[WPF 学习] 5. 2 C#8.0的几个有用的知识点

1. 索引和范围
以下 .net 类型同时支持索引和范围：array、string、span 和 readonlyspan。 list 支持索引，但不支持范围
例一、获取身份证号码的生日

        datetime getbirthdayfromidno(string idno)
        {
            if (idno.length != 18)
                throw new exception("身份证号码不正确");
            return new datetime(int.parse(idno.substring(6, 4)), int.parse(idno.substring(10, 2)), int.parse(idno.substring(12, 2)));
        }
        datetime getbirthdayfromidno2(string idno)
        {
            if (idno.length != 18)
                throw new exception("身份证号码不正确");
            return new datetime(int.parse(idno[6..10]), int.parse(idno[10..12]), int.parse(idno[12..14]));
        }

例二、获取字符串最后一位的内容

            var idno = "330726197303273114";

            var s1 = idno.substring(idno.length - 1);
            var s2 = idno.last();
            var s3 = idno[^1];

例三、移除最后最后一位的内容

           var idno = "330726197303273114";

            var s1 = idno.substring(0,idno.length - 1);
            var s2 = idno.remove(idno.length - 1);
            var s3 = idno[..^1];

2. switch
表达式**

public enum rainbow
{
    red,
    orange,
    yellow,
    green,
    blue,
    indigo,
    violet
}

public static rgbcolor fromrainbow(rainbow colorband) =>
    colorband switch
    {
        rainbow.red    => new rgbcolor(0xff, 0x00, 0x00),
        rainbow.orange => new rgbcolor(0xff, 0x7f, 0x00),
        rainbow.yellow => new rgbcolor(0xff, 0xff, 0x00),
        rainbow.green  => new rgbcolor(0x00, 0xff, 0x00),
        rainbow.blue   => new rgbcolor(0x00, 0x00, 0xff),
        rainbow.indigo => new rgbcolor(0x4b, 0x00, 0x82),
        rainbow.violet => new rgbcolor(0x94, 0x00, 0xd3),
        _              => throw new argumentexception(message: "invalid enum value", paramname: nameof(colorband)),
    };

属性模式

public static decimal computesalestax(address location, decimal saleprice) =>
    location switch
    {
        { state: "wa" } => saleprice * 0.06m,
        { state: "mn" } => saleprice * 0.75m,
        { state: "mi" } => saleprice * 0.05m,
        // other cases removed for brevity...
        _ => 0m
    };

元组模式

public static string rockpaperscissors(string first, string second)
    => (first, second) switch
    {
        ("rock", "paper") => "rock is covered by paper. paper wins.",
        ("rock", "scissors") => "rock breaks scissors. rock wins.",
        ("paper", "rock") => "paper covers rock. paper wins.",
        ("paper", "scissors") => "paper is cut by scissors. scissors wins.",
        ("scissors", "rock") => "scissors is broken by rock. rock wins.",
        ("scissors", "paper") => "scissors cuts paper. scissors wins.",
        (_, _) => "tie"
    };

位置模式
某些类型包含 deconstruct 方法，该方法将其属性解构为离散变量。 如果可以访问 deconstruct 方法，就可以使用位置模式 检查对象的属性并将这些属性用于模式。 考虑以下 point 类，其中包含用于为 x 和 y 创建离散变量的 deconstruct 方法：

public class point
{
    public int x { get; }
    public int y { get; }

    public point(int x, int y) => (x, y) = (x, y);

    public void deconstruct(out int x, out int y) =>
        (x, y) = (x, y);
}

此外，请考虑以下表示象限的各种位置的枚举：

public enum quadrant
{
    unknown,
    origin,
    one,
    two,
    three,
    four,
    onborder
}

下面的方法使用位置模式 来提取 x 和 y 的值。 然后，它使用 when 子句来确定该点的 quadrant：

static quadrant getquadrant(point point) => point switch
{
    (0, 0) => quadrant.origin,
    var (x, y) when x > 0 && y > 0 => quadrant.one,
    var (x, y) when x < 0 && y > 0 => quadrant.two,
    var (x, y) when x < 0 && y < 0 => quadrant.three,
    var (x, y) when x > 0 && y < 0 => quadrant.four,
    var (_, _) => quadrant.onborder,
    _ => quadrant.unknown
};

3. null 合并赋值

list<int> numbers = null;
int? i = null;

numbers ??= new list<int>();
numbers.add(i ??= 17);
numbers.add(i ??= 20);

console.writeline(string.join(" ", numbers));  // output: 17 17
console.writeline(i);  // output: 17

**4. 构造函数表达式

        public class test1
        {
            public int x { get; }
            public int y { get; }
            public test1(int x, int y)
            {
                x = x;
                y = y;
            }
        }
        
        public class test2
        {
            public int x { get; }
            public int y { get; }
            public test2(int x, int y) => (x, y) = (x, y);
        }