C#3 到C#9，每个版本添加的功能，待更新

C#2:

　　1.泛型

C#3：

       1.主要是linq

　　2.拉姆达表达式

C#4：

　　1.可选参数

　　2.异步 async ... await ...

　　异步的例子：

C#5:

C#6:

　　1.  ？.操作符，和??操作符类似

　　2. using静态指令。using static <静态方法的类名>，就可以在下文中，只写静态方法，而不用带出类名。

　　参考地址：https://docs.microsoft.com/zh-cn/dotnet/csharp/language-reference/keywords/using-static

　　3.

using System;

public class Circle
{
   public Circle(double radius)
   {
      Radius = radius;
   }

   public double Radius { get; set; }

   public double Diameter
   {
      get { return 2 * Radius; }
   }

   public double Circumference
   {
      get { return 2 * Radius * Math.PI; }
   }

   public double Area
   {
      get { return Math.PI * Math.Pow(Radius, 2); }
   }
}

使用Math类使用using静态指令后，使用Math类中的静态方法和静态成员就不用了带类名了。

using System;
using static System.Math;

public class Circle
{
   public Circle(double radius)
   {
      Radius = radius;
   }

   public double Radius { get; set; }

   public double Diameter
   {
      get { return 2 * Radius; }
   }

   public double Circumference
   {
      get { return 2 * Radius * PI; }
   }

   public double Area
   {
      get { return PI * Pow(Radius, 2); }
   }
}

3. case () when 语句

using System;

public abstract class Shape
{
   public abstract double Area { get; }
   public abstract double Circumference { get; }
}

public class Rectangle : Shape
{
   public Rectangle(double length, double width)
   {
      Length = length;
      Width = width;
   }

   public double Length { get; set; }
   public double Width { get; set; }

   public override double Area
   {
      get { return Math.Round(Length * Width,2); }
   }

   public override double Circumference
   {
      get { return (Length + Width) * 2; }
   }
}

public class Square : Rectangle
{
   public Square(double side) : base(side, side)
   {
      Side = side;
   }

   public double Side { get; set; }
}

public class Example
{
   public static void Main()
   {
      Shape sh = null;
      Shape[] shapes = { new Square(10), new Rectangle(5, 7),
                         new Rectangle(10, 10), sh, new Square(0) };
      foreach (var shape in shapes)
         ShowShapeInfo(shape);
   }

   private static void ShowShapeInfo(Object obj)
   {
      switch (obj)
      {
         case Shape shape when shape.Area == 0:
            Console.WriteLine($"The shape: {shape.GetType().Name} with no dimensions");
            break;
         case Square sq when sq.Area > 0:
            Console.WriteLine("Information about the square:");
            Console.WriteLine($"   Length of a side: {sq.Side}");
            Console.WriteLine($"   Area: {sq.Area}");
            break;
         case Rectangle r when r.Area > 0:
            Console.WriteLine("Information about the rectangle:");
            Console.WriteLine($"   Dimensions: {r.Length} x {r.Width}");
            Console.WriteLine($"   Area: {r.Area}");
            break;
         case Shape shape:
            Console.WriteLine($"A {shape.GetType().Name} shape");
            break;
         case null:
            Console.WriteLine($"The {nameof(obj)} variable is uninitialized.");
            break;
         default:
            Console.WriteLine($"The {nameof(obj)} variable does not represent a Shape.");
            break;
      }
   }
}
// The example displays the following output:
//       Information about the square:
//          Length of a side: 10
//          Area: 100
//       Information about the rectangle:
//          Dimensions: 5 x 7
//          Area: 35
//       Information about the rectangle:
//          Dimensions: 10 x 10
//          Area: 100
//       The obj variable is uninitialized.
//       The shape: Square with no dimensions

catch(  )  when  语句

using System;
using System.Net.Http;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        Console.WriteLine(MakeRequest().Result);
    }

    public static async Task<string> MakeRequest()
    {
        var client = new HttpClient();
        var streamTask = client.GetStringAsync("https://localHost:10000");
        try
        {
            var responseText = await streamTask;
            return responseText;
        }
        catch (HttpRequestException e) when (e.Message.Contains("301"))
        {
            return "Site Moved";
        }
        catch (HttpRequestException e) when (e.Message.Contains("404"))
        {
            return "Page Not Found";
        }
        catch (HttpRequestException e)
        {
            return e.Message;
        }
    }
}

5.表达式主体定义

public override string ToString() => $"{fname} {lname}".Trim();
//等效于
public override string ToString()
{
   return $"{fname} {lname}".Trim();
}

6. $ 特殊字符将字符串文本标识为内插字符串

string name = "Mark";
var date = DateTime.Now;

// Composite formatting:
Console.WriteLine("Hello, {0}! Today is {1}, it's {2:HH:mm} now.", name, date.DayOfWeek, date);
// String interpolation:
Console.WriteLine($"Hello, {name}! Today is {date.DayOfWeek}, it's {date:HH:mm} now.");
// Both calls produce the same output that is similar to:
// Hello, Mark! Today is Wednesday, it's 19:40 now.

C#7:

1.元组(Tuple)

(string Alpha, string Beta) namedLetters = ("a", "b");
Console.WriteLine($"{namedLetters.Alpha}, {namedLetters.Beta}");

或者变量声明用var

var alphabetStart = (Alpha: "a", Beta: "b");
Console.WriteLine($"{alphabetStart.Alpha}, {alphabetStart.Beta}");

构造函数中有Deconstruct,在生成对象时，可以提取你需要的属性值

public class Point
{
    public Point(double x, double y)
        => (X, Y) = (x, y);

    public double X { get; }
    public double Y { get; }

    public void Deconstruct(out double x, out double y) =>
        (x, y) = (X, Y);
}

var p = new Point(3.14, 2.71);
(double X, double Y) = p;

弃元

当提取出来的数据不需要时，用‘_’占位符号，来放弃当前的变量

using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        var (_, _, _, pop1, _, pop2) = QueryCityDataForYears("New York City", 1960, 2010);

        Console.WriteLine($"Population change, 1960 to 2010: {pop2 - pop1:N0}");
    }

    private static (string, double, int, int, int, int) QueryCityDataForYears(string name, int year1, int year2)
    {
        int population1 = 0, population2 = 0;
        double area = 0;

        if (name == "New York City")
        {
            area = 468.48;
            if (year1 == 1960)
            {
                population1 = 7781984;
            }
            if (year2 == 2010)
            {
                population2 = 8175133;
            }
            return (name, area, year1, population1, year2, population2);
        }

        return ("", 0, 0, 0, 0, 0);
    }
}
// The example displays the following output:
//      Population change, 1960 to 2010: 393,149

is 运算符添加了一些功能，如下，如果input可以转换为int类型，就把转换后的值赋值给count.

if (input is int count)
    sum += count;

switch( )增加了一些新功能

public static int SumPositiveNumbers(IEnumerable<object> sequence)
{
    int sum = 0;
    foreach (var i in sequence)
    {
        switch (i)
        {
            case 0:
                break;
            case IEnumerable<int> childSequence:
            {
                foreach(var item in childSequence)
                    sum += (item > 0) ? item : 0;
                break;
            }
            case int n when n > 0:
                sum += n;
                break;
            case null:
                throw new NullReferenceException("Null found in sequence");
            default:
                throw new InvalidOperationException("Unrecognized type");
        }
    }
    return sum;
}

async main,main方法前面可以添加async修饰符号

static int Main()
{
    return DoAsyncWork().GetAwaiter().GetResult();
}
可以写成下面
static async Task<int> Main()
{
    // This could also be replaced with the body
    // DoAsyncWork, including its await expressions:
    return await DoAsyncWork();
}
如果没有返回证人数字，可以返回一个task
static async Task Main()
{
    await SomeAsyncMethod();
}

本地函数

public static IEnumerable<char> AlphabetSubset3(char start, char end)
{
    if (start < 'a' || start > 'z')
        throw new ArgumentOutOfRangeException(paramName: nameof(start), message: "start must be a letter");
    if (end < 'a' || end > 'z')
        throw new ArgumentOutOfRangeException(paramName: nameof(end), message: "end must be a letter");

    if (end <= start)
        throw new ArgumentException($"{nameof(end)} must be greater than {nameof(start)}");

    return alphabetSubsetImplementation();

    IEnumerable<char> alphabetSubsetImplementation()
    {
        for (var c = start; c < end; c++)
            yield return c;
    }
}

throw表达式

private static void DisplayFirstNumber(string[] args)
{
   string arg = args.Length >= 1 ? args[0] :
                              throw new ArgumentException("You must supply an argument");
   if (Int64.TryParse(arg, out var number))
      Console.WriteLine($"You entered {number:F0}");
   else
      Console.WriteLine($"{arg} is not a number.");
}

默认文本表达式

Func<string, bool> whereClause = default(Func<string, bool>);
可以写成这样
Func<string, bool> whereClause = default;

C#4中有了命名参数，C#7.2中方法的命名参数只要和参数的默认顺序相同，也是可以不写出参数名称的

PrintOrderDetails("Gift Shop", 31, "Red Mug");

PrintOrderDetails(orderNum: 31, productName: "Red Mug", sellerName: "Gift Shop");
PrintOrderDetails(productName: "Red Mug", sellerName: "Gift Shop", orderNum: 31);

也可以这样写
PrintOrderDetails(sellerName: "Gift Shop", 31, productName: "Red Mug");
// This generates CS1738: Named argument specifications must appear after all fixed arguments have been specified.
PrintOrderDetails(productName: "Red Mug", 31, "Gift Shop");

in 参数修饰符，表明该参数被作为引用类型来传递，且不能被方法修改

static void M(S arg);
static void M(in S arg);

C#8:

switch语句的升级

public static RGBColor FromRainbow(Rainbow colorBand) =>
    colorBand switch
    {
        Rainbow.Red    => new RGBColor(0xFF, 0x00, 0x00),
        Rainbow.Orange => new RGBColor(0xFF, 0x7F, 0x00),
        Rainbow.Yellow => new RGBColor(0xFF, 0xFF, 0x00),
        Rainbow.Green  => new RGBColor(0x00, 0xFF, 0x00),
        Rainbow.Blue   => new RGBColor(0x00, 0x00, 0xFF),
        Rainbow.Indigo => new RGBColor(0x4B, 0x00, 0x82),
        Rainbow.Violet => new RGBColor(0x94, 0x00, 0xD3),
        _              => throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand)),
    };

和下面的一样的功能
public static RGBColor FromRainbowClassic(Rainbow colorBand)
{
    switch (colorBand)
    {
        case Rainbow.Red:
            return new RGBColor(0xFF, 0x00, 0x00);
        case Rainbow.Orange:
            return new RGBColor(0xFF, 0x7F, 0x00);
        case Rainbow.Yellow:
            return new RGBColor(0xFF, 0xFF, 0x00);
        case Rainbow.Green:
            return new RGBColor(0x00, 0xFF, 0x00);
        case Rainbow.Blue:
            return new RGBColor(0x00, 0x00, 0xFF);
        case Rainbow.Indigo:
            return new RGBColor(0x4B, 0x00, 0x82);
        case Rainbow.Violet:
            return new RGBColor(0x94, 0x00, 0xD3);
        default:
            throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand));
    };
}

public static string RockPaperScissors(string first, string second)
    => (first, second) switch
    {
        ("rock", "paper") => "rock is covered by paper. Paper wins.",
        ("rock", "scissors") => "rock breaks scissors. Rock wins.",
        ("paper", "rock") => "paper covers rock. Paper wins.",
        ("paper", "scissors") => "paper is cut by scissors. Scissors wins.",
        ("scissors", "rock") => "scissors is broken by rock. Rock wins.",
        ("scissors", "paper") => "scissors cuts paper. Scissors wins.",
        (_, _) => "tie"
    };

using语句，在作用于的结尾，对象会被销毁

static int WriteLinesToFile(IEnumerable<string> lines)
{
    using var file = new System.IO.StreamWriter("WriteLines2.txt");
    int skippedLines = 0;
    foreach (string line in lines)
    {
        if (!line.Contains("Second"))
        {
            file.WriteLine(line);
        }
        else
        {
            skippedLines++;
        }
    }
    // Notice how skippedLines is in scope here.
    return skippedLines;
    // file is disposed here
}
以前的写法需要把using语句写在小括号里

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

引用类型也可为空，类似于可为空的值类型int？，string？

数组的索引和范围

var words = new string[]
{
                // index from start    index from end
    "The",      // 0                   ^9
    "quick",    // 1                   ^8
    "brown",    // 2                   ^7
    "fox",      // 3                   ^6
    "jumped",   // 4                   ^5
    "over",     // 5                   ^4
    "the",      // 6                   ^3
    "lazy",     // 7                   ^2
    "dog"       // 8                   ^1
};              // 9 (or words.Length) ^0

Console.WriteLine($"The last word is {words[^1]}");
// writes "dog"

var quickBrownFox = words[1..4]; 从第二个到第5个

var lazyDog = words[^2..^0]; //从倒数第3个到倒数第一个

var allWords = words[..]; // contains "The" through "dog".
var firstPhrase = words[..4]; // contains "The" through "fox"
var lastPhrase = words[6..]; // contains "the", "lazy" and "dog"

Range phrase = 1..4;
var text = words[phrase];

??=操作符，只有左边为null时，才把??=右边的值赋值给左边

List<int> numbers = null;
int? i = null;

numbers ??= new List<int>();
numbers.Add(i ??= 17);
numbers.Add(i ??= 20);

Console.WriteLine(string.Join(" ", numbers));  // output: 17 17
Console.WriteLine(i);  // output: 17

处理字符串的$ 和 @可以嵌套使用，例如$@"..."

C#9

使用new生成对象时，可以不写类型

private List<WeatherObservation> _observations = new();

判断条件的优化

public static bool IsLetter(this char c) =>
    c is >= 'a' and <= 'z' or >= 'A' and <= 'Z';

public static bool IsLetterOrSeparator(this char c) =>
c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z') or '.' or ',';

if (e is not null)
{
// ...
}