How to get the rowindex / value within an unbounded gridview in edit event?

I fill the data into a gridview as below:
dt = New DataTable()
            dt.Columns.Add(New DataColumn("Username", GetType(String)))
            dt.Columns.Add(New DataColumn("EnglishName", GetType(String)))
            dt.Columns.Add(New DataColumn("ChineseName", GetType(String)))
            dt.Columns.Add(New DataColumn("Role", GetType(String)))
            dt.Columns.Add(New DataColumn("ValidDate", GetType(String)))
            dt.Columns.Add(New DataColumn("EDIT", GetType(Button)))

            dr = dt.NewRow()

            dr.Item(0) = "chantaiman"
            dr.Item(1) = "CHAN Tai-man"
            dr.Item(2) = "YYY"
            dr.Item(3) = "System Administrator"
            dr.Item(4) = "1/1/2007 - 1/1/2009"

            dt.Rows.Add(dr)
            dr = Nothing
            dr = dt.NewRow()
            dr.Item(0) = "Hello World"
            dr.Item(1) = "How Are You"
            dr.Item(2) = "XXX"
            dr.Item(3) = "User"
            dr.Item(4) = "1/1/2008 - 31/12/2046"
            dt.Rows.Add(dr)

However, when in this gridview gv_userList_RowCommand , I want to know the button of which row is pressed. I want to know the row index or some cell values.

Thanks