C# Online Compiler | .NET Fiddle

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;
		});
	}
}

 
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",