函数1

    class Program
    {
        //格式一：没有返回值，没有参数
        public void qiuhe()
        {
            Console.Write("请输入一个数：");
            int a = int.Parse(Console.ReadLine());
            int sum = 0;
            for (int i = 1; i <= a; i++)
            {
                sum += i;
            }
            Console.WriteLine(sum);
        }
        //格式二：没有返回值，有参数
        public void qiuhe(int a)
        {
            int sum = 0;
            for (int i = 1; i <= a; i++)
            {
                sum += i;
            }
            Console.WriteLine(sum);
        }
        //格式三：有返回值，但无参数
        public int qiuhe0()
        {
            Console.Write("请输入一个数：");
            int a = int.Parse(Console.ReadLine());
            int sum = 0;
            for (int i = 1; i <= a; i++)
            {
                sum += i;
            }
            return sum;
        }
        //格式四：有返回值，且有参数
        public int qiuhe1(int a)
        {
            int sum = 0;
            for (int i = 1; i <= a; i++)
            {
                sum += i;
            }
            return sum;
        }
        static void Main(string[] args)
        {
            Program function = new Program(); //初始化这个类，是为了调用这个类里面的函数
            function.qiuhe(); //函数只有调用才会执行。无返回值，无参数    （调用格式一）
            Console.Write("请输入一个数：");
            int a = int.Parse(Console.ReadLine());
            function.qiuhe(a); //无返回值，有参数    （调用格式二）
            int sum0 = function.qiuhe0(); //无返回值，无参数    （调用格式三）
            Console.WriteLine(sum0);
            int sum1 = function.qiuhe1(a); //有返回值，有参数     （调用格式四）
            Console.WriteLine(sum1);
            Console.ReadLine();            
        }

    class Program
    {
        /// <summary>
        /// 两个值进行比较，返回较大值，若相等返回任意一个。
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <returns></returns>
        public double max(double a, double b)
        {
            if (a > b)
                return a;
            else
                return b;
        }
        static void Main(string[] args)
        {
            Program function = new Program();
            double a = 4, b = 5, c = 3;
            Console.WriteLine(function.max(function.max(a, b), c));
            Console.ReadLine();
        }

        static void Main(string[] args)
        {
            Program method = new Program();
            method.sum();//格式一
            Console.WriteLine();
            Console.Write("请输入：");//格式二
            int num = int.Parse(Console.ReadLine());
            method.sum(num);
            Console.WriteLine();
            int sum = method.Sum();//格式三
            Console.WriteLine(sum);
            Console.WriteLine();
            Console.Write("请输入：");//格式四
            int Num = int.Parse(Console.ReadLine());
            int Sum = method.Sum(Num);
            Console.WriteLine(Sum);
            Console.ReadLine();
        }
        public void sum()//格式一
        {
            Console.Write("请输入：");
            int num = int.Parse(Console.ReadLine());
            int sum = 0;
            for (int i = 1; i <= num; i++)
            {
                int factorial = 1;
                for (int j = 1; j <= i; j++)
                {
                    factorial *= j;
                }
                sum += factorial;
            }
            Console.WriteLine(sum);
        }
        public void sum(int num)//格式二
        {
            int sum = 0;
            for (int i = 1; i <= num; i++)
            {
                int factorial = 1;
                for (int j = 1; j <= i; j++)
                {
                    factorial *= j;
                }
                sum += factorial;
            }
            Console.WriteLine(sum);
        }
        public int Sum()//格式三
        {
            Console.Write("请输入：");
            int num = int.Parse(Console.ReadLine());
            int sum = 0;
            for (int i = 1; i <= num; i++)
            {
                int factorial = 1;
                for (int j = 1; j <= i; j++)
                {
                    factorial *= j;
                }
                sum += factorial;
            }
            return sum;
        }
        public int Sum(int num)//格式四
        {
            int sum = 0;
            for (int i = 1; i <= num; i++)
            {
                int factorial = 1;
                for (int j = 1; j <= i; j++)
                {
                    factorial *= j;
                }
                sum += factorial;
            }
            return sum;
        }