Need help with hide/unhide Headertext

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VB

ASP.NET

I have a DetailsView with a list of Headertext. The PERM CODE headertext is hidden and will only be displayed when the DROPDOWN option choosen is DETAIL.

Here is my aspx and vb code behind which is not working.
ASPX code:

<asp:TemplateField HeaderText = "DATAITEM:">
    <itemtemplate>
      <asp:Label ID="LblDataItem" runat="server" 
               Text= '<%# eval("dataitem") %>' />
    </itemtemplate>
    <edititemtemplate>
       <asp:DropDownList ID="DDLDataItemnew" runat="server"
          AutoPostBack ="true"
         OnSelectedIndexChanged = "DDLDataItemnew_selectedindexchanged"
        >
         <asp:ListItem>Choose One
         <asp:ListItem>DETAIL
         <asp:ListItem>SUMMARY
         <asp:ListItem>CUSTOM         
       </edititemtemplate>
<asp:TemplateField HeaderText = "PERM CODE:" Visible="false" >
    <edititemtemplate>
        <asp:DropDownList ID="DDLPermCode" runat="server"  />
        </edititemtemplate>

VB code behind

Protected Sub DDLDataItemnew_selectedindexchanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim tmp_dataitem As DropDownList = DirectCast(DTLVwAddeduser.FindControl("DDLDataItemnew"), DropDownList)
        Dim dataitem As String = Trim(tmp_dataitem.Text)
        If dataitem = "DETAIL" Then
            If DTLVwAddeduser.CurrentMode = DetailsViewMode.Edit Then
                //get the index of the DATAITEM headertext
                Dim i As Integer = GetIndex("DATAITEM:",DTLVwAddeduser) 
                DTLVwAddeduser.Fields(i).Visible = True
            End If
        End If
End Sub

Can someone please assist?