C#7元祖和弃元，C#8索引和范围(数组切片)，C#9记录类型

C#7元祖和弃元

/// 

    /// 元组和弃元
    /// 
    internal class C7
    {
        static (string, double, double) GetLocation()
        {
            var city = "西安市";
            var lat = 33.42d;
            var lon = 107.40d;
            return (city, lon, lat);
        }

        public void Demo1()
        {
            //元组
            (string Alpha, string Beta) namedLetters = ("a", "b");
            Console.WriteLine($"{namedLetters.Alpha}, {namedLetters.Beta}");
            //元组
            (string city, double lon, double lat) = GetLocation();
            Console.WriteLine($"{city},({lon},{lat})");
            //弃元,在 C# 中可以使用下划线_来表示要舍弃的元，是为弃元，怎么样？你学会了吗？
            (string city1, _, _) = GetLocation();
            Console.WriteLine($"{city1}");
        }

        /// 

        /// 我们可以将判断和赋值两个步骤合二为一：
        /// 
        /// 
        /// 
        /// 
        public static double ComputeAreaModernIs(object shape)
        {
            if (shape is Square s)
                return s.Side * s.Side;
            else if (shape is Circle c)
                return c.Radius * c.Radius * Math.PI;
            else if (shape is Rectangle r)
                return r.Height * r.Length;
            // elided
            throw new ArgumentException(
                message: "shape is not a recognized shape",
                paramName: nameof(shape));
        }
        /// 

        /// 主要打破了传统 switch 语句的常量模式
        /// 
        /// 
        /// 
        /// 
        public static double ComputeArea_Version3(object shape)
        {
            switch (shape)
            {
                case Square s when s.Side == 0:
                case Circle c when c.Radius == 0:
                    return 0;
                case Square s:
                    return s.Side * s.Side;
                case Circle c:
                    return c.Radius * c.Radius * Math.PI;
                default:
                    throw new ArgumentException(
                        message: "shape is not a recognized shape",
                        paramName: nameof(shape));
            }
        }

        #region 引发表达式,这个主要是针对 throw 关键字的增强
        //场景A：条件运算符
        string arg = args.Length >= 1 ? args[0] : throw new ArgumentException("You must supply an argument");
        //场景B：Null合并运算符
        public string Name
        {
            get => name;
            set => name = value ??
                throw new ArgumentNullException(
                  paramName: nameof(value),
                  message: "Name cannot be null");
        }
        //场景C：Lambda表达式
        DateTime ToDateTime(IFormatProvider provider) => throw new InvalidCastException("Conversion to a DateTime is not supported.");
        #endregion
    }

C#8数组切片

/// 

/// 正序下标从零开始
/// 倒序下标从一开始
/// 
public void Demo3()
{
    // 正序下标从零开始
    // 倒序下标从一开始
    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
    };

    //取最后一个元素
    Console.WriteLine($"The last word is {words[^1]}");//dog

    //获取下标是一的元素到下标是三的元素
    var quickBrownFox = words[1..4];
    Console.WriteLine(quickBrownFox);//quick,brown,fox

    //获取倒数第二个元素到倒数第一个元素
    var lazyDog = words[^2..^0];
    Console.WriteLine(lazyDog);//lazy,dog

    //获取倒数第二个元素到结束
    var lazyDog2 = words[^2..];
    Console.WriteLine(lazyDog2);//lazy,dog

    //获取全部元素
    var all = words[..];
    Console.WriteLine(all);

    //获取开始到下标是四的元素
    var firstPhrase = words[..4];
    Console.WriteLine(firstPhrase);//The,quick,brown,fox

    //获取下标是六的元素到结束
    var lastPhrase = words[6..];
    Console.WriteLine(lastPhrase);//the,lazy,dog
}

C#9记录类型

internal class C9
    {
        /// 

        /// 实体 通常都有一个唯一的标识并且在整个生命周期中具有连续性，这一类角色通过 class 来实现一直都工作得很好。
        /// 例如，每一个 User 都会有一个唯一的UserId ，我们使用 UserId 来判断其相等性。
        /// 而 值对象 则是指那些没有唯一的标识、不可变的、通过属性来判断相等性。
        /// 例如，我们有一个地址 Address，它由省、市、区、县和详细地址组成，那么，问题来了，
        /// 如果两个 Address 的省、市、区、县和详细地址都相同，这两个 Address 是不是同一个地址呢？
        /// 常识告诉我们：不会，因为它们是不同的实例。
        /// 
        /// 这就是 record 出现的原因。
        /// 
        record Address
        {
            public string Province { get; set; }
            public string City { get; set; }
            public string District { get; set; }
            public string County { get; set; }
        }

        public record Person
        {
            public string LastName { get; }
            public string FirstName { get; set; }
            public Person(string first, string last) => (FirstName, LastName) = (first, last);
        }

       

        public void Demo()
        {
            var addr1 = new Address() { Province = "陕西省", City = "西安市", District = "雁塔区" };
            var addr2 = new Address() { Province = "陕西省", City = "西安市", District = "雁塔区" };
            Console.WriteLine($"addr1 == addr2：{addr1 == addr2}");// addr1 == addr2：True

            var person = new Person("Bill", "Wagner");
            Console.WriteLine(person);// Person { LastName = Wagner, FirstName = Bill }
            Person brother = person with { FirstName = "Paul" }; // 修改FirstName的副本
            Console.WriteLine(brother);// Person { LastName = Wagner, FirstName = Paul }
            Person clone = person with { }; // 空集副本
            Console.WriteLine(clone);// Person { LastName = Wagner, FirstName = Bill }
　　　　　　　//模式匹配增强
            object e;
            if (e is not null)
            {
                // ...
            }
        }
　　　　// 模式匹配增强
        public static bool IsLetter(this char c) => c is >= 'a' and <= 'z' or >= 'A' and <= 'Z';
        public static bool IsLetterOrSeparator(this char c) => c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z') or '.' or ',';

    }