int c=5;
string d = Convert.ToString(c,2).PadLeft(4,'0');

   static void Main(string[] args)
        {
            int[] a = erjinzhi(2021);
            for (int i = 0; i < a.Length; ++i)
            {
                Console.Write(a[i]);
            }
 
        }
        public static int[] erjinzhi(double d)//輸入一個(gè)數(shù)字，將數(shù)字轉(zhuǎn)成二進(jìn)制
        {
            int[] gen = new int[12];//10個(gè)長(zhǎng)度
 
            for (int i = 0; d > 0; ++i)//商=0時(shí)候停止
            {
 
                gen[i] = (int)d % 2;//將余數(shù)轉(zhuǎn)為int類型，存到gen[]數(shù)組中
                d = d / 2;//老商/2作為新被除數(shù)
                d = Math.Floor(d);//向下取整
            }
            Array.Reverse(gen);//將數(shù)組倒敘就得到二進(jìn)制了
            return gen;
        }