Stuck at: ` Object someObj; await new Task<Object>(() => someObj);`

Weiqi Chen 101

I'm sorry that I did not explain my intention with more detail demo code at the first place.

Here is more near to my original intention demo code:

I'm am so sorry that I wasted your guys precious time in pervious posted.

using System;
using System.Threading.Tasks;
using System.Collections.Generic;
using Microsoft.VisualStudio.Threading;

namespace ns
{
    using Queue = AsyncQueue<Object>;
    public class Object
    {
        public string Message { get; set; } = string.Empty;
    }

    public class Program
    {
        //This is still a demo code. actually my code is in
        //WPF Desktop App, something like this:
        //  button1.Click += button1_Click;
        //  private async void button1_Click()
        //  {
        //      var queue = new Queue();
        //      await Task.WhenAll(...);
        //  }
        public static async Task Main()
        {
            var queue = new Queue();
            await Task.WhenAll(ProducerAsync(queue), ConsumerAsync(queue));
        }

        static public async Task ProducerAsync(Queue queue)
        {
            int i = 0;
            while (true)
            {
                await Task.Delay(1000);
                queue.Enqueue(new Object() { Message = $"A message with index={i++}." });
            }

        }
        static public async Task ConsumerAsync(Queue queue)
        {

            Object lastObject = null;
            Random rd = new Random();


            EntryPoint:
            //allow some resources, for example TCP connection.
            //

            var timeout = Task.Delay(1000);
            Task<Object> data;

            if (lastObject != null)
            {
                Console.WriteLine("Put pending data into a task for later handling");
                //I Think there is no need to call data.run()(or Task.Run(...))
                //to put the lambda expr into a thread pool run asynchronously
                //since What i want is that just put it into Task and for later 
                //await to receive the object from the task.
                //a task obj should be Scheduling to start when you
                //await it when it not started, is that right?
                data = new Task<Object>(() => lastObject);
                lastObject = null;
            }
            else
            {
                data = queue.DequeueAsync();
            }

            var tasks = new List<Task>() { timeout, data };

            while (tasks.Count > 0)
            {
                var finished = await Task.WhenAny(tasks);
                tasks.Remove(finished);

                if (finished == timeout)
                {
                    try
                    {
                        //Some operations that may throw exceptions,
                        //for example, TCP connection broken.
                        //here is -1, so it won't throw exception for now...
                        //let us focus at data code below.
                        if (rd.Next(0, 3) == -1)
                            throw new Exception("Exception in timeout.");
                    }
                    catch(Exception ex)
                    {
                        Console.WriteLine(ex.Message);
                        if (data.IsCompleted)
                        {
                            lastObject = await data;
                        }
                        goto EntryPoint;
                    }
                    tasks.Add(timeout = Task.Delay(1000));
                }

                if (finished == data)
                {
                    lastObject = await data;

                    try
                    {
                        //Another operations that may throw exceptions
                        //for example, TCP connection broken.
                        if (rd.Next(0, 4) == 0)
                        {
                            throw new Exception("Exception in data.");
                        }
                    }
                    catch(Exception ex)
                    {
                        Console.WriteLine(ex.Message);
                        goto EntryPoint;
                    }

                    Console.WriteLine($"{lastObject.Message} was handled successfully.");

                    //If everything go well.
                    lastObject = null;
                    tasks.Add(data = queue.DequeueAsync());
                }

            }
        }
    }
}

It finally stuck at

A message with index=0. was handled successfully.
A message with index=1. was handled successfully.
A message with index=2. was handled successfully.
A message with index=3. was handled successfully.
Exception in data.
Put pending data into a task for later handling
// Stucked.

Bruce (SqlWork.com) 61,731 Reputation points

2021-07-29T15:38:16.43+00:00
You never called task.Start() so while you defined a task, you never created the thread to run it. But if you want an awaitable running task you use Task<>.Run();

var task = Task.Run(() => "Hello World!"); var str = await task;
Karen Payne MVP 35,386 Reputation points

2021-07-29T22:01:22.957+00:00
Well in regards to your change of code, should had posted this first time.

Get rid of goto, there is zero reasons for using goto when the code is written properly

My recommendation is to

Drop the code you have, start out small, understand how things work then progress to a larger topic to code.

Read the docs on basic code and asynchronous patterns, hone your skills rather than try to go large.
Weiqi Chen 101 Reputation points

2021-08-02T03:19:28.637+00:00

Thanks for @Bruce (SqlWork.com) . I finally noticed that when you type, for example var readed = TcpClient.GetStream().ReadAsync(). That method was actually invoked and it reach it suspension point( set up callback ) then pass control back to caller of the task.

When you create a bare task like var task = new Task<string>(()=>"Hello World!"), and then await task， await will suspend current function and wait for task to resume it. but since there is no callback was setup or create a thread to resume, await will stuck forever.

Task in C# and Coroutine in C++ is more alike then coroutine in Python. ( The concept of coroutines I first learned from python , So I misunderstand tasks in C#.)

Accepted answer

Weiqi Chen 101 Reputation points

2021-08-03T10:59:01.323+00:00

I finally find out the answer Task.FromRexult

https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.fromresult?view=net-5.0
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3 additional answers

AgaveJoe 27,696

I think you are looking for a pattern like below. Reference doc.

    class Program  
    {  
        static async Task Main(string[] args)  
        {  
            Console.WriteLine("Hello World!");  
            Console.WriteLine("Stuck at \"await task\" ...");  
  
            string str = await ExampleMethodAsync();  
  
            Console.WriteLine("return from await");  
            Console.WriteLine(str);  
        }  
  
        private static async Task<string> ExampleMethodAsync()  
        {  
            await Task.Delay(10);  
            return "Hello World";  
        }  
  
    }

Karen Payne MVP 35,386

Here is an example working with a method for int and a method for string.

Full source

Example 1

using System;
using System.Threading.Tasks;

namespace AsyncMain
{
    class Program
    {
        public static async Task<int> Main(string[] args)
        {
            static Task<string> Simple() => Task.Run(async () =>
                {
                    await Task.Delay(2000);
                    return $"{nameof(Simple)}";
                });


            Console.WriteLine(await Simple());

            Console.ReadKey();
            return 0;
        }


    }
}

Example 2

using System;
using System.Threading.Tasks;

namespace AsyncMain
{
    class Program
    {
        public static async Task<int> Main(string[] args)
        {
            Console.Title = "async Task<int> Main";
            int number1 = 5, number2 = 10;
            Console.WriteLine($"Sum of {number1} and {number2} is: {await AdditionAsync(number1, number2)}");

            Console.WriteLine(await Simple());

            Console.WriteLine("Press any key to exist.");
            Console.ReadKey();
            return 0;
        }
        private static Task<int> AdditionAsync(int value1, int value2)
        {
            return Task.Run(() => SumIt(value1, value2));

            int SumIt(int x, int y)
            {
                return x + y;
            }
        }

        private static Task<string> Simple() => Task.Run(() => $"{nameof(Simple)}");
    }
}

Weiqi Chen 101

A working demo code below

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Diagnostics;
using System.Collections.Generic;
using Microsoft.VisualStudio.Threading;


namespace ns
{
    using Queue = AsyncQueue<Object>;
    public class Object
    {
        public string Message { get; set; } = string.Empty;
    }

    public class Program
    {
        //This is still a demo code. actually my code is in
        //WPF Desktop App, something like this:
        //  button1.Click += button1_Click;
        //  private async void button1_Click()
        //  {
        //      var queue = new Queue();
        //      await Task.WhenAll(...);
        //  }
        public static async Task Main()
        {
            var queue = new Queue();
            await Task.WhenAll(ProducerAsync(queue), ConsumerAsync(queue));
        }

        static public async Task ProducerAsync(Queue queue)
        {
            int i = 0;
            while (true)
            {
                await Task.Delay(1000);
                queue.Enqueue(new Object() { Message = $"A message with index={i++}." });
            }

        }
        static public async Task ConsumerAsync(Queue queue)
        {

            Object lastObject = null;
            Random rd = new Random();


            EntryPoint:
            //allow some resources, for example TCP connection.
            //
            var cts = new CancellationTokenSource();
            var timeout = Task.Delay(1000);
            Task<Object> data=null;

            if (lastObject != null)
            {
                //Console.WriteLine("Put pending data into a task for later handling");
                data = Task.FromResult(lastObject);
            }
            else
            {
                data = queue.DequeueAsync(cts.Token);
            }

            var tasks = new List<Task>() { timeout, data };

            while (tasks.Count > 0)
            {
                var finished = await Task.WhenAny(tasks);
                tasks.Remove(finished);

                if (finished == timeout)
                {
                    try
                    {
                        //Some operations that may throw exceptions,
                        //for example, TCP connection broken.
                        //here is -1, so it won't throw exception for now...
                        //let us focus at data code below.
                        if (rd.Next(0, 2) == 0)
                            throw new Exception("Exception in timeout.");
                    }
                    catch (Exception ex)
                    {
                        Console.WriteLine(ex.Message);
                        cts.Cancel();
                        try
                        {
                            lastObject =  await data;

                        }catch when (cts.IsCancellationRequested)
                        {
                            lastObject = null;
                        }
                        goto EntryPoint;
                    }
                    tasks.Add(timeout = Task.Delay(1000));
                }

                if (finished == data)
                {
                    var ret = await data;

                    try
                    {
                        //Another operations that may throw exceptions
                        //for example, TCP connection broken.
                        if (rd.Next(0, 2) == 0)
                        {
                            throw new Exception("Exception in data.");
                        }
                    }
                    catch (Exception ex)
                    {
                        lastObject = ret;
                        Console.WriteLine(ex.Message);
                        goto EntryPoint;
                    }

                    Console.WriteLine($"{ret.Message} was handled successfully.");

                    //If everything go well.
                    lastObject = null;
                    tasks.Add(data = queue.DequeueAsync());
                }

            }
        }
    }
}

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Stuck at: ` Object someObj; await new Task<Object>(() => someObj);`

3 additional answers