Obter access_token do facebook após o callback de login usando a lib Facebook C# SDK (https://github.com/facebook-csharp-sdk/facebook-csharp-sdk/)

gistfile1.html

@{
    string permalink = Request.Url.Scheme + "://" + Request.Url.Authority + Request.ApplicationPath + "/MeuController/actionCallback/";
    string appId = "9999990999";
}

<!--Login Oauth-->
<p><a href="https://www.facebook.com/dialog/oauth/?client_id=@appId&redirect_uri=@permalink&scope=photo_upload&display=page&state=logged" >Login</a></p>

MeuController.cs

using Facebook;

....

public ActionResult actionCallback()
{
  try
  {
    if (String.IsNullOrEmpty(Request["code"]))
        throw new Exception("Não foi possível efetuar login na aplicação.");
        
    dynamic result = fb.Get("oauth/access_token", new
    {
        client_id = "279271518842322",
        client_secret = "e58e3f943cb8e81afdd1ed8252f442cf",
        code = Request["code"],
        redirect_uri = Request.Url.Scheme + "://" + Request.Url.Authority + Request.ApplicationPath + "/MeuController/actionCallback/"
    });
    
    fb.AccessToken = result.access_token;
    Session["FbAcessToken"] = (string)result.access_token;
  } 
  catch(Exception e)
  {
      ViewBag.Msg = e.Message.ToString();
      //ViewBag.Msg = "Ocorreu um erro ao compartilhar a imagem.";
      return View();
  }
}