public sealed class CacheManager
{
	// one day into the future
	private static TimeSpan DefaultTimeSpan = new TimeSpan((long)24 * 60 * 60 * TimeSpan.TicksPerSecond);

	private static readonly ConcurrentDictionary<string, Tuple<DateTime, object>> _objectByString =
		new ConcurrentDictionary<string, Tuple<DateTime, object>>(StringComparer.OrdinalIgnoreCase);

	public void Clear()
	{
		_objectByString.Clear();
	}

	public Task<T> GetOrCreateObjectAsync<T>(object cacheKey, TimeSpan? expiresIn, Func<Task<T>> getFunc)
	{
		var key = cacheKey.ToString();

		if (_objectByString.TryGetValue(key, out var tuple) && tuple.Item1 >= DateTime.UtcNow)
		{
      // cache hit
			return Task.FromResult((T)tuple.Item2);
		}

    // cache miss
		return CreateAndCacheAsync(key, expiresIn, getFunc);

		async Task<T> CreateAndCacheAsync(string stringKey, TimeSpan? expires, Func<Task<T>> getter)
		{

			var val = await getter();
			var expirationTime = expires == TimeSpan.MaxValue
				? DateTime.MaxValue
				: DateTime.UtcNow.Add(expires ?? DefaultTimeSpan);
			var tuple2 = Tuple.Create(expirationTime, (object)val);
			_objectByString.AddOrUpdate(stringKey, tuple2, (k, v) => tuple2);
			return val;
		}
	}
}

int cacheKey = 0;
var cachedValue = _cacheManager.GetOrCreateObjectAsync(cacheKey, TimeSpan.MaxValue, 
  () => FetchDataAsync(cacheKey));

// WithoutCapturedVariable
public Task<T> GetOrCreateObjectV2Async<TKey, T>(TKey cacheKey, TimeSpan? expiresIn, Func<TKey, Task<T>> getFunc)
{
	var key = cacheKey.ToString();

	//...

	async Task<T> CreateAndCacheAsync(string stringKey, TKey rawkey, TimeSpan? expires, Func<TKey, Task<T>> getter)
	{
		var val = await getter(rawkey);
		//...
		return val;
	}
}