.NET Google api 1.7 beta authenticating with refresh token

If I understand you correctly, you are asking how can you create a new Google service, based on an existing refresh token.

So, you can do the following:

var token = new TokenResponse { RefreshToken = "YOUR_REFRESH_TOKEN_HERE" }; 
var credentials = new UserCredential(new GoogleAuthorizationCodeFlow(
    new GoogleAuthorizationCodeFlow.Initializer 
    {
      ClientSecrets = [your_client_secrets_here]
    }), "user", token);

Then you can pass your credentials to the service's initializer.

By doing the above, GoogleAuthorizationCodeFlow will get a new access token based on you refresh token and client secrets.

Note that you must have client secrets here, without that, you won't be able to get an access token.

To use refresh token:

var init = new GoogleAuthorizationCodeFlow.Initializer
{
    ClientSecrets = new ClientSecrets
    {
        ClientId = "OAuth_Client_ID_From_GoogleAPI",
        ClientSecret = "OAuth_Client_Secret"
    },
    Scopes = new string[] {"MY_SCOPES"}
};
var token = new TokenResponse { RefreshToken = "MY_REFRESH_TOKEN" };
var credential = new UserCredential(new Google.Apis.Auth.OAuth2.Flows.AuthorizationCodeFlow(init), "", token);

//init your Google API service, in this example Google Directory
var service = new DirectoryService(new BaseClientService.Initializer()
{
    HttpClientInitializer = credential,
    ApplicationName = "",
});

What if you don't have a refresh token? The easiest is to follow the instructions on Google SDK docs. First download your credentials from Google API project. Name the file credentials.json. Then run the code:

using (var stream =
    new FileStream("credentials.json", FileMode.Open, FileAccess.Read))
{
    // The file token.json stores the user's access and refresh tokens, and is created
    // automatically when the authorization flow completes for the first time.
    string credPath = "token.json";
    credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
        GoogleClientSecrets.Load(stream).Secrets,
        Scopes,
        "user",
        CancellationToken.None,
        new FileDataStore(credPath, true)).Result;
    Console.WriteLine("Credential file saved to: " + credPath);
}

This should create a folder token.json and inside folder is another json file that has your refresh_token.

{
    "access_token" : "asdf",
    "token_type" : "Bearer",
    "expires_in" : 3600,
    "refresh_token" : "XXX",
    "scope" : "https://www.googleapis.com/auth/admin.directory.user.readonly",
    "Issued" : "2019-02-08T12:37:06.157-08:00",
    "IssuedUtc" : "2019-02-08T20:37:06.157Z"
}

I prefer not to use the GoogleWebAuthorizationBroker because it auto launches a web browser when the token is not found. I prefer the old school way of getting the refresh token by access code. This is very similiar to the using the Google OAuthUtil.CreateOAuth2AuthorizationUrl and OAuthUtil.GetAccessToken in Google's legacy OAuth API.

var a = new Google.Apis.Auth.OAuth2.Flows.GoogleAuthorizationCodeFlow.Initializer
{
    ClientSecrets = new ClientSecrets
    {
        ClientId = "asdf",
        ClientSecret = "hij"
    },
    Scopes = Scopes
};
var flow = new Google.Apis.Auth.OAuth2.Flows.AuthorizationCodeFlow(a);
var url = flow.CreateAuthorizationCodeRequest(GoogleAuthConsts.InstalledAppRedirectUri).Build().AbsoluteUri;
Console.WriteLine("Go to this URL and get access code: " + url);

Console.Write("Enter access code: ");
var code = Console.ReadLine();

Console.WriteLine("Fetching token for code: _" + code + "_");
var r = flow.ExchangeCodeForTokenAsync("user", code, GoogleAuthConsts.InstalledAppRedirectUri, CancellationToken.None).Result;
Console.WriteLine(Newtonsoft.Json.JsonConvert.SerializeObject(r));