Torsten,

When you are adding a variable number of controls to the same container, and
they all have the same name, you want the container to implement the
INamingContainer interface. That way, when you set the ID property of one of
the added controls, the ClientID will be prefixed by the ID of the
containing control. Because DataGridItem, DataListItem and RepeaterItem all
implement this interface, controls you place in the templates of the
DataGrid, DataList or Repeater controls all have their ID's prefixed.

Now, if you needed to, you could create your own container which does this
by creating a NamingContainer control:

public class NamingContainer : PlaceHolder, INamingContainer
{
}

then:

Dim nc as New NamingContainer()
tCell.Controls.Add(nc)
Dim DropDownBox As New DropDownList()
DropDownBox.AutoPostBack = True
nc.Controls.Add(DropDownBox)
...

I haven't tried this myself, so please let me know if it works!