代码演示C#各版本新功能

代码演示C#各版本新功能

C#各版本新功能其实都能在官网搜到，但很少有人整理在一起，并通过非常简短的代码将每个新特性演示出来。

C# 2.0版 - 2005

泛型

Java中的泛型不支持值类型，且会运行时类型擦除，这一点.NET更优秀。

// Declare the generic class.
public class GenericList<T>
{
    public void Add(T input) { }
}
class TestGenericList
{
    private class ExampleClass { }
    static void Main()
    {
        // Declare a list of type int.
        GenericList<int> list1 = new GenericList<int>();
        list1.Add(1);

        // Declare a list of type string.
        GenericList<string> list2 = new GenericList<string>();
        list2.Add("");

        // Declare a list of type ExampleClass.
        GenericList<ExampleClass> list3 = new GenericList<ExampleClass>();
        list3.Add(new ExampleClass());
    }
}

分部类型

拆分一个类、一个结构、一个接口或一个方法的定义到两个或更多的文件中是可能的。 每个源文件包含类型或方法定义的一部分，编译应用程序时将把所有部分组合起来。

public partial class Employee
{
    public void DoWork()
    {
    }
}

public partial class Employee
{
    public void GoToLunch()
    {
    }
}

匿名方法

Func<int, int, int> sum = delegate (int a, int b) { return a + b; };
Console.WriteLine(sum(3, 4));  // output: 7

可以为null的值类型

double? pi = 3.14;
char? letter = 'a';

int m2 = 10;
int? m = m2;

bool? flag = null;

// An array of a nullable type:
int?[] arr = new int?[10];

迭代器

static void Main()
{
    foreach (int number in SomeNumbers())
    {
        Console.Write(number.ToString() + " ");
    }
    // Output: 3 5 8
    Console.ReadKey();
}

public static System.Collections.IEnumerable SomeNumbers()
{
    yield return 3;
    yield return 5;
    yield return 8;
}

协变和逆变

在 C# 中，协变和逆变能够实现数组类型、委托类型和泛型类型参数的隐式引用转换。 协变保留分配兼容性，逆变则与之相反。

// Assignment compatibility.   
string str = "test";  
// An object of a more derived type is assigned to an object of a less derived type.   
object obj = str;  
  
// Covariance.   
IEnumerable<string> strings = new List<string>();  
// An object that is instantiated with a more derived type argument   
// is assigned to an object instantiated with a less derived type argument.   
// Assignment compatibility is preserved.   
IEnumerable<object> objects = strings;  
  
// Contravariance.             
// Assume that the following method is in the class:   
// static void SetObject(object o) { }   
Action<object> actObject = SetObject;  
// An object that is instantiated with a less derived type argument   
// is assigned to an object instantiated with a more derived type argument.   
// Assignment compatibility is reversed.   
Action<string> actString = actObject;

C# 3.0版 - 2007

自动实现的属性

// This class is mutable. Its data can be modified from
// outside the class.
class Customer
{
    // Auto-implemented properties for trivial get and set
    public double TotalPurchases { get; set; }
    public string Name { get; set; }
    public int CustomerID { get; set; }

    // Constructor
    public Customer(double purchases, string name, int ID)
    {
        TotalPurchases = purchases;
        Name = name;
        CustomerID = ID;
    }

    // Methods
    public string GetContactInfo() { return "ContactInfo"; }
    public string GetTransactionHistory() { return "History"; }

    // .. Additional methods, events, etc.
}

class Program
{
    static void Main()
    {
        // Intialize a new object.
        Customer cust1 = new Customer(4987.63, "Northwind", 90108);

        // Modify a property.
        cust1.TotalPurchases += 499.99;
    }
}

匿名类型

var v = new { Amount = 108, Message = "Hello" };  
  
// Rest the mouse pointer over v.Amount and v.Message in the following  
// statement to verify that their inferred types are int and n .  
Console.WriteLine(v.Amount + v.Message);

查询表达式（LINQ）

LINQ允许你可以像写SQL一样写C#代码，像这样：

from p in persons
where p.Age > 18 && p.IsBeatiful
select new
{
    p.WeChatId, 
    p.PhoneNumber
}

LINQ的意义在于让C#做出了重大调整，本章中说到的lambda表达式、扩展方法、表达式树、匿名类型、自动属性等，都是LINQ的必要组成部分。

由于用扩展方法的形式也能得到一致的结果，而且还能让代码风格更加一致，所以我平时用LINQ语法较少：

// 与上文代码相同，但改成了扩展方法风格：
persons
    .Where(x => x.Age > 18 && x.IsBeatiful)
    .Select(x => new 
    {
        x.WeChatId, 
        x.PhoneNumber, 
    });

Lambda表达式

Func<int, int> square = x => x * x;
Console.WriteLine(square(5));
// Output:
// 25

表达式树

这个是LINQ的基础之一，它的作用是将代码像数据一样，保存在内存中；然后稍后对这些“代码数据”进行重新解释/执行。

Entity Framework就是一个经典场景，它先将表达式树保存起来，然后执行时，将其翻译为SQL发给数据库执行。

注意：表达式树并不能表示所有的代码，C# 3.0之后的语法，包含??、?.、async await、可选参数等，都无法放到表达式树中。据说官方准备更新它，但迟迟没有进展。

扩展方法

扩展方法使你能够向现有类型“添加”方法，而无需创建新的派生类型、重新编译或以其他方式修改原始类型。

static void Main()
{
	Console.WriteLine ("Perth".IsCapitalized());	
	// Equivalent to:
	Console.WriteLine (StringHelper.IsCapitalized ("Perth"));	
	
	// Interfaces can be extended, too:
	Console.WriteLine ("Seattle".First());   // S
}

public static class StringHelper
{
	public static bool IsCapitalized (this string s)
	{
		if (string.IsNullOrEmpty(s)) return false;
		return char.IsUpper (s[0]);
	}
	
	public static T First<T> (this IEnumerable<T> sequence)
	{
		foreach (T element in sequence)
			return element;
		
		throw new InvalidOperationException ("No elements!");
	}
}

var

var i = 10; // Implicitly typed.
int i = 10; // Explicitly typed.

分部方法

namespace PM
{
    partial class A
    {
        partial void OnSomethingHappened(string s);
    }

    // This part can be in a separate file.
    partial class A
    {
        // Comment out this method and the program
        // will still compile.
        partial void OnSomethingHappened(String s)
        {
            Console.WriteLine("Something happened: {0}", s);
        }
    }
}

对象和集合初始值设定项

public class Cat
{
    // Auto-implemented properties.
    public int Age { get; set; }
    public string Name { get; set; }

    public Cat()
    {
    }

    public Cat(string name)
    {
        this.Name = name;
    }
}

C# 4.0版 - 2010

dynamic

这个是特性使得CLR不得不进行一次修改。有了这个，C#也能像js、php、python等弱类型语言一样写代码了。

dynamic a = 3;
a = 3.14;
a = "Hello World";
a = new[] { 1, 2, 3, 4, 5 };
a = new Func<int>(() => 3);
a = new StringBuilder();
Console.WriteLine(a.GetType().Name); // StringBuilder

注意dynamic可以表示任何东西，包含数组、委托等等。滥用dynamic容易让程序变得很难维护。

命名参数/可选参数

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

public void ExampleMethod(int required, string optionalstr = "default string",
    int optionalint = 10)

泛型中的协变和逆变

IEnumerable<Derived> d = new List<Derived>();
IEnumerable<Base> b = d;

Action<Base> b = (target) => { Console.WriteLine(target.GetType().Name); };
Action<Derived> d = b;
d(new Derived());

类型等效、内置互操作类型

这个主要是为了和COM进行交互。之前需要引用一些COM类型相关的程序集，现在可以直接引用COM。
具体可以参见：https://docs.microsoft.com/zh-cn/dotnet/framework/interop/type-equivalence-and-embedded-interop-types

C# 5.0版 - 2012

async/await

private DamageResult CalculateDamageDone()
{
    // Code omitted:
    //
    // Does an expensive calculation and returns
    // the result of that calculation.
}

calculateButton.Clicked += async (o, e) =>
{
    // This line will yield control to the UI while CalculateDamageDone()
    // performs its work.  The UI thread is free to perform other work.
    var damageResult = await Task.Run(() => CalculateDamageDone());
    DisplayDamage(damageResult);
};

async/await的本质是状态机，像IEnumerable<T>一样。以前游戏引擎Unity只支持C# 3.0，因此当时它用状态机发Http请求是用的IEnumerable<T>。

async/await有两个好处，一是可以避免UI线程卡顿，二是提高系统吞吐率，最终提高性能。

调用方信息

public void DoProcessing()
{
    TraceMessage("Something happened.");
}

public void TraceMessage(string message,
        [CallerMemberName] string memberName = "",
        [CallerFilePath] string sourceFilePath = "",
        [CallerLineNumber] int sourceLineNumber = 0)
{
    System.Diagnostics.Trace.WriteLine("message: " + message);
    System.Diagnostics.Trace.WriteLine("member name: " + memberName);
    System.Diagnostics.Trace.WriteLine("source file path: " + sourceFilePath);
    System.Diagnostics.Trace.WriteLine("source line number: " + sourceLineNumber);
}

// Sample Output:
//  message: Something happened.
//  member name: DoProcessing
//  source file path: c:\Visual Studio Projects\CallerInfoCS\CallerInfoCS\Form1.cs
//  source line number: 31

注意这个是编译期生成的，因此比StackTrace更能保证性能。

C# 6.0版 - 2015

静态导入

终于可以不用写静态类名了。

using static System.Math;
using static System.Console;

WriteLine(Sin(3.14)); // 0.00159265291648683

异常筛选器

在try-catch时，可以按指定的条件进行catch，其它条件不catch。

public static async Task<string> MakeRequest()
{
    WebRequestHandler webRequestHandler = new WebRequestHandler();
    webRequestHandler.AllowAutoRedirect = false;
    using (HttpClient client = new HttpClient(webRequestHandler))
    {
        var stringTask = client.GetStringAsync("https://docs.microsoft.com/en-us/dotnet/about/");
        try
        {
            var responseText = await stringTask;
            return responseText;
        }
        catch (System.Net.Http.HttpRequestException e) when (e.Message.Contains("301"))
        {
            return "Site Moved";
        }
    }
}

自动初始化表达式

public ICollection<double> Grades { get; } = new List<double>();

Expression-bodied 函数成员

public override string ToString() => $"{LastName}, {FirstName}";

Null传播器

var first = person?.FirstName;

字符串内插

public string GetGradePointPercentage() =>
    $"Name: {LastName}, {FirstName}. G.P.A: {Grades.Average():F2}";

nameof表达式

有时字符串值和某个变量名称一致，尤其是在做参数验证时。这里nameof就能在编译期，自动从变量名生成一个字符串。

if (IsNullOrWhiteSpace(lastName))
    throw new ArgumentException(message: "Cannot be blank", paramName: nameof(lastName));

索引初始值设定项

使集合初始化更容易的另一个功能是对 Add 方法使用扩展方法 。 添加此功能的目的是进行 Visual Basic 的奇偶校验。 如果自定义集合类的方法具有通过语义方式添加新项的名称，则此功能非常有用。

C# 7.0版本 - 2017

out变量

if (int.TryParse(input, out int result))
    Console.WriteLine(result);
else
    Console.WriteLine("Could not parse input");

元组和解构

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

如上代码所示，解构可以将元组拆分为多个变量。

模式匹配

现在可以在匹配一个类型时，自动转换为这个类型的变量，如果转换失败，这个变量就赋值为默认值（null或0）。

极简版：

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

switch/case版：

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;
}

本地函数

这个主要是方便，javascript就能这样写。

比lambda的好处在于，这个可以定义在后面，而lambda必须定义在前面。

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;
    }
}

更多的expression-bodied成员

该功能可以让一些函数写成表达式的形式，非常的方便。

// Expression-bodied constructor
public ExpressionMembersExample(string label) => this.Label = label;

// Expression-bodied finalizer
~ExpressionMembersExample() => Console.Error.WriteLine("Finalized!");

private string label;

// Expression-bodied get / set accessors.
public string Label
{
    get => label;
    set => this.label = value ?? "Default label";
}

Ref 局部变量和返回结果

此功能允许使用并返回对变量的引用的算法，这些变量在其他位置定义。 一个示例是使用大型矩阵并查找具有某些特征的单个位置。

这个功能主要是为了提高值类型的性能，让它真正发挥其作用。C++就有类似的功能。

public static ref int Find(int[,] matrix, Func<int, bool> predicate)
{
    for (int i = 0; i < matrix.GetLength(0); i++)
        for (int j = 0; j < matrix.GetLength(1); j++)
            if (predicate(matrix[i, j]))
                return ref matrix[i, j];
    throw new InvalidOperationException("Not found");
}

ref var item = ref MatrixSearch.Find(matrix, (val) => val == 42);
Console.WriteLine(item);
item = 24;
Console.WriteLine(matrix[4, 2]);

弃元

通常，在进行元组解构或使用out参数调用方法时，必须定义一个其值无关紧要且你不打算使用的变量。 为处理此情况，C#增添了对弃元的支持 。 弃元是一个名为_的只写变量，可向单个变量赋予要放弃的所有值。 弃元类似于未赋值的变量；不可在代码中使用弃元（赋值语句除外）。

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

二进制文本和数字分隔符

这个用于使数字和二进制更可读。

// 二进制文本：
public const int Sixteen =   0b0001_0000;
public const int ThirtyTwo = 0b0010_0000;
public const int SixtyFour = 0b0100_0000;
public const int OneHundredTwentyEight = 0b1000_0000;

// 数字分隔符：
public const long BillionsAndBillions = 100_000_000_000;
public const double AvogadroConstant = 6.022_140_857_747_474e23;
public const decimal GoldenRatio = 1.618_033_988_749_894_848_204_586_834_365_638_117_720_309_179M;

throw表达式

throw之前必须是一个语句，因此有时不得不写更多的代码来完成所需功能。但7.0提供了throw表达式来使代码更简洁，阅读更轻松。

void Main()
{
	// You can now throw expressions in expressions clauses.
	// This is useful in conditional expressions:
	
	string result = new Random().Next(2) == 0 ? "Good" : throw new Exception ("Bad");
	result.Dump();
	
	Foo().Dump();
}

public string Foo() => throw new NotImplementedException();

C# 8.0 版 - 2019

Readonly 成员

public readonly override string ToString() =>
    $"({X}, {Y}) is {Distance} from the origin";

默认接口方法

接口中也能定义方法了，这个新功能经常受到争论。但想想，有时是先定义接口，而实现接口需要实现很多相关、但又繁琐的功能，如ASP.NET Core中的ILogger，谁用谁知道，特别多需要实现的方法，但又都差不多。因此所以这个功能其实很有必要。

void Main()
{
	ILogger foo = new Logger();
	foo.Log (new Exception ("test"));	
}

class Logger : ILogger
{	
	public void Log (string message) => Console.WriteLine (message);
}

interface ILogger
{
	void Log (string message);	
	
	// Adding a new member to an interface need not break implementors:
	public void Log (Exception ex) => Log (ExceptionHeader + ex.Message);
	
	// The static modifier (and other modifiers) are now allowed:
	static string ExceptionHeader = "Exception: ";
}

模式匹配增强

这个是为简化代码、函数式编程而生的，我个人非常喜欢。

属性模式

public static decimal ComputeSalesTax(Address location, decimal salePrice) =>
    location switch
    {
        { State: "WA" } => salePrice * 0.06M,
        { State: "MN" } => salePrice * 0.75M,
        { State: "MI" } => salePrice * 0.05M,
        // other cases removed for brevity...
        _ => 0M
    };

Tuple模式

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"
    };

位置模式

static Quadrant GetQuadrant(Point point) => point switch
{
    (0, 0) => Quadrant.Origin,
    var (x, y) when x > 0 && y > 0 => Quadrant.One,
    var (x, y) when x < 0 && y > 0 => Quadrant.Two,
    var (x, y) when x < 0 && y < 0 => Quadrant.Three,
    var (x, y) when x > 0 && y < 0 => Quadrant.Four,
    var (_, _) => Quadrant.OnBorder,
    _ => Quadrant.Unknown
};

switch表达式

这个功能能使代码从大量的if/else或switch/case变成“一行代码”，符合函数式编程的思想，非常好用！

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)),
    };

using声明

static int WriteLinesToFile(IEnumerable<string> lines)
{
    using var file = new System.IO.StreamWriter("WriteLines2.txt");
    // Notice how we declare skippedLines after the using statement.
    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
}

静态本地函数

相比非静态本地函数，静态本地函数没有闭包，因此生成的代码更少，性能也更容易控制。

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

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

异步流

这个功能和IEnumerable<T>、Task<T>对应，一个经典的表格如下：

 单值多值
同步 T IEnumerable
异步 Task ?

其中，这个问号?终于有了答案，它就叫异步流——IAsyncEnumerable<T>：

public static async System.Collections.Generic.IAsyncEnumerable<int> GenerateSequence()
{
    for (int i = 0; i < 20; i++)
    {
        await Task.Delay(100);
        yield return i;
    }
}

不像IEnumerable<T>，IAsyncEnumerable<T>系统还没有内置扩展方法，因此可能没有IEnumerable<T>方便，但是可以通过安装NuGet包f来实现和IEnumerable<T>一样（或者更爽）的效果。

索引和范围

和Python中的切片器一样，只是-用^代替了。

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

var quickBrownFox = words[1..4];
var lazyDog = words[^2..^0];
var allWords = words[..]; // contains "The" through "dog".
var firstPhrase = words[..4]; // contains "The" through "fox"
var lastPhrase = words[6..]; // contains "the", "lazy" and "dog"

Null合并赋值

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

非托管构造类型

与任何非托管类型一样，可以创建指向此类型的变量的指针，或针对此类型的实例在堆栈上分配内存块

Span<Coords<int>> coordinates = stackalloc[]
{
    new Coords<int> { X = 0, Y = 0 },
    new Coords<int> { X = 0, Y = 3 },
    new Coords<int> { X = 4, Y = 0 }
};

嵌套表达式中的 stackalloc

Span<int> numbers = stackalloc[] { 1, 2, 3, 4, 5, 6 };
var ind = numbers.IndexOfAny(stackalloc[] { 2, 4, 6 ,8 });
Console.WriteLine(ind);  // output: 1

附录/总结

这么多功能，你印象最深刻的是哪个呢？

参考资料：C#发展历史 - C#指南 | Microsoft Docs https://docs.microsoft.com/zh-cn/dotnet/csharp/whats-new/csharp-version-history

本文内容和代码由肖鹏整理，有修改；转载已获得肖鹏本人授权。肖鹏是我公司从Java转.NET的同事。原文链接为：https://akiyax.github.io/new-features-in-csharp/。

喜欢的朋友请关注我的微信公众号：【DotNet骚操作】

 

 

出处：https://www.cnblogs.com/sdflysha/p/20200228-csharp-history-code-demo.html

posted on 2021-05-06 10:34  jack_Meng  阅读(177)  评论(0)  编辑  收藏  举报

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