1. 声明两个变量：int n1 = 10, n2 = 20;要求将两个变量交换，最后输出n1为20,n2为10。扩展（*）：不使用第三个变量如何交换？

int n1 = 10;
int n2 = 20;
int temp;

temp = n1;
n1 = n2;
n2 = temp;

▲ 借用中间量

int n1 = 10;
int n2 = 20;


n1 = n1 + n2;
n2 = n1 - n2;
n1 = n1 - n2;

▲ 不用中间量

2. 用方法来实现：将上题封装一个方法来做，方法有两个参数分别为num1,num2,将num1与num2交换。提示：方法有两个参数n1,n2,在方法中将n1与n2进行交换，使用ref。（*）

static void Main(string[] args)
{
    int n1 = 10;
    int n2 = 20;

    Console.WriteLine("交换前：n1 = {0}, n2 = {1}", n1, n2);


    SwapNum<int>(ref n1, ref n2);

    Console.WriteLine("交换前：n1 = {0}, n2 = {1}", n1, n2);
    Console.ReadKey();
}

static void SwapNum<T> (ref T a, ref T b)
{
    T temp;
    
    temp = a;
    a = b;
    b = temp;
}

运行结果：

交换前：n1 = 10, n2 = 20
交换前：n1 = 20, n2 = 10

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参考：

1.link-01