Error: Object Reference is not set to an instance of object

[Dodoria]

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• What is a NullReferenceException in .NET? 8 answers

I'm currently developing an application that will give user capability of downloading demo software from link provided by us in his email address. After 24 hours, now I have done through link shown in his email address, and he can download but in the coding of disable of this link .. I'm getting error my code is ..\[code\]protected void btn_sub_Click(object sender, EventArgs e){ cn.Open(); objInquiry.Name = txt_name.Text.ToString().Trim(); objInquiry.MobileNo = txtMobileNo.Text.ToString().Trim(); objInquiry.EmailId = txt_eid.Text.ToString().Trim(); objInquiry.InquiryFor = "Agriculture Product Marketing comity System".ToString().Trim(); objInquiry.Message = txt_msg.Text.ToString().Trim(); using (DataSet ds = objInquiry.InsertInquiry()) { Msg.Visible = true; Msg.Text = "Thank U For Inquiry We Will Send Demo Link To Your Email Please Check Your Email Regularly"; } try { DateTime dt1 = Convert.ToDateTime(Request.QueryString["period"].ToString()); DateTime dt2 = DateTime.Now; TimeSpan ts = dt2 - dt1; if (ts.TotalMinutes > 5) { Response.Write("Download time is over"); } else { MailMessage mail = new MailMessage(); mail.From = new MailAddress("[email protected]"); mail.To.Add(txt_eid.Text); mail.Subject = txtInquiryFor.Text; mail.IsBodyHtml = true; mail.Body = "Welcome Mr." + txt_name.Text + "<br><br>"; mail.Body += "To ShreeHans Webnology" + "<br><br>"; mail.Body += "Thank u for putting inquiry for" + txtInquiryFor.Text + "<br><br>"; mail.Body += "Please Click on Following Link To Download Your Demo" + "<br><br>"; mail.Body += "<a href=http://stackoverflow.com/"http://www.test.co.in/ConatctUs.aspx?period=" + DateTime.Now + "'\">Download Demo Software</a>"; SmtpClient smtp = new SmtpClient(); smtp.Host = "smtp.gmail.com"; smtp.Port = 587; smtp.EnableSsl = true; smtp.Credentials = new System.Net.NetworkCredential("[email protected]", "*****"); smtp.DeliveryMethod = SmtpDeliveryMethod.Network; smtp.Send(mail); } } catch (Exception ex) { ex.ToString(); }\[/code\]