Gmail Authentication

Question

Hi, i am working with gmail authentication in application using asp.net, Here in button click event i am getting error. so,please anyone having idea about the error please let me know....

Here is my code:

aspx page:

<asp:Button ID="btnGmail" runat="server" Text="Sign In With Google" OnCommand="btnGmail_Click">

.cs page

protected void Page_Load(object sender, EventArgs e)
   {
  HandleOpenIDProviderResponse();
  }

protected void HandleOpenIDProviderResponse()
  {
  var response = openid.GetResponse();
 
  if (response != null)
  {
  switch (response.Status)
  {
  case AuthenticationStatus.Authenticated:
  //NotLoggedIn.Visible = false;
  //btngmaillogout.Visible = true;
 
   var fetchResponse = response.GetExtension<FetchResponse>();
  Session["FetchResponse"] = fetchResponse;
  var response2 = Session["FetchResponse"] as FetchResponse;
 
  lblemail.Text = response2.GetAttributeValue(WellKnownAttributes.Contact.Email);
  lblname.Text = GetFullname(response2.GetAttributeValue(WellKnownAttributes.Name.First), response2.GetAttributeValue(WellKnownAttributes.Name.Last));
   lblbirthdate.Text = response2.GetAttributeValue(WellKnownAttributes.BirthDate.WholeBirthDate);
  lblphone.Text = response2.GetAttributeValue(WellKnownAttributes.Contact.Phone.Mobile);
  lblgender.Text = response2.GetAttributeValue(WellKnownAttributes.Person.Gender);
 
  Session["Email"] = lblemail.Text;
  Session["UserName"] = lblname.Text;
 
  value = Session["UserName"].ToString();
 
  break;
  }
  }
  else
  {
  return;
 
  }
 
  }
 
  protected void btnGmail_Click(object sender, CommandEventArgs e)
  {
  string discoveryUri = e.CommandArgument.ToString();
  var b = new UriBuilder("https://www.facebook.com/") { Query = "" };
  var req = openid.CreateRequest(discoveryUri, b.Uri, b.Uri);
 
  var fetchRequest = new FetchRequest();
  fetchRequest.Attributes.AddRequired(WellKnownAttributes.Contact.Email);
  fetchRequest.Attributes.AddRequired(WellKnownAttributes.Name.First);
  fetchRequest.Attributes.AddRequired(WellKnownAttributes.Name.Last);
  fetchRequest.Attributes.AddRequired(WellKnownAttributes.Person.Gender);
  fetchRequest.Attributes.AddRequired(WellKnownAttributes.Contact.Phone.Mobile);
  fetchRequest.Attributes.AddRequired(WellKnownAttributes.BirthDate.WholeBirthDate);
  req.AddExtension(fetchRequest);
  req.RedirectToProvider();
  }
 
  private static string GetFullname(string first, string last)
  {
  var _first = first ?? "";
  var _last = last ?? "";
 
  if (string.IsNullOrEmpty(_first) || string.IsNullOrEmpty(_last))
  return "";
 
  return _first + " " + _last;
  }


Getting error in btnGmail_Click Event,
var req = openid.CreateRequest(discoveryUri, b.Uri, b.Uri); //in this line getting error