如何在一个页面上使用两个服务器控件？

Question

我已经创建了一个服务器控件并在页面上调用了两次。

下面是代码

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Web;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;

namespace DNWebComponent {
    [DefaultProperty("Text")]
    [ToolboxData("<{0}:DNdropdown runat=server></{0}:DNdropdown>")]
    public class DNdropdown : WebControl, IDNComponent {

        private string _label;
        private TextBox _txtBox;
        public String _ConnectedField;
        private string _connectedField;
        private string _MultiSeekField;
        private List<String> _selectedValue;
        private string _InputTable;
        private CheckBoxList _Checkbox;
        Panel div;

        public String ConnectedField {
            get { return _connectedField; }
            set { _connectedField = value; }
        }
        public DNdropdown(string label)
            : this() {
            this._label = label;
        }
        public String Label {
            get { return _label; }
            set { _label = value; }
        }
        public DNdropdown() {
            _selectedValue = new List<string>();
            _txtBox = new TextBox();
            _Checkbox = new CheckBoxList();
            div = new Panel();
        }

        public String InputTable {
            get { return _InputTable; }
            set { _InputTable = value; }
        }
        public String MultiSeekField {
            get { return _MultiSeekField; }
            set { _MultiSeekField = value; }
        }
        public CheckBoxList AspChekcBoxes {
            get { return _Checkbox; }
            set { _Checkbox = value; }
        }
        public List<String> SelectedValue {

            get {
                foreach (ListItem item in _Checkbox.Items) {

                    if (item.Selected)
                        _selectedValue.Add(item.Value);
                }
                return _selectedValue;
            }
            set {
                if (value != null) {
                    _selectedValue = value;
                }
            }
        }
        public string DivCss { get; set; }

        protected List<String> GetPropertyValue<v>(string SelectedValue, List<String> nullValue) {
            if (ViewState[SelectedValue] == null) {
                return nullValue;
            }
            return (List<String>)ViewState[SelectedValue];
        }

        protected override void OnInit(EventArgs e) {
            div = new Panel { CssClass = DivCss };
            FillControl();
            div.Controls.Add(new Literal { Text = Label + ": " });
            Panel dvTxtBox = new Panel { CssClass = "dt" };
            _txtBox.Text = "Select";
            _txtBox.ReadOnly = true;
            dvTxtBox.Controls.Add(_txtBox);
            Panel dvChkBox = new Panel { CssClass = "divco" };
            dvTxtBox.Style.Add("float", "right;");
            dvTxtBox.Style.Add("margin-right", "700px;");
            dvChkBox.Style.Add("margin-right", "700px;");
            dvChkBox.Style.Add("float", "right;");
            dvChkBox.Controls.Add(_Checkbox);
            div.Controls.Add(dvTxtBox);
            div.Controls.Add(dvChkBox);
            Controls.Add(div);
        }
        protected override void RenderContents(HtmlTextWriter output) {
            div.RenderControl(output);
        }
        public bool FillControl() {
            _Checkbox.Items.Add("a");
            _Checkbox.Items.Add("b");
            return true;
        }

    }
}





     <asp:PlaceHolder ID="Place1" runat="server">
            <div id="DivDrop" runat="server">
            </div>
            <div id="divbtn">
           <div><cc:DNdropdown ID="dropNew" runat="server" Label="DropDown" />
           <cc:DNdropdown  ID="newcontrol" runat="server"  Label="New Dropdown"/></div>
            <div ><asp:Button ID="btnSave" Text="Save" runat="server" OnClick="btnSave_Clicked" /></div> </div>


        </asp:PlaceHolder>

我在上面的Sample.aspx页面中注册了这个控件，我在那里使用这个控件.dropNew控件工作正常，但每当我向Sample.aspx页面添加一个新控件时问题开始。我点击一个下拉列表第二个自动打开