新匹配功能

Question

我下载了 Enterprise 2015 Preview 3。如何让这个程序在 C#7 下运行？

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

abstract class Animal { }
class Dog : Animal
{
    public string BarkLikeCrazy()
    {
        return "WOOF WOOF WOOF";
    }
}

class Cat : Animal { }
class Swan : Animal { }

class Program
{
    static void Main(string[] args)
    {
        var animals = new Animal[] { new Dog(), new Cat(), new Swan() };

        var organizedAnimals = from animal in animals
                               let sound = animal match(
                                    case Dog d: "woof... " + d.BarkLikeCrazy()
                                    case Cat c: "meow"
                                    case * : "I'm mute.."
                              )
                              select new { Type = animal, Sound = sound };

        foreach (var animal in organizedAnimals)
        {
            Console.WriteLine($"{animal.Type.ToString()} - {animal.Sound}");
        }

        Console.ReadKey();
    }
}

Answer 1

将您的 match 关键字更改为 switch。

var organizedAnimals = from animal in animals
                       let sound = animal switch(
                            case Dog d: "woof... " + d.BarkLikeCrazy()
                            case Cat c: "meow"
                            case * : "I'm mute.."
                      )
                      select new { Type = animal, Sound = sound };

您可以在 discussion on GitHub 中阅读有关此内容的演变（在合并到模式匹配规范之前）。

这是来自GitHub feature discussion的一个例子：

var areas =
    from primitive in primitives
    let area = primitive switch (
        case Line l: 0,
        case Rectangle r: r.Width * r.Height,
        case Circle c: Math.PI * c.Radius * c.Radius,
        case *: throw new ApplicationException()
    )
    select new { Primitive = primitive, Area = area };

Answer 2

如何让这个程序在 C#7 下运行？

你没有。基于表达式的模式匹配在 C# 7.0 的当前预览版中不可用，也不计划在 C# 7.0 的最终版本中使用。

The form that's currently planned for "C# 7.0 + 1" 看起来像这样：

var organizedAnimals = from animal in animals
                       let sound = animal switch (
                            case Dog d: "woof... " + d.BarkLikeCrazy(),
                            case Cat c: "meow",
                            case *: "I'm mute.."
                        )
                        select new { Type = animal, Sound = sound };