GridView讀取隱藏列ID值

this.gvProduct.DataSource = ds;
 this.gvProduct.DataKeyNames =new string []{"ProductID"};//關鍵語句...
 this.gvProduct.DataBind();

protected void gvProduct_RowDataBound(object sender, GridVIEwRowEventArgs e)// 讀取1
    {
            .........gvProduct.DataKeys[e.Row.RowIndex]["ProductID"].ToString()........
           讀取
    }

foreach (GridVIEwRow gvr in gvProduct.Rows) // 讀取2
        {
            if (gvr.RowType == DataControlRowType.DataRow)
            {
                string id = gvProduct.DataKeys[gvr.RowIndex]["ProductID"].ToString();
            }

        }