on code-behind page: 2 steps:
1)
Private Sub myDatalist_ItemDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.DataListItemEventArgs) Handles myDatalist.ItemDataBound
If e.Item.ItemType = ListItemType.EditItem Then
CType(e.Item.FindControl("lbtnItemUpdate"

, LinkButton).CommandArgument = e.Item.ItemIndex
end if
end sub
2)
Private Sub myDatalist_ItemCommand(ByVal source As Object, ByVal e As System.Web.UI.WebControls.DataListCommandEventArgs) Handles myDatalist.ItemCommand
If e.CommandName = "Update" Then
If e.Item.ItemIndex = e.CommandArgument And e.Item.ItemType = ListItemType.EditItem Then
Ctype(e.Item.FindControl("lbtnItemUpdate"

, linkbutton).causesValidation = false
end if
end if
end sub