How can we handle the onselectedindexchanged event of a dropdownlist in a gridview/datalist?

Question

I have a simple datalist with a dropdownlist and a textbox.

When the dropdownlist selected index changes, I want to load a value into the textbox in that listbox item (i.e., the textbox on that particular row).

<ItemTemplate>
  <asp:DropDownList runat="server" 
    ID="ddlCategory" AutoPostBack="true"
    DataTextField="category"
    DataValueField="category_code" 
    OnSelectedIndexChanged="ddlCategory_SelectedIndexChanged" />                      
    <br />
    Code<asp:TextBox 
      runat="server" 
      ID="txtOutputCode" 
      Text='<%# Bind("output_code") %>' />
</ItemTemplate>

How do I do this?

The challenge I'm facing is how to find the corresponding textbox to update.

E.g. for a button I'd pass a commandname, and command arguments. And then I'd handle the event in the gridview or datalist to find the corresponding textbox and update the text. What can we do in case of a selectedindex change of a dropdownlist?

Thanks!