title: updating existing row in a DataTable
body: <p>iam developing a invoice program</p>
  
  <p>in invoice form i have textboxes and a datagridview
  here is my sample code</p>
  
  <pre><code>DataTable dt = new DataTable();
  dt.Columns.Add("productCode");
  dt.Columns.Add("qty");
  dt.Columns.Add("price");
  dt.Columns.Add("total");
  string prodCode = txtProductCode.Text;
  decimal qty = Convert.ToInt32(txtQty.Text);
  decimal price = Convert.ToInt32(txtPrice.Text);
  decimal total = qty*price;
  dt.Rows.Add(prodCode,qty,price,total);
  dataGridView1.DataSource = dt;
  </code></pre>
  
  <p>what i want to do is if i add same prodCode again i want to update qty and total in existing row instead of add new row</p>