DataSet data = new DataSet();
         data.ReadXml(@"\abcd.xml");
 for (int i = 0; i < data.Tables[0].Rows.Count; i++)
                     {
                         DR = data.Tables[0].Rows[i];
                         string barcode = DR["barcode"].ToString();
                         if (barcode == textBox1.Text)
                         {
                             textBox2.Text = DR["productname"].ToString();
                             menuItem1.Enabled = false;
                             menuItem2.Enabled = true;
                         }
                     }

data!=null&&data.Tables[0].Rows.Count>0

------解决方案--------------------
判断data.Tables的count