Auto Login user to website when user is logged in

I am using DotNetOpenAuth to log in users with FB and Google.

When a user visits the website, is there a way to detect that he/she has already logged in to FB or Google and log them automatically?

Thank you

Yes, if your user has already explicitly logged into your web site using Google or Facebook before. For privacy reasons neither provider will allow you to implicitly log their users in without each user explicitly signing into your web site at least once (and clicking "remember me" at their login page while doing so).

Once the user has approved that, it becomes possible but still isn't very simple. The approach involves creating a hidden iframe on your page (perhaps when the user session is first created, you add the iframe to whatever page the user is viewing, or just your popular landing page). This iframe is pointed at a URL on your site that does nothing but initiate the no-user-interaction login flow. For Google, this means your server makes an OpenIdRelyingParty.CreateRequest call with the Google OP Identifier, and you set IAuthenticationRequest.Mode = AuthenticationRequestMode.Immediate on the returned object before calling IAuthenticationRequest.RedirectToProvider. The Immediate bit tells the Provider to not expect to interact with the user (since your iframe is invisible) and to either give an immediate yes or no response. Then in the redirect back to your site (still in the hidden iframe) you'll see the user is logged into Google (or not) and get their Claimed Identifier and log them into your site.

For Facebook which uses an old draft of OAuth 2, the process may be similar, but use the OAuth 2 classes. I haven't done this one though, so I can't really give an overview.