Add list items to dropdownlist in a formview with vb.net

[iamstuart]

I have a dropdownlist in a formview that looks like this:

<asp:DropDownList ID="DropDownList5" runat="server" AppendDataBoundItems="True" 
DataSourceID="SqlDataSource6" DataTextField="StaffName" 
DataValueField="StaffName" SelectedValue='<%# Bind("LetterName") %>' 
Width="155px">
<asp:ListItem Value=""> </asp:ListItem>
</asp:DropDownList>

I am trying to add the value from a textbox which is also in the formview into the dropdownlist as a list item. I currently have this code for it:

    Protected Sub Formview1_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles FormView1.PreRender
 
        Dim dropdown As DropDownList
        Dim listvalue As TextBox
        dropdown = FormView1.FindControl("dropdownlist5")
        listvalue = FormView1.FindControl("textbox1")
 
        dropdown.Items.Add(New ListItem(listvalue.Text, listvalue.Text))
    End Sub

This code works and adds the new item to the dropdownlist if I have no SelectedValue set for my dropdownlist. However when I set the selectedValue I get the error message:
"'DropDownList5' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value"
This is the bit I dont understand because the value does now exist in the list of items as it was added in from the textbox value. Does it try to do the selectedValue before adding my list item? If so how can I add the list item first, or do I need to set the selectedValue in the code after adding the list item?

[aspdotnetuser]

Just use this example to fit to your requirement

Dim ddlValue As New ListItem

ddlValue.Text = "Value Visible in Dropdown"
ddlValue.Value = "Value on Selection"

DropDownList1.Items.Add(ddlValue)

[iamstuart]

This code does not work any better, I still get the error:
'DropDownList5' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value
As it is still trying to get the SelectedValue before adding the list item

[iamstuart]

I now have this working by removing the SelectedValue from the dropdownlist and setting the SelectedValue to the textbox.text in the VB code after adding the new value to the dropdownlist.

    Protected Sub Formview1_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles FormView1.PreRender
 
        Dim dropdown As DropDownList
        Dim listvalue As TextBox
        Dim ddlValue As New ListItem
 
        dropdown = FormView1.FindControl("dropdownlist5")
        listvalue = FormView1.FindControl("textbox1")
 
        ddlValue.Text = listvalue.Text
        ddlValue.Value = listvalue.Text
 
        dropdown.Items.Add(ddlValue)
 
        dropdown.SelectedValue = listvalue.Text
 
    End Sub