按顺序填写下拉列表，并将每个下拉列表设置在其第一个值之前

在我的项目中，我正在制作一个包含三个下拉列表的表单，每个下拉列表用于公司，部门和空缺"选择"作为默认文本。我感觉公司下拉列表在表单加载事件，但默认文本是"选择"现在当用户选择公司，第二个列表应该填写选定公司的部门，但默认文本应该是"选择"。同样，当用户选择部门时，第三个列表应该填充所选公司所选部门的空缺，但默认文本应该是"select"。我正在尝试将另外两个下拉列表分别填入公司列表和部门列表的"selected index change"事件。但是它没有像我想的那样工作。

这是我的代码。aspx页。

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <table width="100%">
        <tr>
            <td colspan="4">
                &nbsp;</td>
        </tr>
        <tr>
            <td align="center" class="tdtitle" colspan="4">
                Search Candidates
            </td>
        </tr>
        <tr>
            <td colspan="4">
                &nbsp;</td>
        </tr>
        <tr>
            <td class="RowHeight" width="20%">
                Select Company</td>
            <td width="30%">
                <asp:DropDownList ID="companyList" runat="server" AutoPostBack="True" 
                 onselectedindexchanged="companyList_SelectedIndexChanged" Width="150px">
                <asp:ListItem>-Select Company-</asp:ListItem></asp:DropDownList>
            </td>
            <td width="20%">
                Select Department</td>
            <td width="30%">
                <asp:DropDownList ID="deptList" runat="server" AutoPostBack="True" 
                    Width="150px" onselectedindexchanged="deptList_SelectedIndexChanged" onclick="Validate();">
                <asp:ListItem>-Select Department-</asp:ListItem></asp:DropDownList>
            </td>
        </tr>
        <tr>
            <td class="RowHeight" width="20%">
                Select Vacancy</td>
            <td colspan="3" width="*">
                <asp:DropDownList ID="vacanyList" runat="server" Width="200px">
                <asp:ListItem>-Select Vacancy-</asp:ListItem></asp:DropDownList>
            </td>
        </tr>
        <tr>
            <td colspan="4">
                &nbsp;</td>
        </tr>
        <tr>
            <td colspan="4">
                &nbsp;</td>
        </tr>
        <tr>
            <td colspan="4">
                &nbsp;</td>
        </tr>
        <tr>
            <td colspan="4">
                &nbsp;</td>
        </tr>
        <tr>
            <td align="center" colspan="4">
                <asp:Label ID="notifyLbl" runat="server" Font-Size="Large" ForeColor="Red" 
                    Text="Label"></asp:Label>
            </td>
        </tr>
        <tr>
            <td colspan="4">
                &nbsp;</td>
        </tr>
    </table>
 <script type="text/javascript">
     function alertOnBadSelection() {
         var select = document.getElementById('companyList');
         if (select.options[select.selectedIndex].value == "-Select Company-") {
             alert('Please Select Company!');
             return false;
         }
     }
    </script>
</asp:Content>

下面是我的。aspx.cs页面的代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
public partial class HR_Department_searcAppForVac : System.Web.UI.Page
{
    DataOperation oDo = new DataOperation();
    protected void Page_Load(object sender, EventArgs e)
    {
       // deptList.Attributes.Add("onchange", "alertOnBadSelection();");
        notifyLbl.Visible = false;
        if (!IsPostBack)
        {
            try
            {
                DataTable objCmpnyTable = oDo.DropDownList("select * from tblCompanyMaster");
                if (objCmpnyTable.Rows.Count > 0)
                {
                    foreach (DataRow Row in objCmpnyTable.Rows)
                    {
                        companyList.Items.Add(new ListItem(Row["CompName"].ToString(), Row["CompId"].ToString()));
                    }
                }
                else
                {
                    notifyLbl.Visible = true;
                    notifyLbl.Text = "There is not any company in the list.";
                }
            }
            catch (Exception)
            {
                throw;
            }
        }
        else
        {
            //deptList.SelectedIndex = -1;
            //vacanyList.SelectedIndex = 1;
        }
    }
    protected void companyList_SelectedIndexChanged(object sender, EventArgs e)
    {
        try
        {
            if (companyList.SelectedIndex > 0)
            {
                deptList.Items.Clear();
                string str = "select * from vwCompWiseList where CompId=" + companyList.SelectedValue;
                DataTable objDeptTable = oDo.DropDownList("select DeptId,DeptName from vwCompWiseDept where CompId= "+companyList.SelectedValue);
                if (objDeptTable.Rows.Count > 0)
                {
                    foreach (DataRow Row in objDeptTable.Rows)
                    {
                        deptList.Items.Add(new ListItem(Row["DeptName"].ToString(), Row["DeptId"].ToString()));
                    }
                }
            }
            else
            {
                notifyLbl.Visible = true;
                notifyLbl.Text = "Select Company....";
            }
        }
        catch (Exception)
        {
            throw;
        }    
    }
    protected void deptList_SelectedIndexChanged(object sender, EventArgs e)
    {
        try
        {
            if (deptList.SelectedIndex > 0)
            {
                vacanyList.Items.Clear();
                DataTable objVacancytbl = oDo.DropDownList("select VacId,VacTitle from tblVacancyMaster where DeptId =" + deptList.SelectedValue + " and CompId=" + companyList.SelectedValue);
                if (objVacancytbl.Rows.Count > 0)
                {
                    foreach (DataRow Row in objVacancytbl.Rows)
                    {
                        vacanyList.Items.Add(new ListItem(Row["VacTitle"].ToString(), Row["VacId"].ToString()));
                    }
                    vacanyList.SelectedIndex = -1;
                    vacanyList.ClearSelection();
                }
                else
                {
                    notifyLbl.Visible = true;
                    notifyLbl.Text = "Ops..!There is no available vacancy....";
                }
            }
            else
            {
                notifyLbl.Visible = true;
                notifyLbl.Text = "Select Department...";
            }
        }
        catch (Exception)
        {
            throw;
        }
    }
}

请展示我的问题和解决方法