Async cache

async_cache.cs

// http://blog.stephencleary.com/2015/04/a-tour-of-task-part-10-promise-tasks.html

public Task<string> GetValueAsync(int key)
{
  string result;
  if (cache.TryGetValue(key, out result))
    return Task.FromResult(result);
  return DoGetValueAsync(key);
}

private async Task<string> DoGetValueAsync(int key)
{
  string result = await ...;
  cache.TrySetValue(key, result);
  return result;
}