using System;

using System.Collections.Generic;

using System.Collections.Concurrent;

using System.Linq;

using System.Net.Http;

using System.Threading.Tasks;

​

public class Program

{

    private static readonly ConcurrentDictionary<string, string> cachedDownloads = new();

    private static readonly HttpClient httpClient = new();

    public static async Task Main()

    {

        var message = await GreetingAsync();

        Console.WriteLine(message);

        string[] urls = new[]

        {

            "https://docs.microsoft.com/aspnet/core",

            "https://docs.microsoft.com/dotnet",

            "https://docs.microsoft.com/dotnet/azure",

            "https://docs.microsoft.com/dotnet/machine-learning",

            "https://docs.microsoft.com/xamarin",

        };

        IEnumerable<Task<string>> downloads1 = urls.Select(DownloadAsync);

        await Task.WhenAll(downloads1);

        IEnumerable<Task<string>> downloads2 = urls.Select(DownloadAsync);

        await Task.WhenAll(downloads2);

    }

    private static Task<string> GreetingAsync() {

        return Task.FromResult("Hello World");

    }

    private static Task<string> DownloadAsync(string address)

    {

        if(cachedDownloads.TryGetValue(address, out string content)){

            Console.WriteLine($"Reading {address} data from cache");

            return Task.FromResult(content);

        }

        return Task.Run(async () => {

            var content = await httpClient.GetStringAsync(address);

            cachedDownloads.TryAdd(address, content);

            Console.WriteLine($"Reading {address} data from web");

            return content;

        });

    }

}