第一次写，还不知道怎么下手，望大家给批评批评！
在服务器端用一个文本框来显示当前所有的选择项，以","进行分割，ViewState["ZYKC"]用来保存所有的选择CheckBox的值

protected void cbl_SubjectList_SelectedIndexChanged(object sender, EventArgs e)
    {
        string sOld = ViewState["ZYKC"].ToString();
        string sNew = "";
        foreach (ListItem eachItem in cbl_SubjectList.Items)
        {
            if (eachItem.Selected)
            {
                if (sNew.Length > 0)
                {
                    sNew = sNew + "," + eachItem.Text;
                }
                else
                {
                    sNew = eachItem.Text;
                }
            }
        }
        ViewState["ZYKC"] = sNew;
        if (sOld.Length < sNew.Length)//当前所做操作为选择
        {
            if (tb_KCMC.Text.Length > 0)
            {
                tb_KCMC.Text += "," + this.getDifferentStr(sNew, sOld);
            }
            else
            {
                tb_KCMC.Text = this.getDifferentStr(sNew, sOld);
            }
        }
        else  //当前所做操作为取消       
      {
            string str = this.getDifferentStr(sOld, sNew);
            tb_KCMC.Text = this.getSelectByOrder(tb_KCMC.Text, str);
        }
    }

//获得当前操作所选择的CheckBox的值

        private string getDifferentStr(string sOld, string sNew)
    {
        System.Text.RegularExpressions.Regex reg = new System.Text.RegularExpressions.Regex(",");
        string[] arr_old = reg.Split(sOld);
        string[] arr_new = reg.Split(sNew);
        for (int i = 0; i < arr_old.Length; i++)
        {
            int flag =0;//存在标志
            for (int j = 0; j < arr_new.Length; j++)
            {
                if (arr_new[j].Equals(arr_old[i]))
                {
                    flag = 1;
                    break;
                }
            }
            if (flag.Equals(0))
            {
                return arr_old[i];
            }
        }
        return "";
    }

    //如果当前所做的操作是取消一个CheckBox的选择，把值取消，保持顺序不变

    private string getSelectByOrder(string sOld, string currentSelect)
    {
        if (!sOld.Equals(currentSelect))
        {
            System.Text.RegularExpressions.Regex reg = new System.Text.RegularExpressions.Regex(",");
            string[] arr_old = reg.Split(sOld);
            int index = 0;
            string str_SelectByOrder = "";
            for (int i = 0; i < arr_old.Length; i++)
            {
                if (!arr_old[i].Equals(currentSelect))
                {
                    if (str_SelectByOrder.Length > 0)
                    {
                        str_SelectByOrder += "," + arr_old[i];
                    }
                    else
                    {
                        str_SelectByOrder = arr_old[i];
                    }
                }
            }
            return str_SelectByOrder;
         }
        else
        {
            return "";
        }
    }