My website uses a common usercontrol on each page to establish the parameters for the data to be viewed. The usercontrol consists of 3 dropdownlists each populated by an SQLDatasource. In addition to the source data, each had 1 list item like "--make your selection --" which is hard coded to the DDL. Therefore each DDL is set for AppendDataBoundItems = True. The contents of DDL2 are controlled by the selection made in DDL1, and the contents of DDL3 are controlled by the selection made in DDL2 in a cascading effect. Thus the selection changed contains code like this to repopulate the cascade.
Protected Sub Streets_DropDownList_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles Streets_DropDownList.SelectedIndexChanged
Session("Street") = Streets_DropDownList.SelectedValue
Session("Location") = "--- Select Location ---"
Session("Count_id" ...
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