GridVeiw DataKey is giving index out of range exception.

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C#

ASP.NET

What i have to check i am getting exception on datakey of a gridview that its getting nul value..

gridview is properly displaying data.. why my code is not getting data key??

here is the code

if (DropDownList1.Text == "Completed")
     {
         for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
         {
             GridViewRow row = GridView1.Rows[i];
             if (row.RowType == DataControlRowType.DataRow)
             {
                 ShoppingCartItem item = new ShoppingCartItem();

                 item.ProductID = (int)GridView1.DataKeys[i].Value; //I am getting index out of range exception here..
                 item.Quantity = int.Parse(((Label)(row.FindControl("txtmainqty"))).Text);





                 dbcon db1 = new dbcon();
                  int orderedQuantity = System.Convert.ToInt32(item.Quantity);

                  string query1 = "update Products set productquantity=productquantity-'" + orderedQuantity + "' where ProductID='" + item.ProductID + "'";
                  //bool b = db.UDI(query);
                  bool c = db1.UDI(query1);
                  if (c == true)
                  {

                     // lblshow.Text = "Product Delivered and Added to DataBase Sucessfully";

                  }

                  else
                  {
                      //lblshow.Text = " Operation Not successful";

                  }

i have no proble with the gridview i have done this in another page.. buts its no working on my current page.. y datakey is giving index out of range exception..