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c# Task.WhenAll to block while awaiting completion

I have a simple Winforms application. I would like to background TCP connections/print requests and check the output of all tasks at a set point in my code.
I would expect ReportOnTasks to block until WaitAll is complete. Please could someone explain why this is not the case? I'm also worried I haven't structured this correctly.
Edit, to clarify my intentions:
I would like to send the print jobs as soon as I receive the data. Then continue with some other DB operations. Once all the print operations are complete, I would like to update the UI to state the result.
I've attempted to simplify the code as much as I can. Maybe too much. HomeController just inits some stuff. There are buttons on the form and file watchers that trigger the main functionality.
public class HomeController
{
public HomeController(){
MessageBox.Show("1");
oPrintController.PrintAsync("192.168.2.213", Encoding.ASCII.GetBytes("string to print"));
MessageBox.Show("2");
// Block here untill tasks are complete
ReportOnTasks();
MessageBox.Show("Report on tasks complete");
}
public async void ReportOnTasks()
{
await Task.WhenAll(oPrintController.Tasks);
foreach(Task<PrintController.PrintResult> PR in oPrintController.Tasks)
{
// do something with the result of task
}
}
}
and the PrintController
public class PrintController
{
public List<Task<PrintResult>> Tasks = new List<Task<PrintResult>>();
public async void PrintAsync(string sIP, List<byte[]> lsToPrint, int iPort = 9100)
{
var s = await Task.Run(() => PrintAsync1(sIP, lsToPrint));
}
public async System.Threading.Tasks.Task<PrintResult> PrintAsync1(string sIP, List<byte[]> lsToPrint, int iPort = 9100)
{
using (TcpClient tc = new TcpClient())
{
await tc.ConnectAsync(sIP, iPort);
using (var ns = tc.GetStream())
{
foreach (byte[] btLabel in lsToPrint)
{
await ns.WriteAsync(btLabel, 0, btLabel.Length);
}
}
}
Thread.Sleep(10000);
return new PrintResult();
}
}
public class PrintResult
{
bool bSuccess = false;
}

You are not awaiting the call to ReportOnTasks()
Moreover, you can't await within a ctor, because they can't be async.
Depending on how your HomeController is used, you could use a static async method which returns an instance of HomeController, created by a private ctor instead:
Something like this:
public class HomeController
{
//notice private - you can't new up a HomeController - you have to use `CreateInstance`
private HomeController(){
MessageBox.Show("1");
//not clear from your code where oPrintController comes from??
oPrintController.PrintAsync("192.168.2.213", Encoding.ASCII.GetBytes("string to print"));
MessageBox.Show("2");
MessageBox.Show("Report on tasks complete");
}
public static async Task<HomeController> CreateInstance() {
var homeController = new HomeController();
await homeController.ReportOnTasks();
return homeController;
}
//don't use async void! Change to Task
public async Task ReportOnTasks()
{
//not clear from your code where oPrintController comes from??
await Task.WhenAll(oPrintController.Tasks);
foreach(Task<PrintController.PrintResult> PR in oPrintController.Tasks)
{
// do something with the result of task
}
}
}
Usage:
var homeControllerInstance = await HomeController.CreateInstance();

It's generally not recommended to perform heavy operations in class constructors, but I suppose you won't change that part, so in order to wait for ReportOnTasks to finish, you need to make it synchronous.
Take into account, that constructor itself doesn't support async/await, it's not possible to mark it async.
Having said that, you won't have real performance enhancement marking void ReportOnTasks as async. In addition, it is not recommended to mark void methods as async due to issues with exceptions handling, which is usually not possible.
So, you can either postpone ReportOnTasks like Alex showed you, or you can synchronously wait until all tasks are finished (which is possible inside ctor).
public void ReportOnTasks()
{
Task.WhenAll(oPrintController.Tasks).GetAwaiter().GetResult(); //synchronously wait
foreach(Task<PrintController.PrintResult> PR in oPrintController.Tasks)
{
// do something with the result of task
}
}
However, I wouldn't suggest this approach, because instance creation will take a while and most importantly block UI thread - and that's usually signal something is really fishy

Load data (async/await)

I'm not sure about this state. I need to get data from database asynchrony.
I have class DB
public class Db{
public async Task<ObservableCollection<Person>> GetAllPerson()
{
using (var context = new Db())
{
// get data and return ObservableCollection<Person>
}
}
}
In the ViewModel I call LoadData function.
public class VM{
public ObservableCollection<Person> Person { get; set; }
private readonly DB sqlRepository;
public VM()
{
sqlRepository=new DB();
LoadData();
}
private async void LoadData()
{
Person= await sqlRepository.GetAllPerson();
}
}
I got warning: Warning CS1998 This async method lacks 'await' operators and will run synchronously.
How can I run my function asynchronously?
Should I use ?
Person=await Task.Run(()=>this.sqlRepository.GetAllPerson());

How can I run my function asynchronously?
You're approaching your problem from the wrong direction. You're trying to go "outside in" - your ViewModel wants to load the database data asynchronously. And that's a fine way of describing the problem, but it's the wrong way to solve it.
To solve it more easily, start at the other end. Whatever methods are actually calling into the database (e.g., Entity Framework calls) should be made asynchronous first, and then let async grow out from there. Eventually you'll end up with something like:
public async Task<ObservableCollection<Person>> GetAllPersonAsync()
{
using (var context = new Db())
{
// This code wasn't shown in the question.
// But from the compiler warning, it was probably using something like
// var people = People.ToList();
// return new ObservableCollection<Person>(people);
// And the async version should be:
var people = await People.ToListAsync();
return new ObservableCollection<Person>(people);
}
}
Which you could consume as:
private async void LoadData()
{
Person = await sqlRepository.GetAllPersonAsync();
}
But I recommend consuming it via NotifyTask as described in my MVVM async data binding article. That approach would give you the ability to data-bind busy spinners and whatnot.
Should I use [Task.Run]?
No. That's "fake asynchrony" - where your code acts like it's asynchronous but it's really just synchronously running on a background thread.

Where to Break the Chain with Task, ContinueWith, Lock

I have MVP application C#, .NET 4, WinForms. It uses Bridge class which communicate with third party app via NamedPipe.
The command flow is like this: View → Presenter → Manager → Bridge → Client
And back in the reverse order. View is prepared for multitasking. I split reverse chain in Manager by rising event with the result, but it doesn't help.
// View class
public void AccountInfo_Clicked() { presenter.RequestAccountInfo(); }
public void UpdateAccountInfo(AccountInfo info)
{
if (pnlInfo.InvokeRequired)
pnlInfo.BeginInvoke(new InfoDelegate(UpdateAccountInfo), new object[] {info});
else
pnlInfo.Update(info);
}
// Presenter class
public void RequestAccountInfo() { manager.RequestAccountInfo(); }
private void Manager_AccountInfoUpdated(object sender, AccountInfoEventArgs e)
{
view.UpdateAccountInfo(e.AccountInfo);
}
// Manager class
public void RequestAccountInfo()
{
AccountInfo accountInfo = bridge.GetAccountInfo();
OnAccountInfoUpdated(new AccountInfoEventArgs(accountInfo));
}
// Bridge class
public AccountInfo GetAccountInfo() { return client.GetAccountInfo(); }
// Client class
public AccountInfo GetAccountInfo()
{
string respond = Command("AccountInfo");
return new AccountInfo(respond);
}
private string Command(string command)
{
var pipe = new ClientPipe(pipeName);
pipe.Connect();
return pipe.Command(command);
}
I want to unfreeze the UI during command processing. There are also other commands that can be executed. Finally all commands reach Command(string command) method in Client.
I tried to break the chain in Manager by using task and ContinueWith but it results to pipe failing to connect. The reason is that client is not thread safe.
// Manager class
public void RequestAccountInfo()
{
var task = Task<AccountInfo>.Factory.StartNew(() => bridge.GetAccountInfo());
task.ContinueWith(t => { OnAccountInfoUpdated(new AccountInfoEventArgs(t.Result)); });
}
My question is: Where to use Task, ContinueWith and where to Lock?
I assume I can lock only Command(string command) because it is the ultimate method.
private string Command(string command)
{
lock (pipeLock)
{
var pipe = new ClientPipe(pipeName);
pipe.Connect();
return pipe.Command(command);
}
}
Can I use Task, Wait in Command in Client class?

I think the problem you are having is that bridge.GetAccountInfo() is trying to extract information from the UI itself - hence the UI thread. This code
public void RequestAccountInfo()
{
var task = Task<AccountInfo>.Factory.StartNew(() => bridge.GetAccountInfo());
task.ContinueWith(t => { OnAccountInfoUpdated(new AccountInfoEventArgs(t.Result)); });
}
is attempting to execute the bridge.GetAccountInfo() method (accessing the UI) from a background thread-pool thread.
My first question here would be how expensive is the call to bridge.GetAccountInfo()? If it is not expensive, it makes no sense to put working into multi-threading this aspect. If it is expensive, you will have to think about a way to make this operation thread safe (I can't advise without more information).
Another thing to do would assess the expense of a move to WCF. This handles most synchronisation problems for you... I am sorry I can't be of more help. I wrote the above before I read your last comment.
I hope this is of some use.
Aside: something to be aware of is SynchronizationContext. Using a TaskScheduler you can launch a Task on the UI thread (this is not what you want here as this again will just block the UI - however, this can be good to know when reporting [in .NET 4.0]. To launch your code above on the UI thread you can do
public void RequestAccountInfo()
{
var task = Task<AccountInfo>.Factory.StartNew(() =>
bridge.GetAccountInfo(),
TaskScheduler.FromCurrentSynchronizationContext());
task.ContinueWith(t => { OnAccountInfoUpdated(new AccountInfoEventArgs(t.Result)); });
}

I locked Command in Client class. It appears that it works perfectly in that way. No blocking UI, no pipe errors. I lock on pipeName because each copy of View is using a unique pipe name.
I applied Task<Type>, ContinueWith to all commands in Manager class.
// Manager class
public void RequestSomeInfo()
{
var task = Task<SomeInfo>.Factory.StartNew(() => bridge.GetSomeInfo());
task.ContinueWith(t => { OnInfoUpdated(new InfoEventArgs(t.Result)); });
}
// Client class
private string Command(string command)
{
lock (pipeName)
{
var pipe = new ClientPipe(pipeName);
pipe.Connect();
return pipe.Command(command);
}
}

Execute function every 5 minutes in background

I have function which reads Data out of an Webservice. With that Data i create Bitmaps. I send the Bitmaps to Panels (Displays) which displays the created Bitmaps. Manually its working like charm. What i need now is, that my Application run this function every 5 min automtically in the Backround.
My Application is running under IIS. How can i do that? Can someone help me with that?

You don't have to be depended on asp.net project, but you can use Cache Callback to do it.
I have found a nice approach, to do it.
actually i don't remember the link so i'll give you a code that i use:
public abstract class Job
{
protected Job()
{
Run();
}
protected abstract void Execute();
protected abstract TimeSpan Interval { get; }
private void Callback(string key, object value, CacheItemRemovedReason reason)
{
if (reason == CacheItemRemovedReason.Expired)
{
Execute();
Run();
}
}
protected void Run()
{
HttpRuntime.Cache.Add(GetType().ToString(), this, null,
Cache.NoAbsoluteExpiration, Interval, CacheItemPriority.Normal, Callback);
}
}
Here is the implementation
public class EmailJob : Job
{
protected override void Execute()
{
// TODO: send email to whole users that are registered
}
protected override TimeSpan Interval
{
get { return new TimeSpan(0, 10, 0); }
}
}

An Asp.Net application is not the correct framework for a task like this.
You should probably create a dedicated service for this type of tasks.
Another option is to create a scheduled task that will run every X minutes
On a side note, if you must do this through your asp.net application, I recommend reading on how to Simulate a Windows Service using ASP.NET to run scheduled jobs

doing scheduled background work in asp.net

I need to perform periodically a certain task in my asp.net app so did like this:
protected void Application_Start()
{
Worker.Start();
}
...
public static class Worker
{
public static void Start()
{
ThreadPool.QueueUserWorkItem(o => Work());
}
public static void Work()
{
while (true)
{
Thread.Sleep(1200000);
//do stuff
}
}
}
is this approach good ?
I saw a blog about the badge awarding on this site is done using an asp.net cache hack:
https://blog.stackoverflow.com/2008/07/easy-background-tasks-in-aspnet/

You can use Timer class for task like this. I'm using this class in my own ASP.NET chat module for closing rooms after some expiration time and it works fine.
And I think, it's better approach than using Thread.Sleep
Below example code:
using System;
using System.IO;
using System.Threading;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
Worker.Start();
Thread.Sleep(2000);
}
public static class Worker
{
private static Timer timer;
public static void Start()
{
//Work(new object());
int period = 1000;
timer = new Timer(new TimerCallback(Work), null, period, period);
}
public static void Work(object stateInfo)
{
TextWriter tw = new StreamWriter(#"w:\date.txt");
// write a line of text to the file
tw.WriteLine(DateTime.Now);
// close the stream
tw.Close();
}
}
}
}

Your method will work, but as Lucasus said, better approach will be using Timer class.
Other than that if you own the computer where your site is running I would recommend using Windows service for your scheduling tasks. This approach will proove itself more beneficial than timers of any kind inside of asp.net infrastructure. That is because everything that is working inside asp.net is going to be managed by asp.net engine and it is not something you want. For example worker process can be recycled and at this moment your task will break.
Detailed information about timers in windows services can be found here: Timers and windows services.
Information about windows services can be found here: Windows services
To hoock timer into windows service you need to create it at the start and handle events that it fires.

If you want do do a scheduled work, why not use Windows Task Scheduler ?
Some info I found, may be useful: http://www.codeproject.com/KB/cs/tsnewlib.aspx
Kris