empty dropdownlist with runtime datasource after Gridview postback

the dropdownlist is placed at Gridview footer
the dropdownlist is empty after gridview postback
however i can check the value of the dropdownlist after databind but the dropdownlist is still empty
i press F11 to see every steps, there is no further step after Last row (Next).
code **************************************
private sub gv_rowcommand(.............
Dim ddlEvent As DropDownList = Gridview.FooterRow.FindControl("DDLEvent")
MsgBox("before GV databind - " & ddlEvent.Items.Count)  ' show 5 items
gridview.databind  ' if i remark this row, the dropdownlist can be shown up
MsgBox("before GV databind - " & ddlEvent.Items.Count)  '  show 5 items
Dim j As Integer     For j = 0 To ddlEvent.Items.Count - 1     MsgBox("ddl event items = " & ddlEvent.Items(j).Text)    ' i ...