Xamarin.Ios BeingBackgroundTask parameters

I'm trying this code from Xamarin. Ios documentation for background tasks: https://docs.microsoft.com/en-us/xamarin/ios/app-fundamentals/backgrounding/ios-backgrounding-techniques/ios-backgrounding-with-tasks. It says that the variable taskId in line 6 has CS0841: Cannot use local variable 'taskId' before it is declared. How should I assign the taskId? Also, how do you declare myFlag? I cannot find any descriptions of how to use this variable.

Task.Factory.StartNew( () => {

    //expirationHandler only called if background time allowed exceeded
    var taskId = UIApplication.SharedApplication.BeginBackgroundTask(() => {
        Console.WriteLine("Exhausted time");
        UIApplication.SharedApplication.EndBackgroundTask(taskId); 
    });
    while(myFlag == true)
    {
        Console.WriteLine(UIApplication.SharedApplication.BackgroundTimeRemaining);
        myFlag = SomeCalculationNeedsMoreTime();
    }
    //Only called if loop terminated due to myFlag and not the expiration of time
    UIApplication.SharedApplication.EndBackgroundTask(taskId);
});