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。
- 代码演示C#各版本新功能
- 附录/总结
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