.NET Gmail OAuth2 for multiple users - gmail

Question

We are building a solution that will need to access our customers Gmail accounts to read/send mail. On account signup, we'd have a pop-up for our customer to do Gmail auth page and then a backend process to periodically read their emails.
The documentation doesn't seem to cover this use case. For example https://developers.google.com/api-client-library/dotnet/guide/aaa_oauth says that client tokens should be stored in client_secrets.json - what if we have 1000s of clients, what then?
Service accounts are for non-user info, but rather application data. Also, if I use the GoogleWebAuthorizationBroker and the user has deleted access or the tokens have expired, I don't want my backend server app to pop open a web brower, as this seems to do.
I would imagine I could use IMAP/SMTP accomplish this, but I don't think it's a good idea to store those credentials in my db, nor do I think Google wants this either.
Is there a reference on how this can be accomplished?

Answer 1

I have this same situation. We are planning a feature where the user is approving access to send email on their behalf, but the actual sending of the messages is executed by a non-interactive process (scheduled task running on an application server).
I think the ultimate answer is a customized IAuthorizationCodeFlow that only supports access with an existing token, and will not execute the authorization process. I would probably have the flow simulate the response that occurs when a user clicks the Deny button on an interactive flow. That is, any need to get an authorization token will simply return a "denied" AuthorizationResult.
My project is still in the R&D phase, and I am not even doing a proof of concept yet. I am offering this answer in the hope that it helps somebody else develop a concrete solution.

Answer 2

While #hurcane's answer more than likely is correct (haven't tried it out), this is what I got working over the past few days. I really didn't want to have to de/serialize data from the file to get this working, so I kinda mashed up this solution
Web app to get customer approval
Using AuthorizationCodeMvcApp from Google.Apis.Auth.OAuth2.Mvc and documentation
Store resulting access & refresh tokens in DB
Use AE.Net.Mail to do initial IMAP access with access token
Backend also uses AE.Net.Mail to access
If token has expired, then use refresh token to get new access token.
I've not done the sending part, but I presume SMTP will work similarly.
The code is based on SO & blog posts:
t = EF object containing token info
ic = new ImapClient("imap.gmail.com", t.EmailAddress, t.AccessToken, AuthMethods.SaslOAuth, 993, true);
To get an updated Access token (needs error handling) (uses the same API as step #1 above)
using (var wb = new WebClient())
{
var data = new NameValueCollection();
data["refresh_token"] = refresh;
data["client_id"] = "(Web app OAuth id)";
data["client_secret"] = "(Web app OAuth secret)";
data["grant_type"] = "refresh_token";
var response = wb.UploadValues(#"https://accounts.google.com/o/oauth2/token", "POST", data);
string Tokens = System.Text.Encoding.UTF8.GetString(response);
var token = Newtonsoft.Json.JsonConvert.DeserializeObject<dynamic>(Tokens);
at = token.access_token;
return at;
}