C# 7.0 中的模式匹配是什么？

C# 7.0 在两种情况下引入了模式匹配：is 表达式和 switch 声明。

模式测试一个值是否具有一定的形状，并且可以从 具有匹配形状时的值。

模式匹配为算法提供了更简洁的语法

您可以对任何数据类型（甚至是您自己的数据类型）执行模式匹配，而 if/else，你总是需要基元来匹配。

模式匹配可以从表达式中提取值。

模式匹配之前 -

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public class PI{
   public const float Pi = 3.142f;
}
public class Rectangle : PI{
   public double Width { get; set; }
   public double height { get; set; }
}
public class Circle : PI{
   public double Radius { get; set; }
}
class Program{
   public static void PrintArea(PI pi){
      if (pi is Rectangle){
         Rectangle rectangle = pi as Rectangle;
         System.Console.WriteLine("Area of Rect {0}", rectangle.Width * rectangle.height);
      }
      else if (pi is Circle){
         Circle c = pi as Circle;
         System.Console.WriteLine("Area of Circle {0}", Circle.Pi * c.Radius * c.Radius);
      }
   }
   public static void Main(){
      Rectangle r1 = new Rectangle { Width = 12.2, height = 33 };
      Rectangle r2 = new Rectangle { Width = 12.2, height = 44 };
      Circle c1 = new Circle { Radius = 12 };
      PrintArea(r1);
      PrintArea(r2);
      PrintArea(c1);
      Console.ReadLine();
   }
}

输出

Area of Rect 402.59999999999997
Area of Rect 536.8
Area of Circle 452.44799423217773

模式匹配后 -

示例

public class PI{
   public const float Pi = 3.142f;
}
public class Rectangle : PI{
   public double Width { get; set; }
   public double height { get; set; }
}
public class Circle : PI{
   public double Radius { get; set; }
}
class Program{
   public static void PrintArea(PI pi){
      if (pi is Rectangle rectangle){
         System.Console.WriteLine("Area of Rect {0}", rectangle.Width *
         rectangle.height);
      }
      else if (pi is Circle c){
         System.Console.WriteLine("Area of Circle {0}", Circle.Pi * c.Radius *
         c.Radius);
      }
   }
   public static void Main(){
      Rectangle r1 = new Rectangle { Width = 12.2, height = 33 };
      Rectangle r2 = new Rectangle { Width = 12.2, height = 44 };
      Circle c1 = new Circle { Radius = 12 };
      PrintArea(r1);
      PrintArea(r2);
      PrintArea(c1);
      Console.ReadLine();
   }
}

输出

Area of Rect 402.59999999999997
Area of Rect 536.8
Area of Circle 452.44799423217773