Forms Data Controls :: Inserting / Updating Sql Data From A Listbox In Formview?

I have a web page with a gridview & formview (master/detail relationship) combo. On page load the gridview shows all records from a SQL table. On my page I have a button that when clicked shows the formview in Insert mode to allow users to add a new record to the data. I have also added a column to my gridview that has a link. When a user clicks this link it hides the gridview and shows the formview in edit mode with the detail from the record selected from the gridview.

In my formview I am using a listbox control to allow users to select multiple values. The choices in the listbox are populated by a SQL data table. I have successfully written code to post the selected listbox values to my SQL data field during Insert. When the user selects a record to edit I have code in the databound event to select values in the listbox from the choices based on the value in the sql data field. This works as well. My problem is that I cannot figure out the correct code to update my SqL data when a user is editing the record and makes changes to the selected listbox value. I have included my design code for my listbox in my formview edittemplate and also my behind code that I have attempted.

<td
style="width: 454px">
<asp:ListBox
ID="lbox_ConstructionType"
runat="server"
DataSourceID="SqlDataConstructionTypes"
DataTextField="Construction_Type"
DataValueField="Construction_Type"
SelectionMode="Multiple"
Width="144px"></asp:ListBox>
<asp:TextBox
ID="tbox_Ctype"
runat="server"
Text='<%# Bind("ConstructionTypeID") %>'></asp:TextBox></td>

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