How can I implement SingleSignOn when connecting to RingCenral using ringcentral-csharp-client SDK?

Our users sign in to the app using SingleSignOn. So we would like create a session with RC under their credentials (phoneNumber and password are used by ringcentral-csharp-client) in order to stay within the limits of tokens and APIs.
We don't want to ask users for their RC credentials.

Our company has implemented SSO for RingCentral access. I also have a test application working with ringcentral-demos-oauth project which redirects to https://service.ringcentral.com.
User then logs in via SSO, but how do I get the token back?

In order to do the 3-legged OAuth login flow, you will need to bring up a pop-up browser window and set the 3-legged OAuth URL there. For the security reason, the 3-legged OAuth flow should never happen inside an iframe, which has been explicitly blocked by x-frame-options. It's very similar to the login flow when you try to login this devcommunity.ringcentral.com app with Google ID, you will see the Google login page is brought up in a pop-up instead of being rendered in an iframe.

The above behavior will be enforced in the new version of login page, so for now, embedding "https://service.ringcentral.com" in an iframe will still work.

The problem you encountered with the error - "Blocked a frame with origin "http://localhost:8080"; from accessing a cross-origin frame." - might be the inconsistency of protocol, where your main page is using "http", but iframe is using "https".

I ended up going with a different solution, as even the SSO required the credentials then storing them locally for what our application needs. Instead of using one admin set of credentials and executing all API calls under it, then possibly running into the limits; we have 4 sets. These are the admins of our RC system. I monitor the API usage per minute, then when we get close to the limit, we switch to a new connection under another set of credentials in round-robin fashion. This is working fine as it rarely happens in peak times. Also, when we first connect to RingCentral, we cache all users and userPresence as reference of extension id's. We use this for processing of incoming event notifications.
Also, ringcentral-csharp-client is going to be extended soon to include values from the header. The header holds number of available API calls, so it will simplify some of my logic as I keep track of it now.