Program.cs

using Microsoft.Win32;
using System;
using System.IO;
using System.Net;
using System.Runtime.InteropServices;
using System.Threading;
using static System.Net.Mime.MediaTypeNames;

namespace DownloadAPOD {
    class Program {
        //TODO: Download all images within a period

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        static extern int SystemParametersInfo(int uAction, int uParam, string lpvParam, int fuWinIni);

        /*
         * -h or --help: help
         * YYMMDD: date in YYMMDD format
         * -nw or --no-wallpaper: don't set as wallpaper
         * -r or --random: random image
         * -d or --default: save image to current directory
         * */
        static void Main(string[] args) {

            //Make sure there are valid arguments
            string dateArg = "";
            bool shouldChooseRandom = false;
            foreach (string arg in args) {
                int dateInt = -1;
                try {
                    dateInt = (Int32.Parse(arg));
                } catch { }

                if(dateInt != -1) {
                    dateArg += arg;
                    continue;
                }
                if (!arg.Equals("-d") && !arg.Equals("--default") && !arg.Equals("-r") && !arg.Equals("--random") && !arg.Equals("-h") && !arg.Equals("--help") && !arg.Equals("-nw") && !arg.Equals("--no-wallpaper")) {
                    Console.WriteLine("Invalid arguments. Use -h or --help to see available arguments");
                    Console.ReadKey();
                    Environment.Exit(0);
                }

            }

            //If -h or --help is used as an argument
            if(Array.Exists(args, element => element.Equals("-h") || element.Equals("--help"))) {
                Console.WriteLine("Usage:   -h   or  --help             Help Menu");
                Console.WriteLine("         -nw  or  --no-wallpaper     Do not set as wallpaper");
                Console.WriteLine("         -r   or  --random           Random image");
                Console.WriteLine("         -d   or  --default          Save in this directory and do not create config file");
                Console.WriteLine("         YYMMDD                      Specify a date in YYMMDD format");
                Environment.Exit(0);
                return;
            }

            //If -r or --random is used as an argument
            if (Array.Exists(args, element => element.Equals("-r") || element.Equals("--random"))) {
                Console.WriteLine("Choosing random image...");
                shouldChooseRandom = true;
            }

            string htmlFileLocation = @"apod.txt";
            string configFileLocation = @"config.txt";
            string imageFileName = "apod.jpg";

            string imageFileLocation = "";
            //If the default flags are used
            if (Array.Exists(args, element => element.Equals("-d") || element.Equals("--default"))) {
                //Use this location and do not create a config file
                imageFileLocation = System.Reflection.Assembly.GetExecutingAssembly().Location.ToString();
            } else {
                
                //Create config file if it does not exist
                if (!File.Exists(configFileLocation)) {
                    Console.WriteLine("Enter the save location for the image or leave blank to store in program directory.");
                    File.WriteAllText(configFileLocation, Console.ReadLine());
                    if (!File.ReadAllText(configFileLocation).EndsWith("\\") && !File.ReadAllText(configFileLocation).Equals("")) {
                        File.WriteAllText(configFileLocation, File.ReadAllText(configFileLocation) + "\\");
                    }
                }

                //Read from config file
                imageFileLocation = File.ReadAllText(configFileLocation);
                if (imageFileLocation.Equals("")) {
                    imageFileLocation = System.Reflection.Assembly.GetExecutingAssembly().Location.ToString();
                    File.WriteAllText(configFileLocation, imageFileLocation);
                }
            }
            
            

            //Set up dateString
            string dateString = "";
            DateTime date;
            if (dateArg.Equals("")) {
                
                if (shouldChooseRandom) {
                    //TODO: Display day that is downloading in file name
                    Random rand = new Random();
                    DateTime firstImage = new DateTime(1995, 6, 16);
                    int range = (DateTime.Today - firstImage).Days;
                    date = firstImage.AddDays(rand.Next(range));
                } else {
                    date = DateTime.Now;
                }
                Console.WriteLine("Image Date: " + date.ToString("d"));
                dateString = date.Year + "";
                dateString = dateString.Substring(2, dateString.Length - 2);
                switch (date.Month) {
                    case 1:
                    case 2:
                    case 3:
                    case 4:
                    case 5:
                    case 6:
                    case 7:
                    case 8:
                    case 9:
                        dateString = dateString + "0" + date.Month;
                        break;
                    case 10:
                    case 11:
                    case 12:
                        dateString = dateString + date.Month;
                        break;
                }
                if (date.Day >= 1 && date.Day <= 9) {
                    dateString = dateString + "0" + date.Day;
                } else {
                    dateString = dateString + date.Day;
                }
            } else {
                dateString += dateArg;
            }

            
            

            //Use dateString to make apodImageToDownload
            string apodImageToDownloadHTML = "https://apod.nasa.gov/apod/ap" + dateString +".html";


            //Download HTML as TXT
            using(WebClient client = new WebClient()) {
                try {
                    client.DownloadFile(apodImageToDownloadHTML, htmlFileLocation);
                } catch {
                    Console.WriteLine("Failed to download image. Make sure you choose a valid date.");
                    Console.ReadKey();
                    Environment.Exit(0);
                }
            }
            Console.WriteLine("Downloading HTML from:  " + apodImageToDownloadHTML);


            //Find part of TXT that has the image url
            string wantedLine = "";
            foreach(var line in File.ReadAllLines(htmlFileLocation)) {
                if(line.Contains("<a href=\"image")) {
                    wantedLine = line;
                    break;
                }
            }


            //Stop execution if there is no image
            if (wantedLine == "") {
                Console.WriteLine("\n\nNo image found on " + apodImageToDownloadHTML);
                Console.WriteLine("Try again tomorrow!");
                Console.ReadLine();
                return;
            }

            //Remove unwanted parts from URL ending
            wantedLine = wantedLine.Trim();
            int beginningIndex = wantedLine.IndexOf('i');
            int endIndex = wantedLine.IndexOf('\"', beginningIndex);
            string imageURLEnd = wantedLine.Substring(beginningIndex, endIndex-beginningIndex);
            Console.WriteLine("Image Downloading:      " + imageURLEnd);

            //Download Image
            string imageURL = "https://apod.nasa.gov/apod/" + imageURLEnd;
            Console.WriteLine("Image URL:              " + imageURL);
            using (WebClient client = new WebClient()) {
                client.DownloadFile(imageURL, imageFileLocation + dateString + imageFileName);
            }

            //Delete HTML file
            File.Delete(htmlFileLocation);
            //Console.WriteLine("\nDeleting:               " + htmlFileLocation);
            Console.WriteLine("Image saved to " + imageFileLocation + dateString + imageFileName);

            //Set wallpaper
            if (!Array.Exists(args, element => element.Equals("-nw") && !element.Equals("--no-wallpaper"))) {
                RegistryKey key = Registry.CurrentUser.OpenSubKey(@"Control Panel\Desktop", true);
                key.SetValue(@"WallpaperStyle", 10.ToString());
                key.SetValue(@"TileWallpaper", 0.ToString());
                SystemParametersInfo(20, 0, imageFileLocation + dateString + imageFileName, 0x01 | 0x02);
                Console.WriteLine("Image set as wallpaper");
                return;
            }

            Environment.Exit(0);
        }


    }
}

/*
if (style == Style.Fill) {
    key.SetValue(@"WallpaperStyle", 10.ToString());
    key.SetValue(@"TileWallpaper", 0.ToString());
}
if (style == Style.Fit) {
    key.SetValue(@"WallpaperStyle", 6.ToString());
    key.SetValue(@"TileWallpaper", 0.ToString());
}
if (style == Style.Span) // Windows 8 or newer only!
{
    key.SetValue(@"WallpaperStyle", 22.ToString());
    key.SetValue(@"TileWallpaper", 0.ToString());
}
if (style == Style.Stretch) {
    key.SetValue(@"WallpaperStyle", 2.ToString());
    key.SetValue(@"TileWallpaper", 0.ToString());
}
if (style == Style.Tile) {
    key.SetValue(@"WallpaperStyle", 0.ToString());
    key.SetValue(@"TileWallpaper", 1.ToString());
}
if (style == Style.Center) {
    key.SetValue(@"WallpaperStyle", 0.ToString());
    key.SetValue(@"TileWallpaper", 0.ToString());
}
*/