How and when to use ‘async’ and ‘await’

Question

From my understanding one of the main things that async and await do is to make code easy to write and read - but is using them equal to spawning background threads to perform long duration logic?

I'm currently trying out the most basic example. I've added some comments inline. Can you clarify it for me?

// I don't understand why this method must be marked as `async`.
private async void button1_Click(object sender, EventArgs e)
{
    Task<int> access = DoSomethingAsync();
    // task independent stuff here

    // this line is reached after the 5 seconds sleep from 
    // DoSomethingAsync() method. Shouldn't it be reached immediately? 
    int a = 1; 

    // from my understanding the waiting should be done here.
    int x = await access; 
}

async Task<int> DoSomethingAsync()
{
    // is this executed on a background thread?
    System.Threading.Thread.Sleep(5000);
    return 1;
}

Senghor

Also, in your example notice that you get a warning when you compile the code above. Pay attention to the warning. It is telling you that this code doesn't make sense. – Ferd

Short answer which might help. async/await is single thread event based model. Which allows you to run code out-of-order until the line of code await. – Hunker

@stephen-cleary's post which he hadn't yet written at the time of his answering below: blog.stephencleary.com/2013/11/there-is-no-thread.html – Ligula

In Raku it would actually wait at the await. sub example { my $p = do-something-async; say 'next line'; await $p; say 'done awaiting'}; sub do-something-async { return Promise.in(5).then: {say 'promise done'}}; example(). Which would print next line. Then after 5 seconds promise done. Followed shortly by done awaiting. – Chickaree

As I understand, async/await is independant of where it is used - client or server. – Vacationist

Answer 1

When using async and await the compiler generates a state machine in the background.

Here's an example on which I hope I can explain some of the high-level details that are going on:

public async Task MyMethodAsync()
{
    Task<int> longRunningTask = LongRunningOperationAsync();
    // independent work which doesn't need the result of LongRunningOperationAsync can be done here

    //and now we call await on the task 
    int result = await longRunningTask;
    //use the result 
    Console.WriteLine(result);
}

public async Task<int> LongRunningOperationAsync() // assume we return an int from this long running operation 
{
    await Task.Delay(1000); // 1 second delay
    return 1;
}

OK, so what happens here:

1. Task<int> longRunningTask = LongRunningOperationAsync(); starts executing LongRunningOperation

2. Independent work is done on let's assume the Main Thread (Thread ID = 1) then await longRunningTask is reached.

   Now, if the longRunningTask hasn't finished and it is still running, MyMethodAsync() will return to its calling method, thus the main thread doesn't get blocked. When the longRunningTask is done then a thread from the ThreadPool (can be any thread) will return to MyMethodAsync() in its previous context and continue execution (in this case printing the result to the console).

A second case would be that the longRunningTask has already finished its execution and the result is available. When reaching the await longRunningTask we already have the result so the code will continue executing on the very same thread. (in this case printing result to console). Of course this is not the case for the above example, where there's a Task.Delay(1000) involved.

Senghor

Any thread can't handle UI issues while in many examples we can access UI in the code after await, you have something wrong in your explanation – Massey

I am a former VB6 programmer, and the await makes me think a little bit about the "Do Events" instruction that I used to tell the UI to refresh when performing an intensive loop. Am I right to think that way? – Clamber

Why do we have an "await" with the "Task.Delay(1000);" in LongRunningOperation async method? – Achromatism

@Clamber In Eric Lippert's comments to the article he linked an introductory article to this topic where he specifically compares DoEvents strategy with async-await – Hepatic

@BenisonSam thread is a bit old, but I had the same question and have been looking for an answer. Reason for the "await" is that if we omit the "await" the LongRunningOperationAsync() will return immediately. In fact the compiler will give a warning if we remove the await. Stephen Cleary's blog post blog.stephencleary.com/2011/09/… gives an execellent of the design discussions. – Lombroso

Ahhh! The dreaded DoEvents() method! blog.codinghorror.com/is-doevents-evil – Checkerberry

@BenisonSam because Task.Delay() implements async so it can be run asynchronously with the await keyword. – Veer

If every async method need to have an await inside of it, and a await can only be done on a methods with async, when does it stop? – Lyonnaise

@JohnB, Task.Delay() isn't implement async modifier (like public static Task Delay(int millisecondsDelay);) but as I understood all methods which in their output type have defined GetAwaiter are considered as asynchronous methods and awaitable as well. for example Task class has a method public TaskAwaiter GetAwaiter();. It means all methods which haveTask return type are awaitable. – Homograph

This answer is clearly wrong. These many upvotes will cause wrong understanding to many users. MS documentation clearly says, no other thread is used when just using async, await. msdn.microsoft.com/en-us/library/mt674882.aspx Please somebody correct the answer. Due to this i wasted one whole day. – Wuhan

Task.Delay() is awaitable method which runs asynchronously. the async method needs to await method but await method could be either async or Task method. – Pyrosis

@KrishnaDeepak when you use an async method, the method is executed synchronously until an await. Then after the await, the method is executed asynchronously. Sometimes you have to run a new thread explicitly with Task.Run and await it. The method runs first synchronously then asynchronously. – Orangery

I think using await Task.Delay in these examples is more confusing than helpful. It's supposed to simulate a long running synchronous operation, but since its awaitable this adds confusion because the method must now have the async modifier added to it. Would make more sense to show a long running do/loop instead. – Contraction

This answer helped a lot, but I still don't understand why the DoSomethingAsync() method in the question example, actually takes the 5 seconds delay in the debugger before going to the next line. Shouldn't this happens async? Can someone clarify? – Lindgren

@KrishnaDeepak Uncle Stephen Cleary's idea is memorized by most of people without pondering that article such that it's conspicuously seen from your comment. There may not be extra thread at OS level but that of kernel. Otherwise, how do you think to handle interrupts e.g. via I/O devices, especially on the cutting-edge smart devices? Just kind of a developer comment not an engineer. We know there may not a need of an extra thread analogously Javascript's handling of callbacks. However, I don't know at all what point you try to pinpoint by sharing just a no-frills link. – Newmann

@ElMac thank you. I still don't understand the await and async until your comment. – Vasiliki

@KrishnaDeepak A task represents some unit of work that should be done. This unit of work can run in a separate thread, and it is also possible to start a task in a synchronized manner, which results in a wait for the calling thread. – Vasiliki

How many calls to MyMethodAsync does a standard CPU can handle without slowing down? – Susansusana

When the longRunningTask is done then a thread from the ThreadPool (can be any thread) will return to MyMethodAsync() - this totally depends on whether it is a windows forms app in which case the same UI thread will be used. Where as if it were a console app then the threadpool thread will be used. Have I understood this right? – Dara

1st statement is false ("...starts executing LongRunningOperation"): LongRunningOperation starts when await longRunningTask is called, not before. – Stringboard

@Stringboard the 1st statement is correct as written - that method is executed immediately. The await longRunningTask step only says don't go any further in the current method until the above task has finished. It's entirely possible (although not in this particular example, given the await Task.Delay(1000);) for a task to be complete before any await for it line has been reached by the CLR. – Lucullus

@BrunoSantos the loop stops when the async method with void return type instead of Task, calls the Task method – Aparri

Answer 2

From my understanding one of the main things that async and await do is to make code easy to write and read.

They're to make asynchronous code easy to write and read, yes.

Is it the same thing as spawning background threads to perform long duration logic?

Not at all.

// I don't understand why this method must be marked as 'async'.

The async keyword enables the await keyword. So any method using await must be marked async.

// This line is reached after the 5 seconds sleep from DoSomethingAsync() method. Shouldn't it be reached immediately?

No, because async methods are not run on another thread by default.

// Is this executed on a background thread?

No.

---

You may find my async/await intro helpful. The official MSDN docs are also unusually good (particularly the TAP section), and the async team put out an excellent FAQ.

Monkhood

So it's not running on a background thread, but it also doesn't block. This is possible due to asynchronous API's which use callbacks instead of juggling with threads. You initiate an (I/O, socket, ..) operation and return to doing your things. When the operation is done, the OS will invoke the callback. This is what Node.js or the Python Twisted framework does and they have some nice explanation, too. – Gutierrez

"The async keyword enables the await keyword. So any method using await must be marked async.", - but why? this answer doesn't help to understand why the method must be marked as async. Can't the compiler just infer that the method is async by looking inside for await keywords? – Zebec

@Stanislav: I have a blog entry that addresses that question. – Monkhood

Suggested clarification: No, because async methods are not run on another thread by default. In your example, the Sleep() call within DoSomethingAsync() blocks the current thread which prevents execution from continuing within button1_Click() until DoSomethingAsync() completes. Note that while Thread.Sleep() blocks the executing thread, Task.Delay() does not. – Karyolysis

Reading this link (learn.microsoft.com/en-us/dotnet/csharp/async) the above anwers are not true, sorry to say. – Continuative

@PeterLarsen'CPH' What part of the docs contradicts these answers? – Monkhood

@StephenCleary Some Task functions does spawn a thread + you'll never know how the task itself has been implemented, right! Maybe I should have said, that I think your answers simplify things, not that they are not true. Sorry about that. – Continuative

@PeterLarsen'CPH': My answers here are "async is not the same as spawning a thread", "async methods are not run on another thread by default", and "Sleep in an async method does not run on another thread". All of these are true and the docs agree. – Monkhood

please clarify if it is necessary to use "promise" with "await" or it's optional ? – Gibbon

@vikramvi: Yes, await requires a promise. The C# name for "promise" is "awaitable". Most of the time, this is a Task / Task<T> / ValueTask / ValueTask<T>, but it could also be other kinds of awaitables. – Monkhood

@StephenCleary thanks for reply, I've seen code where api is called with fetch method and promise is NOT used but just async method with await against fetch method in JS React code. Can you please clarify this – Gibbon

@vikramvi: In JavaScript, the async keyword will create a Promise for you. So you don't need to create one yourself. But if using TypeScript or JSDoc, the return type will need to be specified as a promise type. – Monkhood

Is it worth mentioning that whether thread from threadpool or the UI thread will be used to run the code after await depends on the synchronization context? – Dara

@variable: It's mentioned in all the intro docs. – Monkhood

Stephen- in Javascript, the callback (code after await) is invoked when the awaited function completes and the call stack is empty. Please can you tell me whether c# also requires the call stack be empty or does it immediately run the callback even for example, say if the code running presently is in middle of a for loop execution? – Dara

Note: I am assuming that configure await is not set to false so the callback will be running on the same thread as UI. Does the call stack need to be empty though? – Dara

@variable: C# is different in that regard; it can in some cases invoke the continuation directly, but that only happens if the current thread is compatible with the captured context. So if, e.g., a background thread completes the task that was awaited by the UI thread, then it will behave the same as JS and queue the continuation to the UI thread instead of invoking it directly. – Monkhood

"The async keyword enables the await keyword" - I'm a bit confused. If I want to use, for example, PostAsync. Why doesn't it perform the actual POST if I don't use await? Shouldn't it trigger in a "fire-and-forget" manner? – Hypesthesia

@Maroun: It will perform the POST even if you don't use await. Fire-and-forget is almost always the wrong solution, though. – Monkhood

Answer 3

Explanation

Here is a quick example of async/await at a high level. There are a lot more details to consider beyond this.

Note: Task.Delay(1000) simulates doing work for 1 second. I think it's best to think of this as waiting for a response from an external resource. Since our code is waiting for a response, the system can set the running task off to the side and come back to it once it's finished. Meanwhile, it can do some other work on that thread.

In the example below, the first block is doing exactly that. It starts all the tasks immediately (the Task.Delay lines) and sets them off to the side. The code will pause on the await a line until the 1 second delay is done before going to the next line. Since b, c, d, and e all started executing at almost the exact same time as a (due to lack of the await), they should finish at roughly the same time in this case.

In the example below, the second block is starting a task and waiting for it to finish (that is what await does) before starting the subsequent tasks. Each iteration of this takes 1 second. The await is pausing the program and waiting for the result before continuing. This is the main difference between the first and second blocks.

Example

Console.WriteLine(DateTime.Now);

// This block takes 1 second to run because all
// 5 tasks are running simultaneously
{
    var a = Task.Delay(1000);
    var b = Task.Delay(1000);
    var c = Task.Delay(1000);
    var d = Task.Delay(1000);
    var e = Task.Delay(1000);

    await a;
    await b;
    await c;
    await d;
    await e;
}

Console.WriteLine(DateTime.Now);

// This block takes 5 seconds to run because each "await"
// pauses the code until the task finishes
{
    await Task.Delay(1000);
    await Task.Delay(1000);
    await Task.Delay(1000);
    await Task.Delay(1000);
    await Task.Delay(1000);
}
Console.WriteLine(DateTime.Now);

OUTPUT:

5/24/2017 2:22:50 PM
5/24/2017 2:22:51 PM (First block took 1 second)
5/24/2017 2:22:56 PM (Second block took 5 seconds)

---

Extra info regarding SynchronizationContext

Note: This is where things get a little foggy for me, so if I'm wrong on anything, please correct me and I will update the answer. It's important to have a basic understanding of how this works but you can get by without being an expert on it as long as you never use ConfigureAwait(false), although you will likely lose out on some opportunity for optimization, I assume.

There is one aspect of this which makes the async/await concept somewhat trickier to grasp. That's the fact that in this example, this is all happening on the same thread (or at least what appears to be the same thread in regards to its SynchronizationContext). By default, await will restore the synchronization context of the original thread that it was running on. For example, in ASP.NET you have an HttpContext which is tied to a thread when a request comes in. This context contains things specific to the original Http request such as the original Request object which has things like language, IP address, headers, etc. If you switch threads halfway through processing something, you could potentially end up trying to pull information out of this object on a different HttpContext which could be disastrous. If you know you won't be using the context for anything, you can choose to "not care" about it. This basically allows your code to run on a separate thread without bringing the context around with it.

How do you achieve this? By default, the await a; code actually is making an assumption that you DO want to capture and restore the context:

await a; //Same as the line below
await a.ConfigureAwait(true);

If you want to allow the main code to continue on a new thread without the original context, you simply use false instead of true so it knows it doesn't need to restore the context.

await a.ConfigureAwait(false);

After the program is done being paused, it will continue potentially on an entirely different thread with a different context. This is where the performance improvement would come from -- it could continue on on any available thread without having to restore the original context it started with.

Is this stuff confusing? Hell yeah! Can you figure it out? Probably! Once you have a grasp of the concepts, then move on to Stephen Cleary's explanations which tend to be geared more toward someone with a technical understanding of async/await already.

Harvestman

Lets say if all these task are returning a int and if I am using first task's result in second task(or some calculation) It would be wrong? – Start

@veerendragupta yes. You would consciously choose not to run them asynchronously in that case (because they're not asynchronous). There are also a few other things to realize regarding configuration context which I won't go into here – Harvestman

So await MethodCall() is an absolute waste? You might as well drop the await/async? – Damnify

@Jocie Not quite. When you call await, I think it releases the thread back to the pool instead of holding it. This makes it available for use elsewhere while awaiting the return of the Task – Harvestman

@JoePhillips I think what you just said is the essence of async/await. The calling thread is freed and can be used by other processes on the machine. When the await call is complete, a new thread is used to resume what the caller originally started. The caller is still waiting, but the benefit is that a thread is freed in the meantime. That's the benefit of async/wait? – Jonell

And I like your example in your answer, but what's missing is WHY the first block took 1 second, while the second block took 5. Can someone explain that? – Jonell

@BobHorn I cleaned it up a bit I think. Have a look – Harvestman

attention .NET Core users - it does not have SynchronizationContext, and what may happen to you, async code working in .NET Core may not work properly in 'regular frameworks which do have the SynchronizationContext and you still need to use ConfigureAwait() call. Especially when you call async code from sync code (for example by calling Task.WaitAll() ), it causes deadlocks which are frustrating to figure out why. – Woolcott

Worth noting -- when doing tests like these, you want to use System.Diagnostics.Stopwatch. It will yield more accurate results. – Henricks

Do you want to mention that- whether the ui thread or a threadpool thread will be used to run the code below the await depends on whether it is a windows forms app or console app? – Dara

Answer 4

Further to the other answers, have a look at await (C# Reference)

and more specifically at the example included, it explains your situation a bit

The following Windows Forms example illustrates the use of await in an async method, WaitAsynchronouslyAsync. Contrast the behavior of that method with the behavior of WaitSynchronously. Without an await operator applied to a task, WaitSynchronously runs synchronously despite the use of the async modifier in its definition and a call to Thread.Sleep in its body.

private async void button1_Click(object sender, EventArgs e)
{
    // Call the method that runs asynchronously.
    string result = await WaitAsynchronouslyAsync();

    // Call the method that runs synchronously.
    //string result = await WaitSynchronously ();

    // Display the result.
    textBox1.Text += result;
}

// The following method runs asynchronously. The UI thread is not
// blocked during the delay. You can move or resize the Form1 window 
// while Task.Delay is running.
public async Task<string> WaitAsynchronouslyAsync()
{
    await Task.Delay(10000);
    return "Finished";
}

// The following method runs synchronously, despite the use of async.
// You cannot move or resize the Form1 window while Thread.Sleep
// is running because the UI thread is blocked.
public async Task<string> WaitSynchronously()
{
    // Add a using directive for System.Threading.
    Thread.Sleep(10000);
    return "Finished";
}

Codie

thanks for the answer. But is WaitAsynchronouslyAsync() executed on a separate thread? – Senghor

I do belive so, from the section An await expression does not block the thread on which it is executing. Instead, it causes the compiler to sign up the rest of the async method as a continuation on the awaited task. Control then returns to the caller of the async method. When the task completes, it invokes its continuation, and execution of the async method resumes where it left off. – Codie

According to this MSDN article, "The async and await keywords don't cause additional threads to be created.... an async method doesn't run on its own thread". My understanding is that at await keywords the framework skips ahead (back to the caller) to allow all possible independent code to run while waiting for long operations to finish. I think that means that once all independent code has run, if the long operation hasn't returned, it will block. I am just learning this now, though. – Earpiece

@astander That is incorrect. It does not execute on a different thread. It is just schedules the continuation (the rest of the method) to be called when the timer used by Task.Delay fires. – Mace

This answer is wrong because of the Sleep. See the accepted answer with await Task.Delay(1000); which has the correct behavior. – Evvie

Answer 5

For fastest learning..

• Understand method execution flow(with a diagram): 3 mins

• Question introspection (learning sake): 1 min

• Quickly get through syntax sugar: 5 mins

• Share the confusion of a developer : 5 mins

• Problem: Quickly change a real-world implementation of normal code to Async code: 2 mins

• Where to Next?

Understand method execution flow(with a diagram): 3 mins

In this image, just focus on #6 (nothing more)

At #6 step, execution ran out of work and stopped. To continue it needs a result from getStringTask(kind of a function). Therefore, it uses an await operator to suspend its progress and give control back(yield) to the caller(of this method we are in). The actual call to getStringTask was made earlier in #2. At #2 a promise was made to return a string result. But when will it return the result? Should we(#1:AccessTheWebAsync) make a 2nd call again? Who gets the result, #2(calling statement) or #6(awaiting statement)?

The external caller of AccessTheWebAsync() also is waiting now. So caller waiting for AccessTheWebAsync, and AccessTheWebAsync is waiting for GetStringAsync at the moment. Interesting thing is AccessTheWebAsync did some work(#4) before waiting perhaps to save time from waiting. The same freedom to multitask is also available for the external caller(and all callers in the chain) and this is the biggest plus of this 'async' thingy! You feel like it is synchronous..or normal but it is not.

#2 and #6 is split so we have the advantage of #4(work while waiting). But we can also do it without splitting. So #2 will be: string urlContents = await client.GetStringAsync("...");. Here we see no advantage but somewhere in the chain one function will be splitting while rest of them call it without splitting. It depends which function/class in the chain you use. This change in behavior from function to function is the most confusing part about this topic.

Remember, the method was already returned(#2), it cannot return again(no second time). So how will the caller know? It is all about Tasks! Task was returned. Task status was waited for (not method, not value). Value will be set in Task. Task status will be set to complete. Caller just monitors Task(#6). So 6# is the answer to where/who gets the result. Further reads for later here.

Question introspection for learning sake: 1 min

Let us adjust the question a bit:

How and When to use async and await Tasks?

Because learning Task automatically covers the other two(and answers your question).

The whole idea is pretty simple. A method can return any data type(double, int, object, etc.) but here it is denied and a 'Task' object return is enforced! But we still need the returned data(except void), right? That will be set in a standard property inside 'Task' object eg: 'Result' property. Then we box/unbox(convert to our data type of choice).

Quickly get through syntax sugar: 5 mins

• Original non-async method

internal static int Method(int arg0, int arg1)
        {
            int result = arg0 + arg1;
            IO(); // Do some long running IO.
            return result;
        }

• a brand new Task-ified method to call the above method

internal static Task<int> MethodTask(int arg0, int arg1)
    {
        Task<int> task = new Task<int>(() => Method(arg0, arg1));
        task.Start(); // Hot task (started task) should always be returned.
        return task;
    }

Did we mention await or async? No. Call the above method and you get a task which you can monitor. You already know what the task returns(or contains).. an integer.

• Calling a Task is slightly tricky and that is when the keywords starts to appear. If there was a method calling the original method(non-async) then we need to edit it as given below. Let us call MethodTask()

internal static async Task<int> MethodAsync(int arg0, int arg1)
    {
        int result = await HelperMethods.MethodTask(arg0, arg1);
        return result;
    }

Same code above added as image below:

1. We are 'awaiting' task to be finished. Hence the await(mandatory syntax)
2. Since we use await, we must use async(mandatory syntax)
3. MethodAsync with Async as the prefix (coding standard)

await is easy to understand but the remaining two (async,Async) may not be :). Well, it should make a lot more sense to the compiler though.Further reads for later here

So there are 2 parts.

1. Create 'Task' (only one task and it will be an additional method)

2. Create syntactic sugar to call the task with await+async(this involves changing existing code if you are converting a non-async method)

Remember, we had an external caller to AccessTheWebAsync() and that caller is not spared either... i.e it needs the same await+async too. And the chain continues(hence this is a breaking change which could affect many classes). It can also be considered a non-breaking change because the original method is still there to be called. Change it's access (or delete and move it inside a task) if you want to impose a breaking change and then the classes will be forced to use Task-method. Anyways, in an async call there will always be a Task at one end and only one.

All okay, but one developer was surprised to see Task missing...

Share the confusion of a developer: 5 mins

A developer has made a mistake of not implementing Task but it still works! Try to understand the question and just the accepted answer provided here. Hope you have read and fully understood. The summary is that we may not see/implement 'Task' but it is implemented somewhere in a parent/associated class. Likewise in our example calling an already built MethodAsync() is way easier than implementing that method with a Task (MethodTask()) ourself. Most developers find it difficult to get their head around Tasks while converting a code to Asynchronous one.

Tip: Try to find an existing Async implementation (like MethodAsync or ToListAsync) to outsource the difficulty. So we only need to deal with Async and await (which is easy and pretty similar to normal code)

Problem: Quickly change a real-world implementation of normal code to Async operation: 2 mins

Code line shown below in Data Layer started to break(many places). Because we updated some of our code from .Net framework 4.2.* to .Net core. We had to fix this in 1 hour all over the application!

var myContract = query.Where(c => c.ContractID == _contractID).First();

easypeasy!

1. We installed EntityFramework nuget package because it has QueryableExtensions. Or in other words it does the Async implementation(task), so we could survive with simple Async and await in code.
2. namespace = Microsoft.EntityFrameworkCore

calling code line got changed like this

var myContract = await query.Where(c => c.ContractID == _contractID).FirstAsync();

3. Method signature changed from

Contract GetContract(int contractnumber)

to

async Task<Contract> GetContractAsync(int contractnumber)

4. calling method also got affected: GetContract(123456); was called as GetContractAsync(123456).Result;

Wait! what is that Result? Good catch! GetContractAsync only returns a Task not the value we wanted(Contract). Once the result of an operation is available, it is stored and is returned immediately on subsequent calls to the Result property. We can also do a time-out implementation with a similar 'Wait()'

TimeSpan ts = TimeSpan.FromMilliseconds(150);

if (! t.Wait(ts)) Console.WriteLine("The timeout interval elapsed.");

5. We changed it everywhere in 30 minutes!

But the architect told us not to use EntityFramework library just for this! oops! drama! Then we made a custom Task implementation(yuk!). Which you know how. Still easy! ..still yuk..

Where to Next? There is a wonderful quick video we could watch about Converting Synchronous Calls to Asynchronous in ASP.Net Core. Or have I explained enough? ;)

Fulfillment

Nice answer. You just might want to fix a couple of small things like: (a) mention of ".Net framework 4.2" (no such version that I know of, exists) (b) casing in EntityFrameWork => EntityFramework – Crary

Asynchronous programming is such a blind spot for me no matter how much I read, but this helped. Seeing Task<string> getStringTask = client.GetStringAsync("..."); separated out from the string urlContents = await getStringTask; finally made it make some sense. Almost every example you see just shows something like string urlContents = await client.GetStringAsync("..."); and I've just never understood what the point of that was (maybe there isn't any!). – Overcapitalize

@PhilipStratford one reason I could think of is when a parent class calls it(eg: calling AccessTheWebAsync) and this parent class does not have a requirement to multi task but is forced to use it. But inside AccessTheWebAsync multi tasking is ideal hence the split using tasks(and also the reason for async implementation). – Fulfillment

Answer 6

Showing the above explanations in action in a simple console program:

class Program
{
    static void Main(string[] args)
    {
        TestAsyncAwaitMethods();
        Console.WriteLine("Press any key to exit...");
        Console.ReadLine();
    }

    public async static void TestAsyncAwaitMethods()
    {
        await LongRunningMethod();
    }

    public static async Task<int> LongRunningMethod()
    {
        Console.WriteLine("Starting Long Running method...");
        await Task.Delay(5000);
        Console.WriteLine("End Long Running method...");
        return 1;
    }
}

And the output is:

Starting Long Running method...
Press any key to exit...
End Long Running method...

Thus,

1. Main starts the long running method via TestAsyncAwaitMethods. That immediately returns without halting the current thread and we immediately see 'Press any key to exit' message
2. All this while, the LongRunningMethod is running in the background. Once its completed, another thread from Threadpool picks up this context and displays the final message

Thus, not thread is blocked.

Leggat

"Press any key to exit..." will be showed in which part of output? – Asthenia

and what's the usage of (return 1)? is it necessary? – Asthenia

@StudioX i think it must have return type integer – Lignite

I think the return 1 part deserves some further explanation: the await keyword allows you to return the underlying type of Task<T> directly, thus making it easier to adapt your exiting code to the await/async world. But you don't have to return a value, as it is possible to return a Task without specifying a returning type, which would be the equivalent of a synchronous void method. Mind that C# allows async void methods, but you should avoid doing so unless you're tackling event handlers. – Nitty

Answer 7

I think you've picked a bad example with System.Threading.Thread.Sleep

Point of an async Task is to let it execute in background without locking the main thread, such as doing a DownloadFileAsync

System.Threading.Thread.Sleep isn't something that is "being done", it just sleeps, and therefore your next line is reached after 5 seconds ...

Read this article, I think it is a great explanation of async and await concept: http://msdn.microsoft.com/en-us/library/vstudio/hh191443.aspx

Nutter

Why Sleep is bad example but Download is good example. It is like FooBar kind of thing when I see Thread.Sleep I understand there is some task that takes times. I think his question is relevant – Tver

@Tver Thread.Sleep blocks the thread (the thread cannot do anything else but sit idle), but an async method does not. In the case of DownloadFileAsync, the thread can go and do something else until a reply comes from the remote server. A better placeholder for "some task that takes time" in an async method is Task.Delay, since that is actually asynchronous. – Danuloff

@GabrielLuci my objection is not about Delay vs Sleep; Your answer is looking more like a strawman answer; If you put this as a comment to the question that would be nothing I could object, but as an answer it smells more like a strawman answer. I think it is still fine to use async there even all calls he/she have to be making will be blocking calls; It won't invalidate all the purpose... Even all left will be syntactic sugar it counts as a valid case, – Tver

This wasn't my answer. But to address your point: it depends on the purpose of the method. If he just wanted a method to call, he succeeded. But in this case he was trying to make a method that runs asynchronously. He did that by just using the async keyword. But his method still ran synchronously, and this answer explained perfectly why: because he didn't actually run any asynchronous code. Methods marked async still run synchronously up until you await an incomplete Task. If there is no await, then the method runs synchronously, and the compiler will warn you of that. – Danuloff

Answer 8

Async & Await Simple Explanation

Simple Analogy

A person may wait for their morning train. This is all they are doing as this is their primary task that they are currently performing. (synchronous programming (what you normally do!))

Another person may await their morning train whilst they smoke a cigarette and then drink their coffee. (Asynchronous programming)

What is asynchronous programming?

Asynchronous programming is where a programmer will choose to run some of his code on a separate thread from the main thread of execution and then notify the main thread on it's completion.

What does the async keyword actually do?

Prefixing the async keyword to a method name like

async void DoSomething(){ . . .

allows the programmer to use the await keyword when calling asynchronous tasks. That's all it does.

Why is this important?

In a lot of software systems the main thread is reserved for operations specifically relating to the User Interface. If I am running a very complex recursive algorithm that takes 5 seconds to complete on my computer, but I am running this on the Main Thread (UI thread) When the user tries to click on anything on my application, it will appear to be frozen as my main thread has queued and is currently processing far too many operations. As a result the main thread cannot process the mouse click to run the method from the button click.

When do you use Async and Await?

Use the asynchronous keywords ideally when you are doing anything that doesn't involve the user interface.

So lets say you're writing a program that allows the user to sketch on their mobile phone but every 5 seconds it is going to be checking the weather on the internet.

We should be awaiting the call the polling calls every 5 seconds to the network to get the weather as the user of the application needs to keep interacting with the mobile touch screen to draw pretty pictures.

How do you use Async and Await

Following on from the example above, here is some pseudo code of how to write it:

    //ASYNCHRONOUS
    //this is called using the await keyword every 5 seconds from a polling timer or something.

    async Task CheckWeather()
    {
        var weather = await GetWeather();
        //do something with the weather now you have it
    }

    async Task<WeatherResult> GetWeather()
    {

        var weatherJson = await CallToNetworkAddressToGetWeather();
        return deserializeJson<weatherJson>(weatherJson);
    }

    //SYNCHRONOUS
    //This method is called whenever the screen is pressed
    void ScreenPressed()
    {
        DrawSketchOnScreen();
    }

Additional Notes - Update

I forgot to mention in my original notes that in C# you can only await methods that are wrapped in Tasks. for example you may await this method:

// awaiting this will return a string.
// calling this without await (synchronously) will result in a Task<string> object.
async Task<string> FetchHelloWorld() {..

You cannot await methods that are not tasks like this:

async string FetchHelloWorld() {..

Feel free to review the source code for the Task class here.

Suber

that was a insightful real world analogy! sometimes for some developers understanding the technical parts becomes a lot easy when a real time analogy is made. thanks for it – Countermeasure

This answer is neither simple, nor an explanation. – Comprehension

Thank you for your criticism. People ingest information and learn differently, how could I improve my answer? What was difficult to understand? @Comprehension – Suber

Asynchronous programming is where a programmer will choose to run some of his code on a separate thread from the main thread of execution and then notify the main thread on it's completion. That part could use a little refactoring, async is not parallel and is possible in sigle-threaded languages/frameworks. What it really does is release the thread (even the main one) when it's awaiting some extensive work to be done by anything other than the processor like disk, db, api call etc... When it yelds results some thread (same or another) resumes the processing of your program. – Selmner

Even better, async is mandatory in single-threaded environments – Selmner

@Selmner Okay, but isn't it in MOST CASES like that calling the async task makes it run on another thread, hence isn't it quasi multi-thread ? Although as you suggest the case could be in single-threaded environment, that it just releases THE SAME single thread, which otherwise would just wait for some foreign operation to finish (e.g. DB, or file system storage, network data transmission ...), which could be running in totally different process. (Yet it is sort-of parallel programming even in a single thread world.) – Supplicatory

this is a great answer for a newbie. It helped me make sense of some of the other answers. – Nobile

Answer 9

Here is a quick console program to make it clear to those who follow. The TaskToDo method is your long running method that you want to make async. Making it run async is done by the TestAsync method. The test loops method just runs through the TaskToDo tasks and runs them async. You can see that in the results because they don't complete in the same order from run to run - they are reporting to the console UI thread when they complete. Simplistic, but I think the simplistic examples bring out the core of the pattern better than more involved examples:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace TestingAsync
{
    class Program
    {
        static void Main(string[] args)
        {
            TestLoops();
            Console.Read();
        }

        private static async void TestLoops()
        {
            for (int i = 0; i < 100; i++)
            {
                await TestAsync(i);
            }
        }

        private static Task TestAsync(int i)
        {
            return Task.Run(() => TaskToDo(i));
        }

        private async static void TaskToDo(int i)
        {
            await Task.Delay(10);
            Console.WriteLine(i);
        }
    }
}

Diffidence

Answer 10

All the answers here use Task.Delay() or some other built in async function. But here is my example that use none of those async functions:

// Starts counting to a large number and then immediately displays message "I'm counting...". 
// Then it waits for task to finish and displays "finished, press any key".
static void asyncTest ()
{
    Console.WriteLine("Started asyncTest()");
    Task<long> task = asyncTest_count();
    Console.WriteLine("Started counting, please wait...");
    task.Wait(); // if you comment this line you will see that message "Finished counting" will be displayed before we actually finished counting.
    //Console.WriteLine("Finished counting to " + task.Result.ToString()); // using task.Result seems to also call task.Wait().
    Console.WriteLine("Finished counting.");
    Console.WriteLine("Press any key to exit program.");
    Console.ReadLine();
}

static async Task<long> asyncTest_count()
{
    long k = 0;
    Console.WriteLine("Started asyncTest_count()");
    await Task.Run(() =>
    {
        long countTo = 100000000;
        int prevPercentDone = -1;
        for (long i = 0; i <= countTo; i++)
        {
            int percentDone = (int)(100 * (i / (double)countTo));
            if (percentDone != prevPercentDone)
            {
                prevPercentDone = percentDone;
                Console.Write(percentDone.ToString() + "% ");
            }

            k = i;
        }
    });
    Console.WriteLine("");
    Console.WriteLine("Finished asyncTest_count()");
    return k;
}

Hizar

Thank you! the first answer that actually does some work instead of waiting. – Lavallee

thank you for showing task.Wait(); and how it can be used to avoid async/await hell :P – Thornburg

@Thornburg task.wait() and async/await are not the same concept, be careful. One applies to parallel programming thread syncronization and the other to asyncronous programming thread release. They are oposite. wait() block, await avoid blocking... Problem is C# uses Task to represent them both... so you can use the wrong one... – Selmner

This awnser show cases an use case of parallel programming (processing intensive work) disguised as asyncronous. @Jeffnl, async was created to await, not to do other stuff, its not parallel programing. Take a look at my anwser, it may become clearer, but await is used to release the thread when waiting for something that do not requires processing, like disk read/write, DB querys, API calls etc... that release thread can do other work, but not in the same code, maybe in another request (web) or process in desktop. When your result is done, the same or another thread, resumes your execution. – Selmner

Answer 11

This answer aims to provide some info specific to ASP.NET.

By utilizing async/await in the MVC controller, it is possible to increase thread pool utilization and achieve much better throughput, as explained in the below article,

http://www.asp.net/mvc/tutorials/mvc-4/using-asynchronous-methods-in-aspnet-mvc-4

In web applications that see a large number of concurrent requests at start-up or have a bursty load (where concurrency increases suddenly), making these web service calls asynchronous will increase the responsiveness of your application. An asynchronous request takes the same amount of time to process as a synchronous request. For example, if a request makes a web service call that requires two seconds to complete, the request takes two seconds whether it is performed synchronously or asynchronously. However, during an asynchronous call, a thread is not blocked from responding to other requests while it waits for the first request to complete. Therefore, asynchronous requests prevent request queuing and thread pool growth when there are many concurrent requests that invoke long-running operations.

Davao

Answer 12

Async / Await

Actually, Async / Await is a pair of keywords that are just syntactic sugar for creating a callback of an asynchronous task.

Take by example this operation:

public static void DoSomeWork()
{
    var task = Task.Run(() =>
    {
        // [RUNS ON WORKER THREAD]

        // IS NOT bubbling up due to the different threads
        throw new Exception();
        Thread.Sleep(2000);

        return "Hello";
    });

    // This is the callback
    task.ContinueWith((t) => {
        // -> Exception is swallowed silently
        Console.WriteLine("Completed");

        // [RUNS ON WORKER THREAD]
    });
}

The code above has several disadvantages. Errors are not passed on and it's hard to read. But Async and Await come in to help us out:

public async static void DoSomeWork()
{
    var result = await Task.Run(() =>
    {
        // [RUNS ON WORKER THREAD]

        // IS bubbling up
        throw new Exception();
        Thread.Sleep(2000);

        return "Hello";
    });

    // every thing below is a callback 
    // (including the calling methods)

    Console.WriteLine("Completed");
}

Await calls have to be in Async methods. This has some advantages:

• Returns the result of the Task
• creates automatically a callback
• checks for errors and lets them bubble up in callstack (only up to none-await calls in callstack)
• waits for the result
• frees up the main thread
• runs the callback on the main thread
• uses a worker thread from the threadpool for the task
• makes the code easy to read
• and a lot more

NOTE: Async and Await are used with asynchronous calls not to make these. You have to use Task Libary for this, like Task.Run() .

Here is a comparison between await and none await solutions

This is the none async solution:

public static long DoTask()
{
    stopWatch.Reset();
    stopWatch.Start();

    // [RUNS ON MAIN THREAD]
    var task = Task.Run(() => {
        Thread.Sleep(2000);
        // [RUNS ON WORKER THREAD]
    });
    // goes directly further
    // WITHOUT waiting until the task is finished

    // [RUNS ON MAIN THREAD]

    stopWatch.Stop();
    // 50 milliseconds
    return stopWatch.ElapsedMilliseconds;
}

This is the async method:

public async static Task<long> DoAwaitTask()
{
    stopWatch.Reset();
    stopWatch.Start();

    // [RUNS ON MAIN THREAD]

    await Task.Run(() => {
        Thread.Sleep(2000);
        // [RUNS ON WORKER THREAD]
    });
    // Waits until task is finished

    // [RUNS ON MAIN THREAD]

    stopWatch.Stop();
    // 2050 milliseconds
    return stopWatch.ElapsedMilliseconds;
}

You can actually call an async method without the await keyword but this means that any Exception here is swallowed in release mode:

public static Stopwatch stopWatch { get; } = new Stopwatch();

static void Main(string[] args)
{
    Console.WriteLine("DoAwaitTask: " + DoAwaitTask().Result + " ms");
    // 2050 (2000 more because of the await)
    Console.WriteLine("DoTask: " + DoTask() + " ms");
    // 50
    Console.ReadKey();
}

Async and Await are not meant for parallel computing. They are used to not block your main thread. When it's about asp.net or Windows applications, blocking your main thread due to a network call is a bad thing. If you do this, your app will get unresponsive or even crash.

Check out MS docs for more examples.

Frodin

Answer 13

I'd like to give my two cents to this, I'm sorry if any other answer contains what I will explain, I read most of it and haven't find it, but I could have missed something.

I saw a lot of missconceptions and a lot of good explanations, just want to explain async in terms of how it differs from parallel programming, that I believe will make things easier to understand.

When you need to do long computations, processor intensive work, you should opt to use parallel programming, if it's possible, to optimize cores usage. This opens some threads and process things simultaneosly.

Say you have an array of numbers and want to make some expensive long calculation with every and each one of than. Parallel is your friend.

Asyncronous programming is used in a different use case.

It's used to free your thread when you are waiting for something that do not depend on your processor, like IO for example (writing and reading to/from disk), your thread does nothing when you do IO, same thing when you are awaiting for some result from an expensive query to return from DB.

Async methods free your thread when it's waiting for something long to return results. This thread can be used by other parts of your application (in a web app it process other requests, for example) or can return to OS for other use.

When your result is done, the same thread (or another one) is given back to your application to resume processing.

Async programming is not mandatory (but a good practice) in a multithreaded environment like .net, in a web app other threads will respond to new requests, but if you are in a singlethreaded framework like nodejs it's mandatory, because you can't block your only thread, or you won't be able to anwser any other request.

To summarize, long processor intensive calculations will benefit more from parallel programming and long waiting periods that do not depend on your processor, like IO or DB query or a call to some API will benefit more from async programming.

That's why Entity Framework, for example, has an async api to save, list, find, etc...

Remember that async/await is not the same as wait or waitAll, the contexts are different. Async/await release the thread and are asyncronous programming. wait / waitAll blocks all threads (they are not released) to force syncronization in parallel context... different stuff...

Hope this is usefull for someone...

Selmner

Nice answer! simple and informative – Hardner

Answer 14

To be honest I still think the best explanation is the one about future and promises on the Wikipedia: http://en.wikipedia.org/wiki/Futures_and_promises

The basic idea is that you have a separate pool of threads that execute tasks asynchronously. When using it. The object does however make the promise that it will execute the operation at some time and give you the result when you request it. This means that it will block when you request the result and hasn't finished, but execute in the thread pool otherwise.

From there you can optimize things: some operations can be implemented async and you can optimize things like file IO and network communication by batching together subsequent requests and/or reordering them. I'm not sure if this is already in the task framework of Microsoft - but if it isn't that would be one of the first things I would add.

You can actually implement the future pattern sort-of with yields in C# 4.0. If you want to know how it works exactly, I can recommend this link that does a decent job: http://code.google.com/p/fracture/source/browse/trunk/Squared/TaskLib/ . However, if you start toying with it yourself, you will notice that you really need language support if you want to do all the cool things -- which is exactly what Microsoft did.

Tridentine

Answer 15

See this fiddle https://dotnetfiddle.net/VhZdLU (and improve it if possible) for running a simple console application which shows usages of Task, Task.WaitAll(), async and await operators in the same program.

This fiddle should clear your execution cycle concept.

Here is the sample code

using System;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {               
        var a = MyMethodAsync(); //Task started for Execution and immediately goes to Line 19 of the code. Cursor will come back as soon as await operator is met       
        Console.WriteLine("Cursor Moved to Next Line Without Waiting for MyMethodAsync() completion");
        Console.WriteLine("Now Waiting for Task to be Finished");       
        Task.WaitAll(a); //Now Waiting      
        Console.WriteLine("Exiting CommandLine");       
    }

    public static async Task MyMethodAsync()
    {
        Task<int> longRunningTask = LongRunningOperation();
        // independent work which doesn't need the result of LongRunningOperationAsync can be done here
        Console.WriteLine("Independent Works of now executes in MyMethodAsync()");
        //and now we call await on the task 
        int result = await longRunningTask;
        //use the result 
        Console.WriteLine("Result of LongRunningOperation() is " + result);
    }

    public static async Task<int> LongRunningOperation() // assume we return an int from this long running operation 
    {
        Console.WriteLine("LongRunningOperation() Started");
        await Task.Delay(2000); // 2 second delay
        Console.WriteLine("LongRunningOperation() Finished after 2 Seconds");
        return 1;
    }   

}

Trace coming from Output Window:

Thema

this confused me even more.. – Sucre

you lied! (or this is simply incomplete!) you didn't show usages of Task, Task.WaitAll() (I checked the fiddle - not there either!) – Tucana

Answer 16

On a higher level:

1) Async keyword enables the await and that's all it does. Async keyword does not run the method in a separate thread. The beginning f async method runs synchronously until it hits await on a time-consuming task.

2) You can await on a method that returns Task or Task of type T. You cannot await on async void method.

3) The moment main thread encounters await on time-consuming task or when the actual work is started, the main thread returns to the caller of the current method.

4) If the main thread sees await on a task that is still executing, it doesn't wait for it and returns to the caller of the current method. In this way, the application remains responsive.

5) Await on processing task, will now execute on a separate thread from the thread pool.

6) When this await task is completed, all the code below it will be executed by the separate thread

Below is the sample code. Execute it and check the thread id

using System;
using System.Threading;
using System.Threading.Tasks;

namespace AsyncAwaitDemo
{
    class Program
    {
        public static async void AsynchronousOperation()
        {
            Console.WriteLine("Inside AsynchronousOperation Before AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
            //Task<int> _task = AsyncMethod();
            int count = await AsyncMethod();

            Console.WriteLine("Inside AsynchronousOperation After AsyncMethod Before Await, Thread Id: " + Thread.CurrentThread.ManagedThreadId);

            //int count = await _task;

            Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await Before DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);

            DependentMethod(count);

            Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await After DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
        }

        public static async Task<int> AsyncMethod()
        {
            Console.WriteLine("Inside AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
            int count = 0;

            await Task.Run(() =>
            {
                Console.WriteLine("Executing a long running task which takes 10 seconds to complete, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(20000);
                count = 10;
            });

            Console.WriteLine("Completed AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);

            return count;
        }       

        public static void DependentMethod(int count)
        {
            Console.WriteLine("Inside DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId + ". Total count is " + count);
        }

        static void Main(string[] args)
        {
            Console.WriteLine("Started Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);

            AsynchronousOperation();

            Console.WriteLine("Completed Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);

            Console.ReadKey();
        }

    }
}

Tiana

Answer 17

The way I understand it is also, there should be a third term added to the mix: Task.

Async is just a qualifier you put on your method to say it's an asynchronous method.

Task is the return of the async function. It executes asynchronously.

You await a Task. When code execution reaches this line, control jumps out back to caller of your surrounding original function.

If instead, you assign the return of an async function (ie Task) to a variable, when code execution reaches this line, it just continues past that line in the surrounding function while the Task executes asynchronously.

Glasper

Answer 18

The best example is here,enjoy:

namespace ConsoleTestApp
{
    class Program
    {        
        static async Task Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            Test1Async(3000);
            Test1Async(2000);
            Console.WriteLine("next statement");
            Console.ReadLine();
        }

        public static async Task Test1Async(int t)
        {
            Console.WriteLine("delaying " + t);
            await Task.Delay(t);
            Console.WriteLine("delay " + t + " completed");
        }

    }
}

Vascular

Hi Madhusudhan. Be aware that posting code as an image is generally frowned upon in this site. – Banebrudge

I wouldn't call it 'Best'. What you have here is fire and forget approach and is not recommended. If there are exceptions in Test1Async, they will not be captured in the Main method. – Textualist

Answer 19

Answering your second question - WHEN to use async - here's a fairly easy approach we use:

1. Long-running non-CPU task (disk I/O, network, GPU, Bluetooth, USB you name it) - use async.
2. Long-running CPU-bound task - use parallel execution, threads etc.

Explanation: when you're doing "Type-1" work - sending a network request, reading data from disk, or perform GPU calculations etc - the actual work is done by "external" silicon (network card, disk controller, nVidia chip, Bluetooth controller, USB etc). Once the work is done - that external device's driver will "ping" the OS back, and the OS will execute your continuation code or callback. Until then the CPU is free to do it's own work (and as a bonus you also free up a threadpool thread which is a nice bonus for web app scalability)

Mellifluent

Answer 20

public static void Main(string[] args)
{
    string result = DownloadContentAsync().Result;
    Console.ReadKey();
}

// You use the async keyword to mark a method for asynchronous operations.
// The "async" modifier simply starts synchronously the current thread. 
// What it does is enable the method to be split into multiple pieces.
// The boundaries of these pieces are marked with the await keyword.
public static async Task<string> DownloadContentAsync()// By convention, the method name ends with "Async
{
    using (HttpClient client = new HttpClient())
    {
        // When you use the await keyword, the compiler generates the code that checks if the asynchronous operation is finished.
        // If it is already finished, the method continues to run synchronously.
        // If not completed, the state machine will connect a continuation method that must be executed WHEN the Task is completed.


        // Http request example. 
        // (In this example I can set the milliseconds after "sleep=")
        String result = await client.GetStringAsync("http://httpstat.us/200?sleep=1000");

        Console.WriteLine(result);

        // After completing the result response, the state machine will continue to synchronously execute the other processes.


        return result;
    }
}

Hippel

Answer 21

is using them equal to spawning background threads to perform long duration logic?

This article MDSN:Asynchronous Programming with async and await (C#) explains it explicitly:

The async and await keywords don't cause additional threads to be created. Async methods don't require multithreading because an async method doesn't run on its own thread. The method runs on the current synchronization context and uses time on the thread only when the method is active.

Tideway

Answer 22

Below is code which reads excel file by opening dialog and then uses async and wait to run asynchronous the code which reads one by one line from excel and binds to grid

namespace EmailBillingRates
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            lblProcessing.Text = "";
        }

        private async void btnReadExcel_Click(object sender, EventArgs e)
        {
            string filename = OpenFileDialog();

            Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel.Workbook xlWorkbook = xlApp.Workbooks.Open(filename);
            Microsoft.Office.Interop.Excel._Worksheet xlWorksheet = xlWorkbook.Sheets[1];
            Microsoft.Office.Interop.Excel.Range xlRange = xlWorksheet.UsedRange;
            try
            {
                Task<int> longRunningTask = BindGrid(xlRange);
                int result = await longRunningTask;

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message.ToString());
            }
            finally
            {
                //cleanup  
               // GC.Collect();
                //GC.WaitForPendingFinalizers();

                //rule of thumb for releasing com objects:  
                //  never use two dots, all COM objects must be referenced and released individually  
                //  ex: [somthing].[something].[something] is bad  

                //release com objects to fully kill excel process from running in the background  
                Marshal.ReleaseComObject(xlRange);
                Marshal.ReleaseComObject(xlWorksheet);

                //close and release  
                xlWorkbook.Close();
                Marshal.ReleaseComObject(xlWorkbook);

                //quit and release  
                xlApp.Quit();
                Marshal.ReleaseComObject(xlApp);
            }

        }

        private void btnSendEmail_Click(object sender, EventArgs e)
        {

        }

        private string OpenFileDialog()
        {
            string filename = "";
            OpenFileDialog fdlg = new OpenFileDialog();
            fdlg.Title = "Excel File Dialog";
            fdlg.InitialDirectory = @"c:\";
            fdlg.Filter = "All files (*.*)|*.*|All files (*.*)|*.*";
            fdlg.FilterIndex = 2;
            fdlg.RestoreDirectory = true;
            if (fdlg.ShowDialog() == DialogResult.OK)
            {
                filename = fdlg.FileName;
            }
            return filename;
        }

        private async Task<int> BindGrid(Microsoft.Office.Interop.Excel.Range xlRange)
        {
            lblProcessing.Text = "Processing File.. Please wait";
            int rowCount = xlRange.Rows.Count;
            int colCount = xlRange.Columns.Count;

            // dt.Column = colCount;  
            dataGridView1.ColumnCount = colCount;
            dataGridView1.RowCount = rowCount;

            for (int i = 1; i <= rowCount; i++)
            {
                for (int j = 1; j <= colCount; j++)
                {
                    //write the value to the Grid  
                    if (xlRange.Cells[i, j] != null && xlRange.Cells[i, j].Value2 != null)
                    {
                         await Task.Delay(1);
                         dataGridView1.Rows[i - 1].Cells[j - 1].Value =  xlRange.Cells[i, j].Value2.ToString();
                    }

                }
            }
            lblProcessing.Text = "";
            return 0;
        }
    }

    internal class async
    {
    }
}

Kollwitz

Answer 23

The answers here are useful as a general guidance about await/async. They also contain some detail about how await/async is wired. I would like to share some practical experience with you that you should know before using this design pattern.

The term "await" is literal, so whatever thread you call it on will wait for the result of the method before continuing. On the foreground thread, this is a disaster. The foreground thread carries the burden of constructing your app, including views, view models, initial animations, and whatever else you have boot-strapped with those elements. So when you await the foreground thread, you stop the app. The user waits and waits when nothing appears to happen. This provides a negative user experience.

You can certainly await a background thread using a variety of means:

Device.BeginInvokeOnMainThread(async () => { await AnyAwaitableMethod(); });

// Notice that we do not await the following call, 
// as that would tie it to the foreground thread.
try
{
Task.Run(async () => { await AnyAwaitableMethod(); });
}
catch
{}

The complete code for these remarks is at https://github.com/marcusts/xamarin-forms-annoyances. See the solution called AwaitAsyncAntipattern.sln.

The GitHub site also provides links to a more detailed discussion on this topic.

Reese

From what I understand, async / await is syntactic sugar for callbacks, it has nothing to do with threading. msdn.microsoft.com/en-us/magazine/hh456401.aspx It is for non-CPU bound code, e.g. waiting for input or a delay. Task.Run should only be used for CPU-bound code blog.stephencleary.com/2013/10/… – Cognizance

The term "await" is literal, so whatever thread you call it on will wait for the result of the method before continuing. This is not true - maybe you meant Task.Wait()? When you use await, it sets the rest of the method as a continuation to be executed when whatever you await-ed is complete. It exits the method that you used it in, so the caller can continue. Then when the await-ed line is actually complete, it finishes the remainder of that method on some thread (usually a worker thread). – Bristol

@Cognizance at the very core, async/await is about freeing up .NET Threads. When you await a truly-async operation (such as .NET's File.WriteAsync), it suspends the remainder of the method you used await in, so the caller can continue and potentially finish its purpose. There is no thread blocking or waiting for the await-ed operation. When the operation you awaited is complete, the remainder of the async/await method is put on a thread and executed (similar to a call-back idea). – Bristol

Sorry man, but this is not right... @DonCheadle pointed the mistake, Task.wait is not the same as async/await. There is a lot of confusion between parallel programming and asyncronous programming. Wait or WaitAll blocks to syncronize threads running in parallel, async/await frees the thread to do other work when it's awaiting. – Selmner

I believe the anti-pattern is to async/await for syncronous stuff, there is not point in that...you should only await for async native methods. To point out that this do not apply exclusive to IO, I will use as example Entity Framework's saveAsync or toListAsync (still IO but normally in another server) or an API call made with an async request method. – Selmner

Answer 24

Async just enables us to use the await. It does nothing if we do not use await in the function. when using await we need to write nearby a name of an awaitable, most common is the Task or Task. The await makes the operation of the Task to run until complete. Only when it comletes, the next code of the function is execute.

Retake

Answer 25

The async is used with a function to makes it into an asynchronous function. The await keyword is used to invoke an asynchronous function synchronously. The await keyword holds the JS engine execution until promise is resolved.

We should use async & await only when we want the result immediately. Maybe the result returned from the function is getting used in the next line.

Follow this blog, It is very well written in simple word

Sleet

Answer 26

Maybe my insight is relevant. async tells the compiler to treat a function specially, the function is suspendable/resumable, it saves state in some way. await suspends a function, but is also a way to enforce discipline, is restrictive; you need to specify what you are waiting for, you can't just suspend without cause, which is what makes the code more readable and perhaps also more efficient. This opens up another question. Why not await multiple things, why just one at a time? I believe this is because such a pattern established itself and programmers are following the principle of least astonishment. There exists the possibility of ambiguity: are you satisfied with just one of conditions being fulfilled, or do you want all to be fulfilled, perhaps just some of them?

Urochrome