C#10的特性

时间:2022-05-23 Oleg Kyrylchuk 人气:2

常量的内插字符串

C# 10 允许使用在常量字符串初始化中使用插值, 如下

const string name = "Oleg";
const string greeting = $"Hello, {name}.";

Console.WriteLine(greeting);
// Output: Hello, Oleg.

扩展属性模式

从 C# 10 开始，您可以在适当的模式中引用嵌套的属性或字段，属性模式变得更具可读性并且需要更少的大括号。

Person person = new()
{
    Name = "Oleg",
    Location = new() { Country = "PL" }
};

if (person is { Name: "Oleg", Location.Country: "PL" })
{
    Console.WriteLine("It's me!");
}

class Person
{
    public string Name { get; set; }
    public Location Location { get; set; }
}

class Location
{
    public string Country { get; set; }
}

如果Location为null，则不会匹配模式并返回false。

文件范围的命名空间

C# 10 引入了一种新的命名空间声明方式 - 文件范围的命名空间，减少一个大括号，代码结构更简洁。

namespace FileScopedNamespace;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello World!");
    }
}

全局 Using

一次引用，全局通用

global using System;
global using System.Collections.Generic;
global using System.Linq;
global using System.Threading.Tasks;

List<int> list = new() { 1, 2, 3, 4 };
int sum = list.Sum();
Console.WriteLine(sum);

await Task.Delay(1000);

同一个解构中的赋值和声明

C# 10 可以在同一个解构中进行赋值和声明。

var rgb = (255, 100, 30);

// Initialization & assignment
int r;
(r, int g, int b) = rgb;

Console.WriteLine($"RGB: {r}, {g}, {b}");
// Output: RGB: 255, 100, 30

Record 类型重写 ToString() 时支持密封

Product product = new() { Name = "Bread" };
Console.WriteLine(product.ToString());
// Output: Bread

public record Product
{
    public string Name { get; init; }

    public sealed override string ToString()
    {
        return Name;
    }
}

Record Struct

C# 10 支持 record struct

Person me = new() { FirstName = "Oleg", LastName = "Kyrylchuk" };

Console.WriteLine(me);
// Output: Person { FirstName = Oleg, LastName = Kyrylchuk }

Person otherPerson = me with { FirstName = "John" };
Console.WriteLine(otherPerson);
// Output: Person { FirstName = John, LastName = Kyrylchuk }

Person anotherMe = new() { FirstName = "Oleg", LastName = "Kyrylchuk" };
C onsole.WriteLine(me == anotherMe);
// Output: True

record struct Person
{
    public string FirstName { get; init; }
    public string LastName { get; init; }
}

record struct Product(string Name, decimal Price);

Struct 字段支持初始化

using System;

Person person = new() { Name = "Oleg" };

Console.WriteLine(person.Id + " " + person.Name);
// Output: 0cc6caac-d061-4f46-9301-c7cc2a012e47 Oleg

struct Person
{
    public Guid Id { get; init; } = Guid.NewGuid();
    public string Name { get; set; }
}

Lambda 表达式的 Attributes 支持

C# 9 支持本地函数的 Attributes, C# 10 添加了 Lambda 表达式的 Attributes 支持。

Action a = [MyAttribute] () => { };                
Action<int> b =[return: MyAttribute] (x) => { };  
Action<int> c =[MyAttribute] ([MyAttribute] x) => { };       


class MyAttribute : Attribute
{ }

Lambda 中的显式返回类型

Test<int>();

var l1 = string () => string.Empty;
var l2 = int () => 0;
var l3 = static void () => { };

void Test<T>()
{
    var l4 = T () => default;
}

应用于方法的 AsyncMethodBuilder 特性

从 C# 7 开始，您只能将AsyncMethodBuilder 特性应用于类型, 在 C# 10 中，您还可以将该特性应用于单个方法。

using System.Runtime.CompilerServices;

class Example
{
    [AsyncMethodBuilder(typeof(AsyncVoidMethodBuilder))]
    public void ExampleMethod()
    {
    }
}

结构体中的表达式

C# 10 支持将 with 表达式和 struct 一起使用

Product potato = new() { Name = "Potato", Category = "Vegetable" };
Console.WriteLine($"{potato.Name} {potato.Category}");
// Output: Potato Vegetable

Product tomato = potato with { Name = "Tomato" };
Console.WriteLine($"{tomato.Name} {tomato.Category}");
// Output: Tomato Vegetable

struct Product
{
    public string Name { get; set; }
    public string Category { get; set; }
}

匿名类型中的表达式

C# 10 支持将 with 表达式和匿名类型一起使用

var potato = new { Name = "Potato", Category = "Vegetable" };
Console.WriteLine($"{potato.Name} {potato.Category}");
// Output: Potato Vegetable

var onion = potato with { Name = "Onion" };
Console.WriteLine($"{onion.Name} {onion.Category}");
// Output: Onion Vegetable

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