NuGetDownloader.cs

using NuGet.Common;
using NuGet.Packaging;
using NuGet.Packaging.Core;
using NuGet.Protocol;
using NuGet.Protocol.Core.Types;
using System;
using System.Collections.Concurrent;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
	public class Program
	{
		public static async Task Main()
		{
			if (!Directory.Exists("downloads"))
				Directory.CreateDirectory("downloads");

			var source = Repository.Factory.GetCoreV3("https://aka.ms/skiasharp-eap/index.json");
			var cache = new SourceCacheContext();
			var logger = NullLogger.Instance;

			var listResource = await source.GetResourceAsync<PackageSearchResource>();
			var downloadResource = await source.GetResourceAsync<DownloadResource>();

			var packages = (await listResource.SearchAsync("", new SearchFilter(true, SearchFilterType.IsLatestVersion), 0, 100, logger, default)).ToArray();

			var tasks = new ConcurrentBag<Task>();
			var progress = 0;

			foreach (var package in packages)
			{
				if (package.Identity.Id.StartsWith("_"))
					continue;

				var versions = (await package.GetVersionsAsync()).ToArray();

				for (int verIdx = 0; verIdx < versions.Length; verIdx++)
				{
					var version = versions[verIdx];
					Console.WriteLine($"START {package.Identity.Id}: {version.Version}");

					var outputPath = $"downloads\\{package.Identity.Id}.{version.Version}.nupkg";
					if (File.Exists(outputPath))
					{
						using var reader = new PackageArchiveReader(outputPath);
						try
						{
							await reader.ValidatePackageEntriesAsync(default);
							continue;
						}
						catch
						{
						}
					}

					var d = Task.Run(async () =>
					{
						using var result = await downloadResource.GetDownloadResourceResultAsync(
						new PackageIdentity(package.Identity.Id, version.Version),
						new PackageDownloadContext(cache, "externals", true),
						null,
						logger,
						default);

						using var file = File.Create(outputPath);

						await result.PackageStream.CopyToAsync(file);

						var percent = Interlocked.Increment(ref progress) / (double)tasks.Count;
						Console.WriteLine($"END {package.Identity.Id}: {version.Version} ({percent:P})");
					});
					tasks.Add(d);
				}
			}

			await Task.WhenAll(tasks);
		}
	}
}