1、默认接口方法

之前接口中不能声明方法体，成员也不能有Public修饰符

现在，可以在接口中声明方法体，和抽象类越来越像

interface CustomInterface
    {
        public void Show();

        public void ShowInfo()//该方法有方法体
        {
            Console.WriteLine("this is ShowInfo");
        }

    }

//调用
CustomInterface interface1 = new CustomClass();
interface1.ShowInfo();

2、switch

之前写法

public static string WeekToStringSwitch(WeekInfo week)
        {
            switch (week)
            {
                case WeekInfo.Monday:
                    return "周一";

                case WeekInfo.Tuesday:
                    return "周二";

                case WeekInfo.Wednesday:
                    return "周三";

                case WeekInfo.Thursday:
                    return "周四";

                case WeekInfo.Friday:
                    return "周五";

                case WeekInfo.Saturday:
                    return "周六";

                case WeekInfo.Sunday:
                    return "周七";

                default:
                    throw new NotImplementedException("枚举不存在");

            }
        }

现在可以这样：

public static string WeekToString(WeekInfo week) => week switch //此处是Lambda表达式
        {
            WeekInfo.Monday => "周一",
            WeekInfo.Tuesday => "周二",
            WeekInfo.Wednesday => "周三",
            WeekInfo.Thursday => "周四",
            WeekInfo.Friday => "周五",
            WeekInfo.Saturday => "周六",
            WeekInfo.Sunday => "周七",
            _ => throw new NotImplementedException("枚举不存在"),
        };

3、属性模式

定义如下PropertyPattern类，类中两个属性，

public class PropertyPattern
    {
        public string ProductName { get; set; }

        public double Price { get; set; }
    }

//实例化PropertyPattern类，将两个属性初始化
PropertyPattern product = new PropertyPattern()
            {
                ProductName = "aa",
                Price = 5
            };

C#8.0可以直接将属性应用于Switch结构

public static double PropertyPatternShow(PropertyPattern pattern) => pattern switch
        {
            { ProductName: "aa" } => pattern.Price * 0.5,
            { Price: 234 } => pattern.Price * 0.5, 
            _ => throw new NotImplementedException(),
        };

4、元组模式

string strResult = RockPaperScissors("aa", "bb");

public static string RockPaperScissors(string first, string second)
    => (first, second) switch
    {
        ("aa", "bb") => $"{first}-{second}",
        ("aa", "cc") => $"{first}-{second}",
        ("aa", "dd") => $"{first}-{second}",
        (_, _) => "不匹配"
    };

5、静态本地函数

类似于C#7.0中新增的本地函数语法，函数中调用另一个声明在本函数体中的方法，只不过C#8.0中针对的是静态函数

static int M()
            {
                int y = 5;
                int x = 7;
                return Add(x, y);

                static int Add(int left, int right) => left + right;
            }

6、using 声明

using 声明是前面带 using 关键字的变量声明。 它指示编译器声明的变量应在封闭范围的末尾进行处理。 以下面编写文本文件的代码为例：

之前的写法：

static int WriteLinesToFile(IEnumerable<string> lines)
{
    using (var file = new System.IO.StreamWriter("WriteLines2.txt"))
    {
        int skippedLines = 0;
        foreach (string line in lines)
        {
            ...
        }
        return skippedLines;
    } // file is disposed here
}

更新后：

static int WriteLinesToFile(IEnumerable<string> lines)
{
    using var file = new System.IO.StreamWriter("WriteLines2.txt");
    int skippedLines = 0;
    foreach (string line in lines)
    {
        .......
    }
    return skippedLines;
    // file is disposed here
}