use valid email address. It will work
UserCredential credential;
using (var stream = new FileStream("client_secrets.json", FileMode.Open, FileAccess.Read))
{
credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
GoogleClientSecrets.Load(stream).Secrets,
new[] { TasksService.Scope.Tasks },
"shyam.sundar@gmail.com", CancellationToken.None, new FileDataStore("Tasks.Auth.Store")).Result;
}
//credential=new UserCredential(
// Create the service.
var service = new TasksService(new BaseClientService.Initializer()
{
HttpClientInitializer = credential,
ApplicationName = "Tasks API Sample",
});
ListTaskLists(service)