public class DesktopService    {        [DllImport("user32.dll", EntryPoint = "SystemParametersInfoA")]        static extern Int32 SystemParametersInfo(Int32 uAction, Int32 uParam, string lpvparam, Int32 fuwinIni);        private const string BingUrl = "http://www.bing.com";        private const string BingBasePath = @"E:\BingWallpaper";        private int _idx = -1;        private int idx        {            get => _idx;            set            {                if (value >= 0 && value < 8)                    _idx = value;            }        }        private static DesktopService _instance = null;        public static DesktopService Instance        {            get { return _instance ?? (_instance = new DesktopService()); }        }        /// 
        /// 设置桌面背景图片        /// 
        public Task SetDesktopBackground(bool isNext = true)        {            return Task.Factory.StartNew(() =>            {                if (!Directory.Exists(BingBasePath))                {                    Directory.CreateDirectory(BingBasePath);                }                if (isNext)                    idx++;                else                    idx--;                string bingPath = $@"{BingBasePath}\{DateTime.Now.AddDays(-idx).ToString("yyyy-MM-dd")}.bmp";                if (!File.Exists(bingPath))                {                    var imageUrl = GetImageUrl();                    var image = GetImage(imageUrl);                    image.Save(bingPath, System.Drawing.Imaging.ImageFormat.Bmp);                }                SystemParametersInfo(20, 0, bingPath, 1);            });        }        /// 
        /// 获取图片Url        /// 
        /// 
            private string GetImageUrl()        {            string InfoUrl = $"http://cn.bing.com/HPImageArchive.aspx?idx={idx}&n=1";            string xmlString = HttpGet(InfoUrl);            // 定义正则表达式用来匹配标签            Regex regImg = new Regex("
    
     (?
     
      .*?)
     
    ", RegexOptions.IgnoreCase);            // 搜索匹配的字符串            MatchCollection matches = regImg.Matches(xmlString);            // 取得匹配项列表            string imageUrl = BingUrl + matches[0].Groups["imgUrl"].Value;            return imageUrl;        }        /// 
        /// Get请求        /// 
        ///         /// 
            private string HttpGet(string url)        {            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);            request.Method = "GET";            request.ContentType = "text/html;charset=UTF-8";            string html;            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())            {                Stream myResponseStream = response.GetResponseStream();                using (StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.UTF8))                {                    html = myStreamReader.ReadToEnd();                }            }            return html;        }        /// 
          /// 图片请求          /// 
          /// 要访问的图片所在网址          /// 
              private Image GetImage(string url)        {            Image img;            try            {                WebRequest request = WebRequest.Create(url);                using (WebResponse response = request.GetResponse())                {                    img = Image.FromStream(response.GetResponseStream());                }            }            catch            {                img = null;            }            return img;        }    }