How to populate dropdown list value in edit

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VS2008

VB

Hi,
I've 2 dropdowns 1 is site and another is warehouse. Warehouse dropdown value will be changed based on site dropdown.

I done the insert for that dropdowns but when I try to edit the dropdowns already existing value need to be shown and dropdown value should change based on the site edit control..

How could i do the edit for this 2 controls?
I couldn't find a perfect example for this, can anyone help me in this?

What I have tried:

This is my insert template:

<asp:DropDownList ID= "TxtsiteInsert" runat="server" AutoPostBack="True" OnSelectedIndexChanged="sitedropdown_SelectedIndexChanged"  TabIndex="6">
                                                                           <asp:ListItem Text="Select Site"></asp:ListItem>
                                                                        </asp:DropDownList>
                                                                   
                                                                    </InsertItemTemplate>

<InsertItemTemplate>


                                                                      <asp:DropDownList ID= "TxtWarehouseInsert" runat="server" AutoPostBack="True"  TabIndex="7" >
                                                                          <asp:ListItem Text="Select warehouse"></asp:ListItem>
                                                                       </asp:DropDownList>

                                                                   </InsertItemTemplate>

This is my coding:

Protected Sub sitedropdown_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)

       Dim IdTxtsiteInsert As DropDownList = CType(DetailsView3.Rows(0).FindControl("TxtsiteInsert"), DropDownList)

       Fillwarehouse(IdTxtsiteInsert.Text)

   End Sub

Public Sub Fillwarehouse(ByVal siteid As String)
       'Dim IdTxtWarehouseInsert As DropDownList = CType(DetailsView3.Rows(0).FindControl("TxtWarehouseInsert"), DropDownList)
       Dim ddr1 As DropDownList = CType(DetailsView3.Rows(0).FindControl("TxtWarehouseInsert"), DropDownList)
       Dim strConn As String = WebConfigurationManager.ConnectionStrings("BMGINC_DYNAX09_ProdConnectionString").ConnectionString
       Dim con As New SqlConnection(strConn)
       Dim cmd As New SqlCommand()
       cmd.Connection = con
       cmd.CommandType = CommandType.Text
       cmd.CommandText = "Select InventLocationId, Name from InventLocation where dataareaid='BRM' and InventSiteId =@InventSiteId"
       cmd.Parameters.AddWithValue("@InventSiteId", siteid)
       Dim objDs As New DataSet()
       Dim dAdapter As New SqlDataAdapter()
       dAdapter.SelectCommand = cmd
       con.Open()
       dAdapter.Fill(objDs)
       con.Close()
       If objDs.Tables(0).Rows.Count > 0 Then
           ddr1.DataSource = objDs.Tables(0)
           ddr1.DataTextField = "InventLocationId"
           ddr1.DataValueField = "InventLocationId"
           ddr1.DataBind()
           ddr1.Items.Insert(0, "-Select warehouse-")

       Else
           ddr1.DataTextField = " "
           ddr1.DataValueField = " "
           ddr1.DataBind()
           ddr1.Items.Insert(0, " ")
           LblError.Text = "No Warehouse found"

       End If

   End Sub