C#8.0新特性

只读成员

  private struct Point  {   public Point(double x, double y)   {    X = x;    Y = y;   }   private double X { get; set; }   private double Y { get; set; }   private readonly double Distance => Math.Sqrt(X * X + Y * Y);   public override readonly string ToString() =>    $"({X}, {Y}) is {Distance} from the origin";  }

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使用readonly修饰tostring方法，表示它不可修改

默认接口方法

  /// <summary>  /// 默认接口方法  /// </summary>  private interface IWork  {   public void Work()   {    Console.WriteLine("Work");   }  }

现在可以在接口中定义默认的方法，而不是只能申明void Work();

更多的模式匹配

使用switch表达式的模式匹配

   public enum Rainbow   {    Red,    Orange,    Yellow,    Green,    Blue,    Indigo,    Violet   }   //switch表达式   public static Color FromRainbow(Rainbow colorBand) =>    colorBand switch    {     Rainbow.Red => Color.Red,     Rainbow.Orange => Color.Orange,     Rainbow.Yellow => Color.Yellow,     Rainbow.Green => Color.Green,     Rainbow.Blue => Color.Blue,     Rainbow.Indigo => Color.Indigo,     Rainbow.Violet => Color.Violet,     _ => throw new ArgumentException(message: "invalid enum value", paramName: nameof(colorBand)),    };

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在变量后面使用switch关键字，同时将case:替换为=>，使用_替代default

属性模式

   public static decimal ComputeSalesTax(Address location, decimal salePrice) =>    location switch    {     { State: "WA" } => salePrice * 0.06M,     { State: "MN" } => salePrice * 0.075M,     { State: "MI" } => salePrice * 0.05M,     // other cases removed for brevity...     _ => 0M    };   public struct Address   {    public string State { get; set; }   }

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对location变量的State属性进行模式匹配

元组模式

   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"    };

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位置模式

   //位置模式，使用解构将属性解构的离散变量，如果可以访问 Deconstruct 方法，就可以使用位置模式 检查对象的属性并将这些属性用于模式   public class XPoint   {    public int X { get; set; }    public int Y { get; set; }    public void Deconstruct(out int x, out int y)    {     x = X;     y = Y;    }   }   public int GetNumber(XPoint point) => point switch   {    (0, 0) => 0,    var (x, y) when x > 0 && y > 0 => 1,    var (x, y) when x < 0 && y > 0 => 2,    var (x, y) when x < 0 && y < 0 => 3,    var (x, y) when x > 0 && y < 0 => 4,    var (_, _) => -1,    _ => -2   };

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 using申明

  /// <summary>  /// using 声明，using 表达式的操作数可以实现 IDisposable 或 IAsyncDisposable  /// </summary>  public void UsingStatement()  {   using var file = new System.IO.StreamWriter("WriteLines2.txt");  }

可以对异步可释放类型使用using关键字进行释放

静态本地函数

  /// <summary>  /// 静态本地函数，在本地函数中使用static关键字  /// </summary>  private class StaticLocalFunction  {   int N()   {    int y;    LocalFunction();    return y;    void LocalFunction() => y = 0;   }   int M()   {    int y = 5;    int x = 7;    return Add(x, y);    static int Add(int left, int right) => left + right;   }  }

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可以在本地函数申明中使用static关键字

索引和范围

  private class IndexRange  {   string[] 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   void Test()   {    //>=index_1 && < index_4    var quickBrownFox = words[1..4];    var lazyDog = words[^2..^0];   }  }

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null合并赋值

  /// <summary>  /// null合并赋值  /// </summary>  private void NullMergeAssignment()  {   List<int> list = new List<int>();   List<int> numbers = list.Where(x => x > 20).ToList();      numbers ??= new List<int>();  }

当左操作数计算为 null 时，将??= 右操作数的值分配给左操作数

异步流

public static class AsyncStream  {   public static async System.Collections.Generic.IAsyncEnumerable<int> GenerateSequence()   {    for (int i = 0; i < 20; i++)    {     await Task.Delay(100);     yield return i;    }   }   public static async void GetNumbers()   {    var c = CSharp8.AsyncStream.GenerateSequence();    await foreach (var number in c)    {     Console.WriteLine(number);    }   }  }

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GetNumbers使用await等待返回结果，每隔100ms会输出i++的值

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