how can I solve this error?

 
I wrote this code but I have error.I didn't bind my gridview to sqldatasource.so is it correct that 
i use commandargument?how can reach to value of  one column(for example label of one row)
--------------------------------------------------------------------------------------------

protected void order_del(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "delete")
        {
            string pid = Convert.ToString(e.CommandArgument);
            List<string> templist = (List<string>)Session["basket"];
            templist.Remove(pid);
         &n ...