#region 获取windows桌面背景
        [System.Runtime.InteropServices.DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto, SetLastError = true)]
        public static extern int SystemParametersInfo(int uAction, int uParam, StringBuilder lpvParam, int fuWinIni);

        private const int SPI_GETDESKWALLPAPER = 0x0073;
        #endregion

void WindowsBtn_Click(object sender, RoutedEventArgs e)
        {
            //定义存储缓冲区大小
            StringBuilder s = new StringBuilder(300);
            //获取Window 桌面背景图片地址，使用缓冲区
            SystemParametersInfo(SPI_GETDESKWALLPAPER, 300, s, 0);
            //缓冲区中字符进行转换
            string wallpaper_path = s.ToString(); //系统桌面背景图片路径
         }