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;
        }

------解决方案--------------------
十进制转换二进制string j = Convert.ToString(i, 2);
至于置0，你可以用Replace函数

------解决方案--------------------
C# code
        int value = 100;
        string r = Convert.ToString(value, 2);
        Response.Write(r);

------解决方案--------------------

int x = 100; // 64(16) 1100100
如果想置第2位为1，则
x |= 0x0000010;
置0，则
x &= 0x1111011;

------解决方案--------------------

C# code
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;
        }
    }
}

------解决方案--------------------

可以用扩展方法，读起来比较自然。
C# code