you should save category id for second grid.
case "link":
int categoryId = Convert.ToInt32(e.CommandArgument);
Session["CategoryID"] = categoryId;
myGrid2.DataSource = Service.GetProductsByCategories(categoryId);
myGrid2.DataBind();
break;
and
private void Doldur2()
{
int catid = (int)Session["CategoryID"];
WebHelper.FillGridView(myGrid2, Service.GetProductsByCategories(catid));
}
thats all.