Welcome to the RingCentral Community

It is out date. C#.net SDK released 5.x which was a major upgrade: https://medium.com/ringcentral-developers/ringcentral-net-sdk-5-0-and-its-extensions-2992a5dc5caa

You need to use this extension to build oauthUri:

https://github.com/ringcentral/RingCentral.Net/tree/master/RingCentral.Net.AuthorizeUri

Replace

var oauthUri = rc.AuthorizeUri(RINGCENTRAL_REDIRECT_URL);

with

var authorizeUriExtension = new AuthorizeUriExtension(...);
await rc.InstallExtension(authorizeUriExtension);
var oauthUri = authorizeUriExtension.BuildUri(new AuthorizeRequest{    
    redirect_uri = RINGCENTRAL_REDIRECT_URL
});

We will upgrade docs as soon as possible.