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()); }
        }


        /// <summary>
        /// 设置桌面背景图片
        /// </summary>
        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);
            });
        }

        /// <summary>
        /// 获取图片Url
        /// </summary>
        /// <returns></returns>
        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("<Url>(?<imgUrl>.*?)</Url>", 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;
        }

        /// <summary>
        /// Get请求
        /// </summary>
        /// <param name="url"></param>
        /// <returns></returns>
        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;
        }

        /// <summary>  
        /// 图片请求  
        /// </summary>  
        /// <param name="url">要访问的图片所在网址</param>  
        /// <returns></returns>  
        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;
        }
    }

C#电脑背景图片Bing