Cascading picklists (or dependent dropdowns, however you want to call them) are a pretty common occurrence and there is a common pattern in ASP.NET to address it, it goes like this:

<asp:DropDownList runat="server" ID="cboType"
     AppendDataBoundItems="true" AutoPostBack="true" DataSourceID="dsTypes">
  <asp:ListItem Text="" Value="" />
</asp:DropDownList>
<asp:DropDownList runat="server" ID="cboSubType"
     AppendDataBoundItems="true" AutoPostBack="true" DataSourceID="dsSubTypes">
  <asp:ListItem Text="" Value="" />
</asp:DropDownList>
<asp:ObjectDataSource runat="server" ID="dsSubTypes" TypeName="CustomerDao" SelectMethod="GetSubTypes">
<SelectParameters>
  <asp:ControlParameter ControlID="cboType" Name="type" Type="String" PropertyName="SelectedValue" />
</SelectParameters>
</asp:ObjectDataSource>
<asp:ObjectDataSource runat="server" ID="dsTypes" TypeName="CustomerDao" SelectMethod="GetTypes"/>

What happens if you want to have the dropdown displayed in an editable list of data though, like so:

You wish you could write something like this:

<asp:DropDownList OnDataBound="cboSubType_DataBound" runat="server" ID="cboSubType"
  SelectedValue='<%# Bind("SubTypes") %>'
  AppendDataBoundItems="false" DataSourceID="dsSubTypes">
    <asp:ListItem Text="" Value="" />
</asp:DropDownList>

But in reality this will give 2 kinds of error:

• ‘cboSubType’ has a SelectedValue which is invalid because it does not exist in the list of items
• Databinding methods such as Eval(), XPath(), and Bind() can only be used in the context of a databound control

An additional problem is that if you disable the ViewState for the page (as I tend to routinely do) the "SelectedValue" property of the DropDownList is not correctly updated anymore.

In order to tackle these problems I took inspiration in the code listed at http://webswapp.com/codesamples/viewsource.aspx?file=~/codesamples/aspnet20/dependentlists/datalist.aspx (there are a few other neat tricks on the page so I recommend the read!).

The solution I came up with boils down to 2 elements:

• In the DataBound event of the DropDownList, retrieve the value (either from the postback data, or from the current data) and select it.

DropDownList cboSubType = (DropDownList)sender;
ListViewDataItem parentItem = (ListViewDataItem)cboSubType.NamingContainer;
cboSubType.ClearSelection();
String prevValue = null;
if (IsPostBack)
{
    prevValue = Request.Form[cboSubType.UniqueID];
}
else
{
    if (parentItem.DataItem != null)
    {
        prevValue = ((Customer)parentItem.DataItem).SubType;
    }
}
ListItem li = cboSubType.Items.FindByValue(prevValue);
if (li != null)
    li.Selected = true; 

• In the ItemUpdating event of the listview I explicitely retrieve the value (from the postback because the SelectedValue may not be valid at that point) and set it in the updated values:

DropDownList cboSubType = (DropDownList)lstCustomers.Items[e.ItemIndex].FindControl("cboSubType");
e.NewValues["SubType"] = Request.Form[cboSubType.UniqueID];

This is not quite as bad as the solution I started with (which involved checking something like Request.Form[lstContacts.UniqueId + "$ctrl00$cboSubType"]) but I still think I must be missing something obvious.  Oh well, maybe the light will come on at some point, until then I got my stuff working.

The code is here in case it is of use to anyone (or in case I need a refresher in 2 months, hah!).