Question

When I use the Google.GData.Client .NET library and try to initiate an OAuth session it works fine until the session expires. It seems when I do my initial GetAccessToken request that the RefreshToken value is null.

Step1 (default.aspx):

OAuth2Parameters parameters = new OAuth2Parameters();
//<<build parameters with keys and CLIENT_IDs and such>>
string authorizationUrl = OAuthUtil.CreateOAuth2AuthorizationUrl(parameters);
Response.Redirect(authorizationUrl);

Step2 (oauth.aspx):

Session["ACCESS_CODE"] = Request.QueryString["code"];
Response.Redirect("Results.aspx");

Step3 (results.aspx)

if (Session["ACCESS_CODE"] == null)
  Response.Redirect("Default.aspx");

parameters.AccessCode = Session["ACCESS_CODE"].To_String();
OAuthUtil.GetAccessToken(parameters);
Response.Write("OAuth Access Token: " + parameters.AccessToken);
Session["ACCESS_TOKEN"] = parameters.AccessToken;
Session["REFRESH_TOKEN"] = parameters.RefreshToken;

When I inspect the code at this point my RefreshToken == null. Because of this I cannot call

OAuthUtil.RefreshAccessToken(parameters); 

without getting an error of value cannot be null.

Answer 1

With the OAuth 2.0 offline flow, you only receive a refresh token the first time the user exchanges an authorization code and grants access to your app. You should store that refresh token in a database for later use.

If at any time you need a refresh token for the user and you don’t have it in your database or the one you have is corrupt or revoked, you have to send the user back to the authorization page so that it can grant access again and provide you with a new refresh token.

The Google Drive SDK documentation explains this process thoroughly. Also, there’s is a complete ASP.NET MVC application that you can use as reference.