I'm trying to refactor an existing synchronous app into one that uses Async/Await. This WPF app was written originally using a BackgroundWorker thread calling a bunch of synchronous methods. I went down to the deepest level and worked my way up, converting these methods to async, using await Task.Run(()..) on some of them that didn't seem to be taking too long.
I am stuck on something though. I want to pass in an IProgress parameter, to be able to get feedback into the UI -- and the last, deepest method, the one that takes the longest to execute, is written this way:
public static Task<bool> SaveToFileAsync(string filePath)
{
return Task.Run(() => SaveToFile(filePath));
/*
SaveToFile calls into an unmanaged dll that uses a few loops and writes
to file using fopen/fwrite...
*/
}
private async void CreateObjectAndSaveToFile(/*IProgress progress*/)
{
List<Task> SaveFileTaskArray = new List<Task>();
int index = 0;
foreach (...)
{
//
{
DoSomethingThatTakesSomeTime();
//Start Async Task to save this SomeThing to file
SaveFileTaskArray.Add(SaveToFileAsync(fileName));
}
index++;
}
await Task.WhenAll(SaveFileTaskArray);
}
Now if I call CreateObjectAndSaveToFile() on the UI side like so:
await CreateObjectAndSaveToFile();
I get the desired effect - UI is responsive and files get saved one by one (can be 10-20 files). But for completeness, I'd like to add a progress bar to this (and therefore adding that IProgress parameter all the way down).
I'm not sure how I can do that.
You mention in the comments that you know how to use IProgress already. So if you want report progress as each task finishes, you can use WhenAny() instead of WhenAll(), and remove each task from the list as it finishes. Something like this:
while (SaveFileTaskArray.Count > 0) {
var finishedTask = await Task.WhenAny(SaveFileTaskArray);
SaveFileTaskArray.Remove(finishedTask);
//Report progress of how many are left
progress.Report(SaveFileTaskArray.Count);
}
The Microsoft documentation actually has a whole article about this, if you want to read more: Start Multiple Async Tasks and Process Them As They Complete
I have to call my API and show waiting PictureBox(wait.gif) until API get complete and return the data to bind my custom control into tablepanellayout.
I can't use BackgroundWorker class here because it has cross-thread issue with my sub control in custom control.
Here I have only idea to do that, is call child thread from main thread but until its get executed completely show the PictureBox(wait.gif) and block the rest code to be executed.
Can anybody suggest me how to do it exactly or please provide some code snippet for example.
There are two articles that helped me a lot understanding async-await:
This interview with Eric Lippert. Search somewhere in the middle for async aawait. Eric compares async await with a cook who doesn't wait for the water to boil, but instead looks around if he can do something else in the mean time.
Async Await by the ever so helpful Stephen Cleary
For WinForms you do the following:
If your data query function has async versions to fetch the data, use that function. Think of functions like SqlConnection.OpenAsync, Dapper.QueryAsync etc
If your data querier has no async versions make it async using Task.Run
Every function that calls async functions should be declared async itself
Every async function returns Task instead of void and Task<TResult> instead of TResult.
The only exception is the event handler: this function returns void instead of Task
the calls to other async functions should be awaited before your async returns.
.
// The event handler: make async. The only one that still returns void
async void OnButton1_clicked(object sender, ...)
{
// start fetching the data. Don't await yet, you'll have other things to do
Task<MyData> fetchDataTask = FetchData(...);
// meanwhile: show the user that you are busy:
this.ShowBusy(true); // show picture box?
// if needed do other things you can do before the data is fetched
this.ClearTable();
// once you have nothing meaningful to do, await for your data
MyData fetchedData = await fetchDataTask;
this.ProcessData(fetchedData);
// finished:
this.ShowBusy(false); // remove picture box
}
Async version of the function that fetched the data:
async Task<IQueryable<MyData>> FetchDataAsync(myParams)
{
using (SqlConnection dbConnection = new SqlConnection(...)
{
// open the connection, don't wait yet:
Task taskOpen = sqlCommand.OpenAsync();
// continue while opening:
using (var sqlCommand = new SqlCommand(...))
{
cmd.Parameters.AddWithValue(...);
// before executing the query: wait until OpenAsync finished:
await taskOpen;
// read the data. If nothing to do: await, otherwise use Task similar to Open
SqlDataReader dataReader = await cmd.ExecuteReaderAsync();
foreach (var row in dataReader)
{
... (some Await with GetFieldValueAsync
}
}
}
}
I'm not really familiar with reading SQL data on such a low level, I prefer entity framework and dapper, so there might be an error in my SqlReader stuff. Maybe someone can correct this. Still you'll get the gist.
If you write it like this, your program will be pretty responsive: Whenever the procedure has to await for something, control is given back to the caller of the function who can continue processing until he meets an await, when control is given back to the caller, who continues processing, etc.
Note that this won't help you if your program is not waiting for something. If your main thread does some heavy calculations for several seconds, your program won't be responsive. Consider creating an async function that will do this for your using Task.Run
If you have programmed like this, all threads performing your functions will have the same context: it will be as if only the UI-thread is involved. No need for mutexes, semaphores, InvokeRequired etc.
I have a Winform project that inside of the winform class I have a property called DataBindingTasks like so.
// create a task list to determine when tasks have finished during load
protected List<Task> DataBindingTasks = new List<Task>();
I have several async void methods that I am calling in the winform "Load" event that are all similar to the following.
private async void BindSomething(int millSecToWait)
{
var someTask = Task.Factory.StartNew(() =>
{
// do some work
System.Threading.Thread.Sleep(millSecToWait);
// return some list for binding
return new List<int>();
});
// add the task to the task list
DataBindingTasks.Add(someTask);
// wait until data has loaded
var listToBind = await someTask;
// bind the data to a grid
}
I am calling the BindSomething methods on load.
I say methods because there are several of these binding types of methods that are called on load.
private void Form_Load(object sender, EventArgs e)
{
// async bind something and let UI continue
// fire and forget
BindSomething(5000);
BindSomething(8000);
BindSomething(2000);
BindSomething(2000);
// code to execute when all data binding tasks have completed
Task.WhenAll(DataBindingTasks).ContinueWith((x) =>
{
// Do something after all async binding tasks have completed
});
}
EXCEPT the ContinueWith code is executing even though all the tasks have not completed.
Here is a screen shot showing that all task are not complete.
UPDATED 10/29
The problem is obviously deeper than the sample code above and the sample code above does not fully explain the true scenario.
I will try to explain in greater detail but try to not make it to long.
This is a Winform application.
We have created a base winform "BaseForm" that all other winforms will inherit from.
We have overridden the "OnLoad" event in the "BaseForm" so that we can call a new method that all inherited forms will have called "LoadData".
Since "LoadData" can have async method calls, the base form needs to know when the "LoadData" method is finished.
So in the base form was have some of the following:
protected List<Task> DataBindingTasks = new List<Task>();
public event EventHandler DataBindingTasksComplete;
protected void OnDataBindingTasksComplete(EventArgs e)
{
if (DataBindingTasksComplete != null)
{
DataBindingTasksComplete(this, e);
}
// now clear the list
DataBindingTasks.Clear();
}
// NOTE: this is inside the OnLoad called before base.OnLoad(e)
Task.WhenAll(DataBindingTasks).ContinueWith((x) =>
{
OnDataBindingTasksComplete(EventArgs.Empty);
});
The hope was that all inherited forms would add any of their "async" tasks to this list so that the base form could fire the "DataBindingTasksComplete" event so they would know that form has finished loading.
The problem "as perceived to us at the time of the issue" was that the "WhenAll().ContinueWith" was not waiting until all the tasks on the list had completed.
BUT as someone noted, the list might have changed.
So here is most likely what happened.
There are 4 "BindSomething" methods that are marked async all called from the Form_Load
The 2nd or so line down inside the "BindSomething" method is used to add a task to the "BaseForm.DataBindingTasks" list.
Since each of these calls are marked async, the Form_Load continues to call all 4 as a "fire and forget".
After that, it returns back to the BaseForm OnLoad which then looks at the "DataBindingTasks" list to see if all tasks have completed.
My best guess is that one of the "BindSomething" methods was in the middle of adding its task to the list yet the Base.OnLoad has already started looking at the list.
I could add 4 "fake" tasks (like thread sleep) to the list even before calling the "BindSomething" methods as "place holders" and then inside the "BindSomething" methods swap out the "fake" tasks with the "real" tasks.
This seams messy and most likely will cause other issues.
The most likely fix is to not use a task list / WhenAll.ContinueWith and instead call the load data with "await" and then raise the event on the next line.
The async void methods are called as fire-and-forget, and there is no way to wait for them, that's why your delegate don't wait properly - it simply can't do that. So you need some changes in your code.
Update: #Servy noted the main problem in your code which I've missed, thanks for him:
DataBindingTasks.Add(someTask);
This operation isn't thread-safe! You simply losing some of your tasks during parallel calls for Add method. You need to change this: by using lock, by using ConcurrentCollection or by using data separation: assign a task to array by different indexes so parallel tasks aren't intersect each other.
First of all, you shouldn't use the StartNew in this case, use the Task.Run, otherwise you can met some problems in your app.
Second thing is that you can make the Load method async and await it, so your UI wouldn't freeze, and you can switch the signature for your BindSomething methods to became awaitable, as #digimunk mentioned:
// note that we return the task here
private async Task BindSomething(int millSecToWait)
{
// use Task.Run in this case
var someTask = Task.Run(() =>
{
// Some work
System.Threading.Thread.Sleep(millSecToWait);
// return some list for binding
return new List<int>();
});
DataBindingTasks.Add(someTask);
// wait until data has loaded
var listToBind = await someTask;
// bind the data to a grid
}
// async void for the event handler
private async void Load()
{
// start tasks in fire-and-forget fashion
BindSomething(5000);
BindSomething(8000);
BindSomething(2000);
// code to execute when all data binding tasks have completed
await Task.WhenAll(DataBindingTasks);
// Do something after all binding is complete
}
In this case you can await the Load method safely.
You don't need .ContinueWith(). Just await the Task.WhenAll(), and then put whatever code you want to run after it under it. Also, change the "void" in the method signature to "async Task".
I have a method called from within a program that authorizes card payment which was previously implemented in a synchronous way. However now I want to change it so it runs on a different thread so I can send cancel request once it has started.
It works, but I want to know if this is a good implementation, if there are some things I need to consider like deadlocks etc.
So here is a simplified version of my code:
public void Authorize()
{
lock(object)
{
AuthorizeAsync();
}
}
public async void AuthorizeAsync()
{
// ...
IProgress<string> progressHandler = new Progress<string>(progressString =>
{
UpdateProgressWindow(progressString.ToString());
});
cancellationToken = progressWindow.CancellationTokenSource.Token;
progressWindow.show();
results = await DoAuthorizeAsync(progressHandler, cancellationToken).ConfigureAwait(false);
// ...
}
I mainly want to use async - await because I want the authorization work to run separately from the UI thread so the user can still cancel the operation.
Do you suggest another approach other than async - await?
I am mainly concerned with the fact that I am calling an async method from sync method, I know the best practice is to use async all the way up but I cannot change the rest of the program.
The other concern I have is the lock in Authorize() could it pose any problem? I only want the code to be accessed by one thread at a time.
I am fairly new to this async - await architecture in .net so I'm pretty sure I didn't get it right on my first attempt.
Microsoft just announced the new C# Async feature. Every example I've seen so far is about asynchronously downloading something from HTTP. Surely there are other important async things?
Suppose I'm not writing a new RSS client or Twitter app. What's interesting about C# Async for me?
Edit I had an Aha! moment while watching Anders' PDC session. In the past I have worked on programs that used "watcher" threads. These threads sit waiting for something to happen, like watching for a file to change. They aren't doing work, they're just idle, and notify the main thread when something happens. These threads could be replaced with await/async code in the new model.
Ooh, this sounds interesting. I'm not playing with the CTP just yet, just reviewing the whitepaper. After seeing Anders Hejlsberg's talk about it, I think I can see how it could prove useful.
As I understand, async makes writing asynchronous calls easier to read and implement. Very much in the same way writing iterators is easier right now (as opposed to writing out the functionality by hand). This is essential blocking processes since no useful work can be done, until it is unblocked. If you were downloading a file, you cannot do anything useful until you get that file letting the thread go to waste. Consider how one would call a function which you know will block for an undetermined length and returns some result, then process it (e.g., store the results in a file). How would you write that? Here's a simple example:
static object DoSomeBlockingOperation(object args)
{
// block for 5 minutes
Thread.Sleep(5 * 60 * 1000);
return args;
}
static void ProcessTheResult(object result)
{
Console.WriteLine(result);
}
static void CalculateAndProcess(object args)
{
// let's calculate! (synchronously)
object result = DoSomeBlockingOperation(args);
// let's process!
ProcessTheResult(result);
}
Ok good, we have it implemented. But wait, the calculation takes minutes to complete. What if we wanted to have an interactive application and do other things while the calculation took place (such as rendering the UI)? This is no good, since we called the function synchronously and we have to wait for it to finish effectively freezing the application since the thread is waiting to be unblocked.
Answer, call the function expensive function asynchronously. That way we're not bound to waiting for the blocking operation to complete. But how do we do that? We'd call the function asynchronously and register a callback function to be called when unblocked so we may process the result.
static void CalculateAndProcessAsyncOld(object args)
{
// obtain a delegate to call asynchronously
Func<object, object> calculate = DoSomeBlockingOperation;
// define the callback when the call completes so we can process afterwards
AsyncCallback cb = ar =>
{
Func<object, object> calc = (Func<object, object>)ar.AsyncState;
object result = calc.EndInvoke(ar);
// let's process!
ProcessTheResult(result);
};
// let's calculate! (asynchronously)
calculate.BeginInvoke(args, cb, calculate);
}
Note: Sure we could start another thread to do this but that would mean we're spawning a thread that just sits there waiting to be unblocked, then do some useful work. That would be a waste.
Now the call is asynchronous and we don't have to worry about waiting for the calculation to finish and process, it's done asynchronously. It will finish when it can. An alternative to calling code asynchronously directly, you could use a Task:
static void CalculateAndProcessAsyncTask(object args)
{
// create a task
Task<object> task = new Task<object>(DoSomeBlockingOperation, args);
// define the callback when the call completes so we can process afterwards
task.ContinueWith(t =>
{
// let's process!
ProcessTheResult(t.Result);
});
// let's calculate! (asynchronously)
task.Start();
}
Now we called our function asynchronously. But what did it take to get it that way? First of all, we needed the delegate/task to be able to call it asynchronously, we needed a callback function to be able to process the results, then call the function. We've turned a two line function call to much more just to call something asynchronously. Not only that, the logic in the code has gotten more complex then it was or could be. Although using a task helped simplify the process, we still needed to do stuff to make it happen. We just want to run asynchronously then process the result. Why can't we just do that? Well now we can:
// need to have an asynchronous version
static async Task<object> DoSomeBlockingOperationAsync(object args)
{
//it is my understanding that async will take this method and convert it to a task automatically
return DoSomeBlockingOperation(args);
}
static async void CalculateAndProcessAsyncNew(object args)
{
// let's calculate! (asynchronously)
object result = await DoSomeBlockingOperationAsync(args);
// let's process!
ProcessTheResult(result);
}
Now this was a very simplified example with simple operations (calculate, process). Imagine if each operation couldn't conveniently be put into a separate function but instead have hundreds of lines of code. That's a lot of added complexity just to gain the benefit of asynchronous calling.
Another practical example used in the whitepaper is using it on UI apps. Modified to use the above example:
private async void doCalculation_Click(object sender, RoutedEventArgs e) {
doCalculation.IsEnabled = false;
await DoSomeBlockingOperationAsync(GetArgs());
doCalculation.IsEnabled = true;
}
If you've done any UI programming (be it WinForms or WPF) and attempted to call an expensive function within a handler, you'll know this is handy. Using a background worker for this wouldn't be that much helpful since the background thread will be sitting there waiting until it can work.
Suppose you had a way to control some external device, let's say a printer. And you wanted to restart the device after a failure. Naturally it will take some time for the printer to start up and be ready for operation. You might have to account for the restart not helping and attempt to restart again. You have no choice but to wait for it. Not if you did it asynchronously.
static async void RestartPrinter()
{
Printer printer = GetPrinter();
do
{
printer.Restart();
printer = await printer.WaitUntilReadyAsync();
} while (printer.HasFailed);
}
Imagine writing the loop without async.
One last example I have. Imagine if you had to do multiple blocking operations in a function and wanted to call asynchronously. What would you prefer?
static void DoOperationsAsyncOld()
{
Task op1 = new Task(DoOperation1Async);
op1.ContinueWith(t1 =>
{
Task op2 = new Task(DoOperation2Async);
op2.ContinueWith(t2 =>
{
Task op3 = new Task(DoOperation3Async);
op3.ContinueWith(t3 =>
{
DoQuickOperation();
}
op3.Start();
}
op2.Start();
}
op1.Start();
}
static async void DoOperationsAsyncNew()
{
await DoOperation1Async();
await DoOperation2Async();
await DoOperation3Async();
DoQuickOperation();
}
Read the whitepaper, it actually has a lot of practical examples like writing parallel tasks and others.
I can't wait to start playing with this either in the CTP or when .NET 5.0 finally makes it out.
The main scenarios are any scenario that involves high latency. That is, lots of time between "ask for a result" and "obtain a result". Network requests are the most obvious example of high latency scenarios, followed closely by I/O in general, and then by lengthy computations that are CPU bound on another core.
However, there are potentially other scenarios that this technology will mesh nicely with. For example, consider scripting the logic of a FPS game. Suppose you have a button click event handler. When the player clicks the button you want to play a siren for two seconds to alert the enemies, and then open the door for ten seconds. Wouldn't it be nice to say something like:
button.Disable();
await siren.Activate();
await Delay(2000);
await siren.Deactivate();
await door.Open();
await Delay(10000);
await door.Close();
await Delay(1000);
button.Enable();
Each task gets queued up on the UI thread, so nothing blocks, and each one resumes the click handler at the right point after its job is finished.
I've found another nice use-case for this today: you can await user interaction.
For example, if one form has a button that opens another form:
Form toolWindow;
async void button_Click(object sender, EventArgs e) {
if (toolWindow != null) {
toolWindow.Focus();
} else {
toolWindow = new Form();
toolWindow.Show();
await toolWindow.OnClosed();
toolWindow = null;
}
}
Granted, this isn't really any simpler than
toolWindow.Closed += delegate { toolWindow = null; }
But I think it nicely demonstrates what await can do. And once the code in the event handler is non-trivial, await make programming much easier. Think about the user having to click a sequence of buttons:
async void ButtonSeries()
{
for (int i = 0; i < 10; i++) {
Button b = new Button();
b.Text = i.ToString();
this.Controls.Add(b);
await b.OnClick();
this.Controls.Remove(b);
}
}
Sure, you could do this with normal event handlers, but it would require you to take apart the loop and convert it into something much harder to understand.
Remember that await can be used with anything that gets completed at some point in the future. Here's the extension method Button.OnClick() to make the above work:
public static AwaitableEvent OnClick(this Button button)
{
return new AwaitableEvent(h => button.Click += h, h => button.Click -= h);
}
sealed class AwaitableEvent
{
Action<EventHandler> register, deregister;
public AwaitableEvent(Action<EventHandler> register, Action<EventHandler> deregister)
{
this.register = register;
this.deregister = deregister;
}
public EventAwaiter GetAwaiter()
{
return new EventAwaiter(this);
}
}
sealed class EventAwaiter
{
AwaitableEvent e;
public EventAwaiter(AwaitableEvent e) { this.e = e; }
Action callback;
public bool BeginAwait(Action callback)
{
this.callback = callback;
e.register(Handler);
return true;
}
public void Handler(object sender, EventArgs e)
{
callback();
}
public void EndAwait()
{
e.deregister(Handler);
}
}
Unfortunately it doesn't seem possible to add the GetAwaiter() method directly to EventHandler (allowing await button.Click;) because then the method wouldn't know how to register/deregister that event.
It's a bit of boilerplate, but the AwaitableEvent class can be re-used for all events (not just UI). And with a minor modification and adding some generics, you could allow retrieving the EventArgs:
MouseEventArgs e = await button.OnMouseDown();
I could see this being useful with some more complex UI gestures (drag'n'drop, mouse gestures, ...) - though you'd have to add support for cancelling the current gesture.
There are some samples and demos in the CTP that don't use the Net, and even some that don't do any I/O.
And it does apply to all multithreaded / parallel problem areas (that already exist).
Async and Await are a new (easier) way of structuring all parallel code, be it CPU-bound or I/O bound. The biggest improvement is in areas where before C#5 you had to use the APM (IAsyncResult) model, or the event model (BackgroundWorker, WebClient). I think that is why those examples lead the parade now.
A GUI clock is a good example; say you want to draw a clock, that updates the time shown every second. Conceptually, you want to write
while true do
sleep for 1 second
display the new time on the clock
and with await (or with F# async) to asynchronously sleep, you can write this code to run on the UI thread in a non-blocking fashion.
http://lorgonblog.wordpress.com/2010/03/27/f-async-on-the-client-side/
The async extensions are useful in some cases when you have an asynchronous operation. An asynchronous operation has a definite start and completion. When asynchronous operations complete, they may have a result or an error. (Cancellation is treated as a special kind of error).
Asynchronous operations are useful in three situations (broadly speaking):
Keeping your UI responsive. Any time you have a long-running operation (whether CPU-bound or I/O-bound), make it asynchronous.
Scaling your servers. Using asynchronous operations judiciously on the server side may help your severs to scale. e.g., asynchronous ASP.NET pages may make use of async operations. However, this is not always a win; you need to evaluate your scalability bottlenecks first.
Providing a clean asynchronous API in a library or shared code. async is excellent for reusability.
As you begin to adopt the async way of doing things, you'll find the third situation becoming more common. async code works best with other async code, so asynchronous code kind of "grows" through the codebase.
There are a couple of types of concurrency where async is not the best tool:
Parallelization. A parallel algorithm may use many cores (CPUs, GPUs, computers) to solve a problem more quickly.
Asynchronous events. Asynchronous events happen all the time, independent of your program. They often do not have a "completion." Normally, your program will subscribe to an asynchronous event stream, receive some number of updates, and then unsubscribe. Your program can treat the subscribe and unsubscribe as a "start" and "completion", but the actual event stream never really stops.
Parallel operations are best expressed using PLINQ or Parallel, since they have a lot of built-in support for partitioning, limited concurrency, etc. A parallel operation may easily be wrapped in an awaitable by running it from a ThreadPool thread (Task.Factory.StartNew).
Asynchronous events do not map well to asynchronous operations. One problem is that an asynchronous operation has a single result at its point of completion. Asynchronous events may have any number of updates. Rx is the natural language for dealing with asynchronous events.
There are some mappings from an Rx event stream to an asynchronous operation, but none of them are ideal for all situations. It's more natural to consume asynchronous operations by Rx, rather than the other way around. IMO, the best way of approaching this is to use asynchronous operations in your libraries and lower-level code as much as possible, and if you need Rx at some point, then use Rx from there on up.
Here is probably a good example of how not to use the new async feature (that's not writing a new RSS client or Twitter app), mid-method overload points in a virtual method call. To be honest, i am not sure there is any way to create more than a single overload point per method.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;
namespace AsyncText
{
class Program
{
static void Main(string[] args)
{
Derived d = new Derived();
TaskEx.Run(() => d.DoStuff()).Wait();
System.Console.Read();
}
public class Base
{
protected string SomeData { get; set; }
protected async Task DeferProcessing()
{
await TaskEx.Run(() => Thread.Sleep(1) );
return;
}
public async virtual Task DoStuff() {
Console.WriteLine("Begin Base");
Console.WriteLine(SomeData);
await DeferProcessing();
Console.WriteLine("End Base");
Console.WriteLine(SomeData);
}
}
public class Derived : Base
{
public async override Task DoStuff()
{
Console.WriteLine("Begin Derived");
SomeData = "Hello";
var x = base.DoStuff();
SomeData = "World";
Console.WriteLine("Mid 1 Derived");
await x;
Console.WriteLine("EndDerived");
}
}
}
}
Output Is:
Begin Derived
Begin Base
Hello
Mid 1 Derived
End Base
World
EndDerived
With certain inheritance hierarchies (namely using command pattern) i find myself wanting to do stuff like this occasionally.
here is an article about showing how to use the 'async' syntax in a non-networked scenario that involves UI and multiple actions.