的确用 Set 就可以，开始时想复杂了

public async Task<int[]> GetCachedPostIds(int blogId, int categoryId)
{
    var cacheKey = CacheKeyManager.PostToCategoryMap(blogId, categoryId);

    if (await _redis.ExistsAsync(cacheKey))
    {
        return await _redis.SetMembersAsync<int>(cacheKey);
    }

    return await GetPostIds(blogId, categoryId);
}

private async Task<int[]> GetPostIds(int blogId, int categoryId)
{
    var cacheKey = CacheKeyManager.PostToCategoryMap(blogId, categoryId);
    var postIds = (await GetPostToCategoryIds(blogId, categoryId, false)).ToArray();
    await _redis.SetAddAllAsync(cacheKey, CommandFlags.None, postIds);
    await _redis.UpdateExpiryAsync(cacheKey, TimeSpan.FromHours(3));
    return postIds;
}