3.6C#引用类型之字符串(String)

1.字符串声明

string 标识符 = "值";

2.获取字符串长度

a.Length

3.访问字符串

string str = "test";
char x = str[2];  // x = 's';

4.字符串连接

可使用 + 运算符 或 += 运算符连接字符串。

string userName = "<Type your name here>";
string dateString = DateTime.Today.ToShortDateString();
string str = "Hello " + userName + ". Today is " + dateString + ".";
str += " How are you today?";

5.字符串内插(Interpolation)

若要将字符串标识为字符串内插(Interpolation)，可在该字符串 " 左引号前面加上 $ 美元符号( $ 和 " 之间不能有任何空格)，然后在 " 左引号 与 " 右引号 之间插入各种形式的 {内插表达式} 。

5.1内插表达式形式

//负整数左对齐，正整数右对齐
//整数值代表宽度
{内插表达式[,±整数][:格式]}

5.2字符串内插

string name = "Mark";
var date = DateTime.Now;
Console.WriteLine($"Hello, {name}! Today is {date.DayOfWeek}, it's {date:HH:mm} now.");
// 输出：Hello, Mark! Today is Wednesday, it's 19:40 now.

5.3转义序列

var userName = "Jane";

//支持普通的字符串转义序列
var stringWithEscapes = $"C:\\Users\\{userName}\\Documents";
//以 $ 字符开头，后跟 @(原义) 字符
var verbatimInterpolated = $@"C:\Users\{userName}\Documents";
Console.WriteLine(stringWithEscapes);
Console.WriteLine(verbatimInterpolated);
// C:\Users\Jane\Documents
// C:\Users\Jane\Documents

//两个双引号不会被 @(原义) 输出
string s2 = $@"He said, ""This is the last \u0063hance\x0021""";
输出：He said, "This is the last \u0063hance\x0021"

//若要包含大括号 "{" 或 "}"，在前面加两个大括号 "{{" 或 "}}"。
var xs = new int[] { 1, 2, 7, 9 };
var ys = new int[] { 7, 9, 12 };
Console.WriteLine($"Find the intersection of the {{{string.Join(", ",xs)}}} and {{{string.Join(", ",ys)}}} sets.");
// Find the intersection of the {1, 2, 7, 9} and {7, 9, 12} sets.

5.4三元条件运算符

因为冒号 (:) 在具有内插表达式的项中具有特殊含义，为了在内插表达式中使用三元条件运算符，请将内插表达式放在括号内。

var rand = new Random();
for (int i = 0; i < 7; i++)
{
    Console.WriteLine($"Coin flip: {(rand.NextDouble() < 0.5 ? "heads" : "tails")}");
}

6.复合格式字符串

复合格式字符串不需要加 $美元符号，{内插表达式} 为代表位置的整数索引。

强烈推荐使用字符串内插，因为字符串内插可用于任何字符串，而复合格式字符串必须调用 String.Format() 方法或支持复合格式字符串的其它方法，比如 Console.WriteLine() 或 StringBuilder.AppendFormat() 。

6.1内插表达式形式

//负整数左对齐，正整数右对齐
//整数值代表宽度
{索引[,±整数][:格式]}

6.2复合格式字符串

Console.WriteLine("Hello, {0}! Today is {1}, it's {2:HH:mm} now.", name, date.DayOfWeek, date);
// 同样地输出：Hello, Mark! Today is Wednesday, it's 19:40 now.

6.3转义序列

var userName = "Jane";

//支持普通的字符串转义序列
var stringWithEscapes = "C:\\Users\\{userName}\\Documents";
//字符串前加 @ 字符，表示原义输出
var verbatimInterpolated = @"C:\Users\{userName}\Documents";
Console.WriteLine(stringWithEscapes);
Console.WriteLine(verbatimInterpolated);
// C:\Users\Jane\Documents
// C:\Users\Jane\Documents

//两个双引号不会被 @(原义) 输出
string s2 = @"He said, ""This is the last \u0063hance\x0021""";
输出：He said, "This is the last \u0063hance\x0021"

//若要包含大括号 "{" 或 "}"，在前面加一个大括号 "{" 或 "}"。
string result;
int nOpen = 1;
int nClose = 2;
result = String.Format("The text has {0} {{ characters and {1} }} characters.", 
nOpen, nClose);
Console.WriteLine(result);
//The text has 1 { characters and 2 } characters.

7.字符串转义序列