gistfile1.txt

public static class Helpers
{
  // this method must be non-async because the caller doesn't understand Tasks
  // it also must be static for *reasons*
  public static string GetApplicationValue(string appType, string listType, string key)
  {
    RefreshCache(); // this needs to block so that the cache is refreshed before trying to return a result
    
    // pseudocode
    return Cache.SingleOrDefault(item => item.Key == key);
  }
  
  private static DateTime lastUpdatedTime = DateTime.MinValue;
  
  private static SemaphoreSlim throttle = new SemaphoreSlim(1);
  
  private static async void RefreshCache()
  {
    try
    {
      // stop more than one thread from trying to update the cache at a time
      await throttle.WaitAsync();
      
      if(lastUpdatedTime < DateTime.Now.AddMinutes(-10))
      {
      
        // pseudocode
        // this method must be async so that the HttpClient methods can be awaited
        var data = await new HttpClient().GetAsync("/api/data");
        
        // pseudocode
        Cache.NewData(data);
        
        lastUpdatedTime = DateTime.Now;
      
      }
    }
    finally
    {
      // make sure the semaphore is always released otherwise every thread that enters will be stuck here forever
      throttle.Release();
    }
  }
}