curl.cs

// Build with: csc curl.cs  -r:System.Net.Http.dll /langversion:7.1
using System;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;

using TTask = System.Threading.Tasks.Task;


class App {
	public static async Task Main (string[] urls)
	{
		var tasks   = new TTask [urls.Length];
		using (var client = new HttpClient ()) {
			client.Timeout = TimeSpan.FromHours (3);
			for (int i = 0; i < urls.Length; ++i) {
				tasks [i] = DownloadFile (client, urls [i], Path.Combine (Directory.GetCurrentDirectory(), Path.GetFileName (urls [i])));
			}
			TTask.WaitAll (tasks);
		}		
	}

	static async TTask DownloadFile (HttpClient client, string uri, string destinationFile)
	{
		if (File.Exists (destinationFile)) {
			Console.WriteLine ($"Skipping uri '{uri}' as destination file already exists '{destinationFile}'.");
			return;
		}
		var dp       = Path.GetDirectoryName (destinationFile);
		var dn       = Path.GetFileName (destinationFile);
		var tempPath = Path.Combine (dp, "." + dn + ".download");
		Directory.CreateDirectory(dp);

		Console.WriteLine ($"Downloading `{uri}` to `{tempPath}`.");
		try {
			using (var s = await client.GetStreamAsync (uri))
			using (var o = File.OpenWrite (tempPath)) {
				await s.CopyToAsync (o);
			}
			Console.WriteLine ($"mv '{tempPath}' '{destinationFile}'.");
			File.Move (tempPath, destinationFile);
		}
		catch (Exception e) {
			Console.Error.WriteLine ("Unable to download URL `{0}` to `{1}`: {2}", uri, destinationFile, e.Message);
			Console.Error.WriteLine (e);
		}
	}
}