/// <summary>
        /// 打开包含给定帖子的给定索引处的图片的数据流。
        /// </summary>
        /// <param name="dataFacade"></param>
        /// <param name="postId">帖子 ID</param>
        /// <param name="imageIndex">图像索引</param>
        /// <returns></returns>
        /// <exception cref="ArgumentNullException">
        /// <paramref name="dataFacade"/>为null
        /// </exception>
        /// <exception cref="ArgumentOutOfRangeException">
        /// <paramref name="imageIndex"/>小于0 或 大于等于9
        /// </exception>
        public static async Task <Stream> OpenPostImage(this IDataFacade dataFacade, int postId, int imageIndex)
        {
            if (dataFacade == null)
            {
                throw new ArgumentNullException(nameof(dataFacade));
            }
            if (imageIndex < 0 || imageIndex >= 9)
            {
                throw new ArgumentOutOfRangeException(nameof(imageIndex));
            }

            var(indexEntity, contentEntity) = await FindPost(dataFacade, postId);

            if (imageIndex >= contentEntity.ImageIds.Length)
            {
                return(null);
            }

            var imageId = contentEntity.ImageIds[imageIndex];

            if (imageId == null)
            {
                return(null);
            }

            return(await dataFacade.OpenImage(imageId.Value));
        }