C# switch expression, pattern matching 語法整理, ref→[The evolution of Pattern Matching in C# (from version 6 to 10)](https://www.youtube.com/watch?v=MzNHMJCyU40&ab_channel=NickChapsas)

switch_pattern_matching.cs

///
/// C# switch pattern 語法整理
/// ref→[The evolution of Pattern Matching in C# (from version 6 to 10)](https://www.youtube.com/watch?v=MzNHMJCyU40&ab_channel=NickChapsas)
/// ref→[Pattern matching overview](https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/pattern-matching)
/// ref→[Patterns (C# reference)](https://docs.microsoft.com/zh-tw/dotnet/csharp/language-reference/operators/patterns)
/// 

using System;
using System.Collections.Generic;

namespace ConsoleApp2019
{
    class Program
    {
        static void Main(string[] args)
        {
            var circle = new Circle(5);
            var rectangle = new Rectangle(420, 1337);
            var square = new Rectangle(69, 69);

            var shapes = new List<Shape> { circle, rectangle, square };

            var randomShape = shapes[new Random().Next(shapes.Count)];

            //## Let's go

            #region with C#6
            {
                if (randomShape is Circle)
                {
                    var cir6 = (Circle)randomShape;
                    Console.WriteLine($"Circle with area {cir6.Area}");
                }
            }
            #endregion

            #region with C#7 
            {
                if (randomShape is Circle cir7)
                {
                    Console.WriteLine($"Circle with area {cir7.Area}");
                }

                switch (randomShape)
                {
                    case Circle c:
                        Console.WriteLine($"這是個圓形其面積有{c.Area}。");
                        break;
                    case Rectangle r when r.Height == r.Width:
                        Console.WriteLine($"這是個正方形其面積有{r.Area}");
                        break;
                    default:
                        Console.WriteLine($"沒什麼特別的形狀。");
                        break;
                }
            }
            #endregion

            #region with C#8
            {
                if (randomShape is Circle { Radius: 10, Area: 50 })
                {
                }

                string shapeDetaisl = randomShape switch
                {
                    Circle cir
                        => $"這是個圓形其面積有{cir.Area}。",
                    Rectangle rect when rect.Height == rect.Width
                        => $"這是個正方形其面積有{rect.Area}",
                    { Area: 100 }
                        => $"此面積有100。",
                    { Area: > 100 }
                        => $"此面積大於100。",
                    _ => "什麼特性都沒有。"
                };

                Console.WriteLine(shapeDetaisl);
            }
            #endregion

            #region with C#9
            {
                if (randomShape is not Rectangle)
                {
                }

                if (randomShape is Circle { Radius: > 100 and < 200, Area: >= 1000 })
                {
                }

                if (randomShape is Rectangle { ShapeInShape: { Area: 100 } }) 
                { 
                }

                string shapeDetaisl = randomShape switch
                {
                    Circle { Area: > 100 and < 200 } => "一個神奇的圓形。",
                    Circle cir => $"這是個圓形其面積有{cir.Area}。",
                    Rectangle rect when rect.Height == rect.Width
                        => $"這是個正方形其面積有{rect.Area}",
                    { Area: 100 }
                        => $"此面積有100。",
                    { Area: > 100 }
                        => $"此面積大於100。",
                    _ => "什麼特性都沒有。"
                };

                var areaDetails = randomShape.Area switch
                {
                    >= 100 and <= 200 => "...",
                    _ => "not match"
                };

                Console.WriteLine(shapeDetaisl);
            }
            #endregion

            Console.WriteLine("Press any key to continue.");
            Console.ReadKey();
        }
    }

    public abstract class Shape
    {
        public abstract double Area { get; }

        public Shape ShapeInShape { get; set; }
    }

    public interface ISquare
    {
        double Height { get; set; }
        double Width { get; set; }
    }

    public class Rectangle : Shape, ISquare
    {
        public Rectangle(double height, double width)
        {
            Height = height;
            Width = width;
        }

        public double Height { get; set; }
        public double Width { get; set; }
        public override double Area => Height * Width;
    }

    public class Circle : Shape
    {
        const double PI = Math.PI;

        public Circle(double diameter)
        {
            Diameter = diameter;
        }

        public double Diameter { get; set; }
        public double Radius => Diameter / 2.0;
        public override double Area => PI * Radius * Radius;
    }
}