CalculateDrink(blackTea); //25
var teaPrice2 = CalculateDrink(blackTea); //25

所以可以善用這種方式來讓程式碼可讀性更高，更簡潔。

參考資料

C# 9.0 on the record
C# 9.0 中的新增功能