C# 7.0特性

一.out的形参变量无需再提前声明

befor：

string input = "3";
int numericResult;
if (int.TryParse(input, out numericResult))
    Console.WriteLine(numericResult);
else
    Console.WriteLine("Could not parse input");

after：

string input = "3";
if (int.TryParse(input, out var numericResult))
    Console.WriteLine(numericResult);
else
    Console.WriteLine("Could not parse input");

二.扩展了元组（Tuple的使用，需要Nuget引用 System.ValueTuple）

1.命名的改进：

①.无命名，仅能通过无意义的Item1,Item2进行访问：

var letters = ("a", "b");
Console.WriteLine($"Value is {letters.Item1} and {letters.Item2>}");

befor:

(string first, string second) letters = ("a", "b");
Console.WriteLine($"Value is {letters.first} and {letters.second}");

after:

var letters = (first: "a",second: "b");
Console.WriteLine($"Value is {letters.first} and {letters.second}");

混合型命名：（会有一个编译警告，仅以左侧命名为准）

(string leftFirst,string leftSecond) letters = (first: "a", second: "b");
Console.WriteLine($"Value is {letters.leftFirst} and {letters.leftSecond}");

2.函数返回元组、对象转元组

static void Main(string[] args)
{
    var p = GetData();
    Console.WriteLine($"value is {p.name} and {p.age}");
}
 
private static (string name,int age) GetData()
{
    return ("han mei", 23);
}

三.Local function (本地函数)

static void Main(string[] args)
        {
            var v = Fibonacci(3);
            Console.WriteLine($"value is {v}");
        }
 
        private static int Fibonacci(int x)
        {
            if (x < 0) throw new ArgumentException("Less negativity please!", nameof(x));
            return Fib(x).current;
 
            (int current, int previous) Fib(int i)
            {
                if (i == 0) return (1, 0);
                var (p, pp) = Fib(i - 1);
                return (p + pp, p);
            }
        }

四.Literal improments(字义改进)

1.数字间可以增加分隔符：_ (增加可读性)

2.可以直接声明二进制 （使用二进制的场景更方便，比如掩码、用位进行权限设置等）

var d = 123_456;
var x = 0xAB_CD_EF;
var b = 0b1010_1011_1100_1101_1110_1111;

五.Ref returns and locals (返回引用[返回的变量可以是一个引用])

static void Main(string[] args)
{
    int[] array = { 1, 15, -39, 0, 7, 14, -12 };
    ref int place = ref Find(7, array); // aliases 7's place in the array
    place = 9; // replaces 7 with 9 in the array
    Console.WriteLine(array[4]); // prints 9
}
 
 
private static ref int Find(int number, int[] numbers)
{
    for (int i = 0; i < numbers.Length; i++)
    {
        if (numbers[i] == number)
        {
            return ref numbers[i]; // return the storage location, not the value
        }
    }
    throw new IndexOutOfRangeException($"{nameof(number)} not found");
}

六.More expression bodied members(更多的表达式体的成员)

支持更多的成员使用表达式体，加入了访问器、构造函数、析构函数使用表达式体

class Person
{
    private static ConcurrentDictionary<int, string> names = new ConcurrentDictionary<int, string>();
    private int id = 123;
 
    public Person(string name) => names.TryAdd(id, name); // constructors
    ~Person() => names.TryRemove(id, out var v);              // destructors
    public string Name
    {
        get => names[id];                                 // getters
        set => names[id] = value;                         // setters
    }
}

七.Throw expressions(抛出表达式)

将异常直接作为表达式抛出，不管是用表达式体时，还是普通的return 时可以直接作为一个表达式来写。

class Person
{
    public string Name { get; }
    public Person(string name) => Name = name ?? throw new ArgumentNullException(name);
    public string GetFirstName()
    {
        var parts = Name.Split(' ');
        return (parts.Length > 0) ? parts[0] : throw new InvalidOperationException("No name!");
    }
    public string GetLastName() => throw new NotImplementedException();
}

八.Generalized async return types(全面异步返回类型)

需要Nuget引用System.Threading.Tasks.Extensions。异步时能返回更多的类型。

public async ValueTask<int> Func()
{
    await Task.Delay(100);
    return 5;
}

九.Pattern matching（模式匹配）

1. is 表达式 ，判断类型的同时创建变量

public static int DiceSum2(IEnumerable<object> values)
{
    var sum = 0;
    foreach(var item in values)
    {
        if (item is int val)
            sum += val;
        else if (item is IEnumerable<object> subList)
            sum += DiceSum2(subList);
    }
    return sum;
}

2. switch 表达式，允许case后的条件判断的同时创建变量

public static int DiceSum5(IEnumerable<object> values)
{
    var sum = 0;
    foreach (var item in values)
    {
        switch (item)
        {
            case 0:
                break;
            case int val:
                sum += val;
                break;
            case PercentileDie die:
                sum += die.Multiplier * die.Value;
                break;
            case IEnumerable<object> subList when subList.Any():
                sum += DiceSum5(subList);
                break;
            case IEnumerable<object> subList:
                break;
            case null:
                break;
            default:
                throw new InvalidOperationException("unknown item type");
        }
    }
    return sum;
}

参考内容：https://www.cnblogs.com/dev2007/p/6526261.html