Why can't I get output from Parallel.ForEach in the code below?

public static class Program
{
    private static void Main(string[] args)
    {
        List<string> urls = new List<string> {
            "https://www.microsoft.com",
            "https://www.google.com",
            "https://www.amazon.com"
        };

        List<string> Results = new List<string>();
        Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 1");

        Task.Run(() =>
        {
            Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 2");
            HttpClient client = new HttpClient();
            var p = Parallel.ForEach<string>(urls, (url) =>
            {
                Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 3");
                Console.WriteLine(url);
                var content = client.GetStringAsync(url).Result;
                Results.Add(url + " " + content.Length);
                Console.WriteLine(url + " " + content.Length);
            });
        });

        Console.WriteLine("Main program");
    }
}

When I run the code, I get an output like the one below.

1 1
Main Program

Why is there no output from the places I marked as 2 or 3?

So there is no output from Console.WriteLine in Task.Run() and Parallel.ForEach().

I want to print all thread ids.

Signature of Main function can be updated to return async Task instead of void and use await on Task.Run() as shown below. In the updated code, the Parallel.ForEach was updated as well to use the new Parallel.ForEachAsync() to utilize async/await magic

private static async Task Main(string[] args)
{
    List<string> urls = new List<string> {
        "https://www.microsoft.com",
        "https://www.google.com",
        "https://www.amazon.com"
    };

    List<string> Results = new List<string>();
    Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 1");

    await Task.Run(async () =>
    {
        Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 2");
        HttpClient client = new HttpClient();
        await Parallel.ForEachAsync<string>(urls, async (url, token) =>
        {
            Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 3");
            Console.WriteLine(url);
            var content = await client.GetStringAsync(url);
            Results.Add(url + " " + content.Length);
            Console.WriteLine(url + " " + content.Length);
        });
    });

    Console.WriteLine("Main program");
}

Recommendation would be to remove Task.Run altogether unless it is required for some other reason in code

private static async Task Main(string[] args)
{
    List<string> urls = new List<string> {
        "https://www.microsoft.com",
        "https://www.google.com",
        "https://www.amazon.com"
    };

    List<string> Results = new List<string>();
    Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 1");
    HttpClient client = new HttpClient();
    await Parallel.ForEachAsync<string>(urls, async (url, token) =>
    {
        Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " 2");
        Console.WriteLine(url);
        var content = await client.GetStringAsync(url);
        Results.Add(url + " " + content.Length);
        Console.WriteLine(url + " " + content.Length);
    });

    Console.WriteLine("Main program");
}

The Task.Run method returns a Task object, that represents the completion of the asynchronous operation. You have to wait for the operation to complete, otherwise the main thread will terminate, and shortly after the process will terminate too after aborting all background threads. The simplest way to wait the task to complete is by calling the Wait method:

Task.Run(() =>
{
    //...
}).Wait();

When you call Task.Run you say to thread that it could do other thing when the task is idle. So your Task will not be waited by the Main Thread. Like in the code bellow:

Task.Run(() => { /*task-job*/});

In order to wait the Task be completed to resume your code flow, just put await keyword to synchronize the Task with Thread when its job is done. The Thread will wait till every Taskchecked with await be finished in the queue to die.

await Task.Run(() => { /*task-job*/});

Also, you have to update your Main method signature, to allow async calls:

static async Task Main()