A simple automatic update system, written in C# for a blog post.

AutoUpdateExampleHelper.cs

using System;
using System.Net;
using System.IO;
using System.IO.Compression;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Diagnostics;
using System.Text;
using System.Security.Cryptography;
using System.Security.Permissions;

class Program
{
	static string remoteVersionURL = "http://localhost:8090/program-version.txt";

	static void Main(string[] args)
	{
		if(args.Length == 0)
		{
			Console.WriteLine("This helper program updates the main program. Call it like this:");
			Console.WriteLine("\thelper.exe update");
			return;
		}

		WebClient webClient = new WebClient();

		switch(args[0].Trim().ToLower())
		{
			case "update":
				Console.WriteLine("Checking for updates...");
				// Format:
				//	<version> <url> <hash>
				string remoteVersionText = webClient.DownloadString(remoteVersionURL).Trim();
				string[] remoteVersionParts = (new Regex(@"\s+")).Split(remoteVersionText);
				string remoteUrl = remoteVersionParts[1];
				string remoteHash = remoteVersionParts[2];
				
				if(!File.Exists("version.txt"))
				{
					Console.Write("No version file detected. Calling program to obtain version - ");
					ProcessStartInfo startInfo = new ProcessStartInfo("program.exe");
					startInfo.Arguments = "writeversion";
					Process versionWriter = new Process();
					versionWriter.StartInfo = startInfo;
					versionWriter.Start();
					versionWriter.WaitForExit();
					Console.WriteLine("done.");
				}

				Version localVersion = new Version(File.ReadAllText("version.txt").Trim());
				Version remoteVersion = new Version(remoteVersionParts[0]);

				if(remoteVersion > localVersion)
				{
					// There is a new version on the server!
					Console.WriteLine("There is a new version available on the server.");
					Console.WriteLine("Current Version: {0}, New version: {1}", localVersion, remoteVersion);
					while (true)
					{
						Console.Write("Perform update? ");
						string response = Console.ReadLine().Trim().ToLower();
						if (response.StartsWith("y"))
						{
							PerformUpdate(remoteUrl, remoteHash);
							break;
						}
						else if (response.StartsWith("n"))
						{
							Console.WriteLine("Abort.");
							break;
						}
					}
				}
				break;
			default:
				Console.WriteLine("Unknown command.");
					break;
		}
	}

	static bool PerformUpdate(string remoteUrl, string expectedHash)
	{
		Console.WriteLine("Beginning update.");
		string downloadDestination = Path.GetTempFileName();

		Console.Write("Downloading {0} to {1} - ", remoteUrl, downloadDestination);
		WebClient downloadifier = new WebClient();
		downloadifier.DownloadFile(remoteUrl, downloadDestination);
		Console.WriteLine("done.");
		
		Console.Write("Validating download - ");
		string downloadHash = GetSHA1HashFromFile(downloadDestination);
		if (downloadHash.Trim().ToLower() != expectedHash.Trim().ToLower()) {
			// The downloaded file looks bad!
			// Destroy it quick before it can do any more damage!
			File.Delete(downloadDestination);
			// Tell the user about what has happened
			Console.WriteLine("Fail!");
			Console.WriteLine("Expected {0}, but actually got {1}).", expectedHash, downloadHash);
			Console.WriteLine("The downloaded update may have been modified by an attacker in transit!");
			Console.WriteLine("Nothing has been changed, and the downloaded file deleted.");
			return false;
		}
		else
			Console.WriteLine("ok.");
		
		// Since the download doesn't appear to be bad at first sight, let's extract it
		Console.Write("Extracting archive - ");
		string extractTarget = @"./downloadedFiles";
		ZipFile.ExtractToDirectory(downloadDestination, extractTarget);
		// Copy the extracted files and replace everything in the current directory to finish the update
		// C# doesn't easily let us extract & replace at the same time
		// From http://stackoverflow.com/a/3822913/1460422
		foreach (string newPath in Directory.GetFiles(extractTarget, "*.*", SearchOption.AllDirectories))
			File.Copy(newPath, newPath.Replace(extractTarget, "."), true);
		Console.WriteLine("done.");

		// Clean up the temporary files
		Console.Write("Cleaning up - ");
		Directory.Delete(extractTarget, true);
		Console.WriteLine("done.");

		return true;
	}

	/// <summary>
	/// Gets the SHA1 hash from file.
	/// Adapted from https://stackoverflow.com/a/16318156/1460422
	/// </summary>
	/// <param name="fileName">The filename to hash.</param>
	/// <returns>The SHA1 hash from file.</returns>
	static string GetSHA1HashFromFile(string fileName)
	{
		FileStream file = new FileStream(fileName, FileMode.Open);
		SHA1 sha1 = new SHA1CryptoServiceProvider();
		byte[] byteHash = sha1.ComputeHash(file);
		file.Close();

		StringBuilder hashString = new StringBuilder();
		for (int i = 0; i < byteHash.Length; i++)
			hashString.Append(byteHash[i].ToString("x2"));
		return hashString.ToString();
	}
}

AutoUpdateExampleProgram.cs

using System;
using System.IO;

class Program
{
	static string version = "0.1";
	static void Main(string[] args)
	{
		// Write out our current version
		File.WriteAllText("version.txt", version);
		// Exit now if we were just asked for our version
		if (args.Length > 0 && args[0].Trim().ToLower() == "writeversion")
			return;

		// Continue on with the main body of the program
		Console.WriteLine("Rockets are cool.");
	}
}