请问，NextValue()始终返回的0

        
        private void load_Monitor() 
        {

            listView1.Items.Clear();
            PerformanceCounterCategory category = new PerformanceCounterCategory("Web Service");
            int i;
            i = 0;
            foreach (string str in category.GetInstanceNames())
            {
                i++;
                PerformanceCounter counter1 = new PerformanceCounter(category.CategoryName,"Bytes Sent/sec",str);
                PerformanceCounter counter2 = new PerformanceCounter(category.CategoryName,"Files/sec",str);
                ListViewItem listItem = new ListViewItem();
                listItem.SubItems[0].Text = i.ToString();
                listItem.SubItems.Add(str);
                listItem.SubItems.Add(counter1.NextValue().ToString());
                listItem.SubItems.Add(counter2.NextValue().ToString());
                listView1.Items.Add(listItem);
            }
        }