C#.求 1+2!+3!+...+20!的和

C#代码如下所示：

using System;

namespace FactorialSum
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = 20;
            long sum = 0;
            long factorial = 1;
            for (int i = 1; i <= n; i++)
            {
                factorial *= i;
                sum += factorial;
            }
            Console.WriteLine(sum);
        }
    }
}