How to implement OAuth in google classroom - google-oauth

Question

Presently i am working on google classroom API to integrate classroom into my .NET product.I am using below method for authenticating user.My problem is when i execute this code it asking authentication for first time but when i execute this code next time it directly log in as previous log in credentials.When i try this after many days and many browsers also directly log in as first authenticated user.But for every fresh time execution of code i want it ask for authentication of user rather than directly log in as previous user credentials.How to achieve this...?
I am new to this OAuth and API's.Your valuable answer will help my team a lot.
please any one help me on this...
private ClassroomService getservice()
{
using (var stream =
new FileStream(Server.MapPath("client_secret1.json"), FileMode.Open, FileAccess.Read))
{
credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
GoogleClientSecrets.Load(stream).Secrets,
Scopes,
"user",
CancellationToken.None).Result;
}
var service = new ClassroomService(new BaseClientService.Initializer()
{
HttpClientInitializer = credential,
ApplicationName = ApplicationName,
});
return service;
}

Answer 1

Even if you don't pass in a data store object, the library by default will store the user's credentials in C:\Users\%USERNAME%\AppData\Roaming\Google.Apis.Auth\. If you don't want to store the auth information at all, and instead have the user authorize the application on each run, you'll need to pass in a custom data store object that doesn't actually store the credentials:
class NullDataStore : IDataStore
{
public Task StoreAsync<T>(string key, T value)
{
return Task.Delay(0);
}
public Task DeleteAsync<T>(string key)
{
return Task.Delay(0);
}
public Task<T> GetAsync<T>(string key)
{
return Task.FromResult(default(T));
}
public Task ClearAsync()
{
return Task.Delay(0);
}
}
Then pass an instance of this class into the AuthorizeAsync() method:
credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
GoogleClientSecrets.Load(stream).Secrets,
Scopes,
"user",
CancellationToken.None,
new NullDataStore()).Result;