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 -
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 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();
}
}Area of Rect 402.59999999999997 Area of Rect 536.8 Area of Circle 452.44799423217773
模式匹配后 -
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();
}
}Area of Rect 402.59999999999997 Area of Rect 536.8 Area of Circle 452.44799423217773
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