这是在控制台用的，不过一样可以应用的网页。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace JinZhiTest
{
    public partial class Form1 : Form
    {
        private int CurrBase = 10;//当前进制，开始默认为10
        public Form1()
        {
            InitializeComponent();
        }
        //转换成二进制
        private void rdoEr_Click(object sender, EventArgs e)
        {
            this.JinZhiZhuHuan(CurrBase, 2);
        }
        //转换成八进制
        private void rdoBa_Click(object sender, EventArgs e)
        {
            this.JinZhiZhuHuan(CurrBase, 8);
        }
        //转换成十进制
        private void rdoShi_Click(object sender, EventArgs e)
        {
            this.JinZhiZhuHuan(CurrBase, 10);
        }
        //转换成十六进制
        private void rdoShiLiu_Click(object sender, EventArgs e)
        {
            this.JinZhiZhuHuan(CurrBase, 16);
        }
        /// <summary>
        /// 进制转换
        /// </summary>
        /// <param name="fromBase">原进制</param>
        /// <param name="toBase">目标进制</param>
        private void JinZhiZhuHuan(int fromBase, int toBase)
        {
            //得到文本框中数的十进制
            int num = Convert.ToInt32(this.textBox1.Text.Trim(), fromBase);
            //将数转换成目标进制的字符串形式并显示在文本框中
            this.textBox1.Text = Convert.ToString(num, toBase);
            //将目标进制保存为当前进制
            this.CurrBase = toBase;
        }
    }
}
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前台？

class Program
{
    static void Main(string[] args)
    {
        int shi;//用来保存十进制数
        Console.WriteLine("******** 十进制转二进制 ********\n");
        Console.Write("请输入十进制整数：");
        //从控制台读取十进制整数
        shi = Convert.ToInt32(Console.ReadLine());
        //将输入的十进制数转换为二进制字符串
        string er = Convert.ToString(shi, 2);
        //输出二进制字符串
        Console.WriteLine("二进制为："+er);
        Console.ReadLine();
    }
}