How To Use A Datalist Control To Find Its Child Datalist In C#

how do I use the datalist "DL_Pro_Result" to find the child datalist "DL_Gro_Result" in C#?

For example in the following code, dlii value is null, even though dli != null.

DataList dli = (DataList)Page.FindControl("DL_Pro_Result");
DataList dlii = (DataList)dli.FindControl("DL_Gro_Result");
<div id="ProList">
<asp:DataList ID="DL_Pro_Result" runat="server">
<HeaderTemplate>
<table id="T_Pro_Result_Header" runat="server">
<tr>
<td>
<asp:Label ID="L_Pro_Result_Header" runat="server"></asp:Label>
</td>
</tr>
</table>
</HeaderTemplate>
<ItemTemplate>
<table id="T_Pro_Result_Item" class="table" runat="server">
<tr>
<td>
<asp:Label ID="L_Pro_Result_Item" runat="server"></asp:Label>
<asp:Button ID="B_Pro_Result_Item_1" OnClick="B_Pro_Result_Item_1_Click"/>
</td>
</tr>
<tr>
<td>
<asp:DataList ID="DL_Gro_Result" runat="server">