Access User Info using Google APIs for .NET

Set your scopes to:

• https://www.googleapis.com/auth/userinfo.email
• https://www.googleapis.com/auth/userinfo.profile

At: Google.Apis.Auth.OAuth2.Flows.AuthorizationCodeFlow.Scopes

And use this endpoint address: https://www.googleapis.com/oauth2/v1/userinfo?alt=json

That should help you to acquire the required information.

Here ,I edit my answere. Please look into this. On Default2.aspx page , I am displaying Session["username"] and Session["useremail"] value in label. I hope these will be help for you.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using DotNetOpenAuth.OpenId;
using DotNetOpenAuth.OpenId.RelyingParty;
using DotNetOpenAuth.OpenId.Extensions.SimpleRegistration;
using DotNetOpenAuth.OpenId.Extensions.AttributeExchange;

public partial class _Default : System.Web.UI.Page 
{

   protected void Page_Load(object sender, EventArgs e)
   {

       openIdAuth();

   }

   protected void openIdAuth()
  {
       OpenIdAjaxRelyingParty rp = new OpenIdAjaxRelyingParty();
       var response = rp.GetResponse();

       if (response != null)
       {
         switch (response.Status)
         {
            case AuthenticationStatus.Authenticated:
                NotLoggedIn.Visible = false;
                Session["GoogleIdentifier"] = response.ClaimedIdentifier.ToString();

                var fetchResponse = response.GetExtension<FetchResponse>();
                Session["FetchResponse"] = fetchResponse;
                var response2 = Session["FetchResponse"] as FetchResponse;

                string UserName = response2.GetAttributeValue(WellKnownAttributes.Name.First) ?? "Guest"; // with the OpenID Claimed Identifier as their username.
                string UserEmail = response2.GetAttributeValue(WellKnownAttributes.Contact.Email) ?? "Guest";

                Session["username"] = UserName;
                Session["useremail"] = UserEmail;

                Response.Redirect("Default2.aspx");
                break;

            case AuthenticationStatus.Canceled:
                lblAlertMsg.Text = "Cancelled.";
                break;

            case AuthenticationStatus.Failed:
                lblAlertMsg.Text = "Login Failed.";
                break;
        }

    }
    var CommandArgument = "https://www.google.com/accounts/o8/id";
    string discoveryUri = CommandArgument.ToString();
    OpenIdRelyingParty openid = new OpenIdRelyingParty();

    var url = new UriBuilder(Request.Url) { Query = "" };
    var request = openid.CreateRequest(discoveryUri); // This is where you would add any OpenID extensions you wanted
    var fetchRequest = new FetchRequest(); // to fetch additional data fields from the OpenID Provider

    fetchRequest.Attributes.AddRequired(WellKnownAttributes.Contact.Email);
    fetchRequest.Attributes.AddRequired(WellKnownAttributes.Name.First);
    fetchRequest.Attributes.AddRequired(WellKnownAttributes.Name.Last);
    fetchRequest.Attributes.AddRequired(WellKnownAttributes.Contact.HomeAddress.Country);
    request.AddExtension(fetchRequest);

    request.RedirectToProvider();
   }
 }

You should do the following:

1. In addition to Google.Apis.Auth NuGet package you should install the following page: https://www.nuget.org/packages/Google.Apis.Oauth2.v2

2. Add Google.Apis.Oauth2.v2.Oauth2Service.Scope.UserinfoProfile and also Google.Apis.Oauth2.v2.Oauth2Service.Scope.UserinfoEmail to the scopes list (When you initialize the AppFlowMetadata).

3. Now, add the following code:

if (result.Credential != null)
{
    var oauthSerivce = new Google.Apis.Oauth2.v2.Oauth2Service(
        new BaseClientService.Initializer()
        {
            HttpClientInitializer = credential,
            ApplicationName = "OAuth 2.0 Sample",
        });

    var userInfo = await oauthSerivce.Userinfo.Get().ExecuteAsync();
    // You can use userInfo.Email, Gender, FamilyName, ... 
}