如何解决ComboBox控件的“设置 DataSource 属性后无法修改项集合”问题

　　初次用.Net写Form程序，遇见了这个恶心的问题。自己的一点心得，写出来给初学的朋友参考，也希望初学的朋友能够少走弯路。

　　问题描述：

　　我想实现多个下拉列表联动的功能，例如有三个下拉列表A、B、C，当选择了下拉列表A中的数据后，下拉列表B中的数据发生相应的变化，选择下拉列表的B时下拉列表C的数据发生相应的变化。当然是采用ComboBox的DataSource属性绑定数据啦。结果当我选择下拉列表A的时候，系统抛出异常“设置 DataSource 属性后无法修改项集合”。

 

　　解决问题：

　　于是苦思冥想，上网查找资料，结果都没有满意的答案。最后还是查看帮助手册，也没有明确的答案。但是我从手册上看到这样一句话“实现 IList 接口的对象，如 DataSet 或 Array。默认为空引用（在 Visual Basic 中为 Nothing）。 ”，请注意我标红的那句话。灵感突现，再清除下拉列表项的时候首先将ComboBox.DataSource付空值“null”，问题果然就迎刃而解了。

 

　　我的代码：代码中标红的部分就是出问题的部分。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Collections;

using HRGJ.Logic;

namespace HRGJ
{
    public partial class MemberForm : Form
    {

        #region Private Variables
        private HrMemberBasic parentNode = new HrMemberBasic();
        private HrMemberBasic recommenderNode = new HrMemberBasic();
        private DataTable areaList = null;
        #endregion

        #region Public Properties
        public HrMemberBasic ParentNode
        {
            get { return parentNode; }
            set { parentNode = value; }
        }
        public HrMemberBasic RecommenderNode
        {
            get { return recommenderNode; }
            set { recommenderNode = value; }
        }
        #endregion

        public MemberForm()
        {
            InitializeComponent();
        }

        private void MemberForm_Load(object sender, EventArgs e)
        {
            if (this.areaList == null)
            {
                this.areaList = SysDic.GetAllByListCode("Area");
            }
            this.BindComboMembers();
            this.BindCredentials();
            this.BindArea(this.Combo_Country, 0);
            //int countryID = (int)this.Combo_Country.SelectedValue;
            //this.BindArea(this.Combo_Province, countryID);
            //int provinceID = (int)this.Combo_City.SelectedValue;
            //this.BindArea(this.Combo_City, provinceID);
        }

        #region protected Method
        protected void BindComboMembers()
        {
            ArrayList itemList = new ArrayList();
            for (int i = 1; i <= SysParameter.MaxNodeMembers; i++)
            {
                MyComboItem mItem = new MyComboItem();
                mItem.ItemText = i.ToString() + "个会员资格";
                mItem.ItemValue = i;
                itemList.Add(mItem);
            }
            this.Combo_Members.DataSource = itemList;
            this.Combo_Members.DisplayMember = "ItemText";
            this.Combo_Members.ValueMember = "ItemValue";
        }

        protected void BindCredentials()
        {
            DataTable dt = SysDic.GetAllByListCode("Credentials");
            this.Combo_Credentials.DataSource = dt;
            this.Combo_Credentials.DisplayMember = "DicName";
            this.Combo_Credentials.ValueMember = "DicID";
        }

        /// <summary>
        /// 绑定区域下拉列表
        /// </summary>
        /// <param name="parentID"></param>
        protected void BindArea(ComboBox combo_Area,int parentID)
        {
            if (combo_Area.Items.Count > 0)
            {
                combo_Area.DataSource = null;
                combo_Area.Items.Clear();
            }
            DataRow[] dList = areaList.Select("ParentID = " + parentID.ToString());
            if (dList.Length>0)
            {
                ArrayList da = new ArrayList();
                foreach (DataRow dr in dList)
                {
                    SysDic dic = new SysDic(dr);
                    da.Add(dic);
                }
                combo_Area.DisplayMember = "DicName";
                combo_Area.ValueMember = "DicID";
                combo_Area.DataSource = da;
            }
        }

       
        #endregion

        private void Combo_Country_SelectionChangeCommitted(object sender, EventArgs e)
        {
            int countryId = (int)this.Combo_Country.SelectedValue;
            this.BindArea(this.Combo_Province, countryId);
        }

        private void Combo_Province_SelectionChangeCommitted(object sender, EventArgs e)
        {
            int provinceID = (int)this.Combo_Province.SelectedValue;
            this.BindArea(this.Combo_City, provinceID);
        }

    }
}