WebPart 中DropDownList 丟失狀態 的解決方案

最近比較忙,忙的都沒時間更新部落格了.

所以今天要一口氣寫兩篇.

WebPart  中DropDownList 丟失狀態 的解決方案,十分頭痛.

現象就是DropDownList老是為空白.

當時有些暈，上網搜了好久，要麼沒答案，要麼含糊其詞。

當有些網友說用HiddenField 儲存狀態給我一些啟發，於是我朝HiddenField的方向去想。

原理：HiddenField的是可以直接反映到用戶端的且狀態值不丟失，用HiddenField的值記錄DropDownList的ClientID.
而DropDownList在用戶端反映為ID為ClientID的Select域的Form成員。然後當提交時在伺服器端獲得ID為ClientID的Select域的Form成員
的值即為DropDownList的選中值。

1、用HiddenField的值記錄DropDownList的ClientID
     private DropDownList DlistMember = new DropDownList();
        private TextBox txtCNName = new TextBox();
        private Button btnOnboard = new Button();
        private Label lblMessage = new Label();
        private HiddenField hiddenFiled = new HiddenField();      
       
       protected override void Render(System.Web.UI.HtmlTextWriter writer)
        {
                //填充 DlistMember  的資料
                DlistMember.RenderControl(writer)
                btnOnboard.Click += new EventHandler(btnOnboard_Click);
                txtCNName.RenderControl(writer);
                txtCNName.AutoPostBack = false;
                btnOnboard.RenderControl(writer);
                lblMessage.RenderControl(writer); 
                hiddenFiled.Value = DlistMember.ClientID;
                hiddenFiled.RenderControl(writer);
      }  
      protected override void CreateChildControls()//為了加上btnOnboard的Click事件
        {
            btnOnboard.Click += new System.EventHandler(this.btnOnboard_Click);
            DlistMember.EnableViewState = true;
            //DlistMember.EnableTheming = true;
            //DlistMember.AutoPostBack = true;
            this.Controls.Add(DlistMember);
            this.Controls.Add(btnOnboard);
            this.Controls.Add(hiddenFiled);
        }
2、事件處理
    protected void btnOnboard_Click(object sender, EventArgs e)// process btnOnboard click
        {
              string strValue="";
              strValue = hiddenFiled.Value.ToString().Replace("$","_");
              for (int i = 0; i < this.Page.Request.Form.Count; i++)//因為$和_的混淆，這裡用遍曆Form的域來找到需要的域
                    {
                        string strTemp = this.Page.Request.Form.Keys[i].ToString().Replace("$", "_");
                        if (strTemp == strValue)
                        {
                            strValue = this.Page.Request.Form[i].ToString();
                            isFindFrom = true;
                            break;
                        }
                    }
         }