使用C#从.NET类库中调用方法

相关概念

有状态方法和无状态方法：无状态方法类似JAVA中类的静态方法；有状态方法类似JAVA中的实例方法；
创建类的实例：对于有状态的方法，我们必须创建类的实例，才能调用有状态方法。
方法的返回值：方法完成后返回的值，无返回值则是void方法；
方法参数：调用方法时，传入该方法将用于完成其任务的值，这些值称为参数，通常“形参”指的是方法内使用的变量，“实参”是调用方法时传递的值.C# 中的参数按值传递给函数,因为结构是值类型，所以按值将结构传递给方法时，该方法接收结构参数的副本并在其上运行。类实例是引用类型，而非值类型。按值将引用类型传递给方法时，方法接收对类实例的引用的副本。这两个变量都引用同一对象。

代码示例：找到两个值中较大的一个

int firstValue = 500;
int secondValue = 600;
int largeValue =Math.Max(firstValue,secondValue);
Console.WriteLine($"largeValue :{largeValue}");

选择语句

if语句

整体与JAVA类似

Random random = new Random();
int dayUntilExpiration = random.Next(12);
int discountPercentage = 0;

if (dayUntilExpiration <= 0){
    Console.WriteLine("Your subscription has expired.");
}else if(dayUntilExpiration == 1){
    discountPercentage = 20;
    Console.WriteLine("Your subscription expires within a day!");
    Console.WriteLine($"Renew now and save {discountPercentage}%");
}else if(dayUntilExpiration <= 5){
    discountPercentage = 10;
    Console.WriteLine($"Your subscription expires in {dayUntilExpiration} days!");
    Console.WriteLine($"Renew now and save {discountPercentage}%");
}else if(dayUntilExpiration <= 10){
    Console.WriteLine($"Your subscription will expire soon.Renew now!");
}

switch语句

switch语句中的case可以有以下几种形式：

用于将表达式结果与常量值进行比较；
测试表达式结果是否等于常量。

default语句指定匹配表达式与其他任何case模式不匹配时要执行的语句。

可以为switch语句的一部分指定多个case模式:

DisplayMeasurement(-4);  // Output: Measured value is -4; out of an acceptable range.
DisplayMeasurement(50);  // Output: Measured value is 50.
DisplayMeasurement(132);  // Output: Measured value is 132; out of an acceptable range.

void DisplayMeasurement(int measurement)
{
    switch (measurement)
    {
        case < 0:
        case > 100:
            Console.WriteLine($"Measured value is {measurement}; out of an acceptable range.");
            break;
        
        default:
            Console.WriteLine($"Measured value is {measurement}.");
            break;
    }
}

case guard

case 模式可能表达功能不够，无法指定用于执行 switch 部分的条件。在这种情况下，可以使用 case guard。这是一个附加条件，必须与匹配模式同时满足。 case guard 必须是布尔表达式。可以在模式后面的 when 关键字之后指定一个 case guard，如以下示例所示：

DisplayMeasurements(3, 4);  // Output: First measurement is 3, second measurement is 4.
DisplayMeasurements(5, 5);  // Output: Both measurements are valid and equal to 5.

void DisplayMeasurements(int a, int b)
{
    switch ((a, b))
    {
        case (> 0, > 0) when a == b:
            Console.WriteLine($"Both measurements are valid and equal to {a}.");
            break;

        case (> 0, > 0):
            Console.WriteLine($"First measurement is {a}, second measurement is {b}.");
            break;

        default:
            Console.WriteLine("One or both measurements are not valid.");
            break;
    }
}

数组和循环语句

依然与JAVA类似

for语句

string[] orders = {"B123","C234","A345","C15","B177","G3003","C235","B179"};

foreach (string order in orders){
    if(order[0] =='B'){
        Console.WriteLine(order);
    }
}


for (int counter = 0; counter < 10; counter++) 
{ 
    Console.WriteLine($"Hello World! The counter is {counter}"); 
}

//第二种声明方式
string[] fraudulentOrderIDs = new string[3];
fraudulentOrderIDs[0] = "A123";
fraudulentOrderIDs[1] = "B456";
fraudulentOrderIDs[2] = "C789";

标识符名称 - 规则和约定 - C# | Microsoft Learn

while语句

int counter = 0;
while (counter < 10)
{
  Console.WriteLine($"Hello World! The counter is {counter}");
  counter++;
}

int counter = 0;
do
{
  Console.WriteLine($"Hello World! The counter is {counter}");
  counter++;
} while (counter < 0);

【C#】【微软】【入门第二部门】