下拉选择不触发更改

我正在处理一个项目，我正在进行弹出操作，在弹出操作中，当我弹出时，网格中的数据会填充到文本框/下拉列表中，但实际情况是，我有一个类别下拉列表和另一个作为子类别的下拉列表，我想要的是，当我选择类别时，与类别相关的子类别会被填写在子类别下拉列表中，但发生的是所有的子类别都会自动填写。

<table class="panel_table">
<tr style="background-color:#0095E4">
<td colspan="2"><span class="panel_heading1">Category Details</span></td></tr>
<tr class="row1">
<td  colspan="2">
<span  class="paneltb_column1">BookID--</span>
<asp:Label ID="lblEditbookID" runat="server" Text="book ID"></asp:Label></td>
</tr>
<tr class="row1" >
<td colspan="2">
<span  class="paneltb_column2">Book Name</span>
<asp:TextBox CssClass="txtbox1"  ID="txtEditbookname" runat="server"></asp:TextBox>
</td></tr>
<tr class="row1" >
<td colspan="2">
<span  class="paneltb_column2">Category Name</span> 
<asp:DropDownList ID="drp_editcatname" runat="server" Width="120px"
   AutoPostBack="true"        OnSelectedIndexChanged="drpeditgetvaluesubcategory_onselectinindexchange">       </asp:DropDownList>
 </td></tr>
 <tr class="row1">
 <td colspan="2">
 <span  class="paneltb_column2">SubCategory Name</span>
 <asp:DropDownList ID="drpEditsubcatname" CssClass="txtbox2" 
  runat="server">               </asp:DropDownList>
 </td>
 </tr>
 <tr class="row1" >
 <td colspan="2">
 <span  class="paneltb_column2">Author Name</span>
 <asp:DropDownList ID="drp_editauthorname" runat="server"></asp:DropDownList>
 </td>
 </tr>
  </table> 

背后的代码

 protected void drpgetvaluesubcategory_onselectedindexchange(object sender,EventArgs e)
 {
   sql = "select subcat_id,subcategoryname from library_subcategory where    cat_id='"+drp_categoryname.SelectedItem.Value+"'";
   ds = obj.openDataset(sql, Session["SCHOOLCODE"].ToString());
    drp_subcategoryname.Items.Clear();  
    ListItem li = new ListItem();
    li.Text = "Select SubCategory";
    li.Value = "0";
    drp_subcategoryname.Items.Add(li);
    for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
    {
        li = new ListItem();
        li.Text = ds.Tables[0].Rows[i]["subcategoryname"].ToString();
        li.Value = ds.Tables[0].Rows[i]["subcat_id"].ToString();
        drp_subcategoryname.Items.Add(li);
    }
}

 OnSelectedIndexChanged="drpeditgetvaluesubcategory_
     onselectinindexchange"

 OnSelectedIndexChanged="drpgetvaluesubcategory_onselectedindexchange"