title: FacebookApplication.VerifyAuthentication(_httpContext, GenerateLocalCallbackUri()) return null on Facebook
body: <p>I've developed an mvc 5 application using nopcommerce and i use facebook login using External callback it was working but now it is not working and i can't find out actual problem. And using this below code</p>
  
  <p><code>this.FacebookApplication.VerifyAuthentication(_httpContext, GenerateLocalCallbackUri());</code></p>
  
  <p>and it's returning me always null and authentication status failed i searched on web an do every thing and followed that steps but still i can't login with facebook.</p>
  
  <p>My code is like this in FacebookProviderAuthorizer.cs</p>
  
  <pre><code>private AuthorizeState VerifyAuthentication(string returnUrl)
  {
     var authResult = DotNetOpenAuth.AspNet.Clients.FacebookApplication.VerifyAuthentication(_httpContext, GenerateLocalCallbackUri());
  
     if (authResult.IsSuccessful)
     {
     }
  }
  </code></pre>
  
  <p>And then write Call back method</p>
  
  <pre><code>private Uri GenerateLocalCallbackUri()
  {
      string url = string.Format("{0}plugins/externalauthFacebook/logincallback/", _webHelper.GetStoreLocation());
      return new Uri(url);            
  }
  </code></pre>
  
  <p>Then generate service login url</p>
  
  <pre><code>private Uri GenerateServiceLoginUrl()
  {
     //code copied from DotNetOpenAuth.AspNet.Clients.FacebookClient file
     var builder = new UriBuilder("https://www.facebook.com/dialog/oauth");
     var args = new Dictionary&lt;string, string&gt;();
     args.Add("client_id", _facebookExternalAuthSettings.ClientKeyIdentifier);
     args.Add("redirect_uri", GenerateLocalCallbackUri().AbsoluteUri);
     args.Add("response_type", "token");
     args.Add("scope", "email");
     AppendQueryArgs(builder, args);
     return builder.Uri;
  }
  </code></pre>