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

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?

async

await

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;



var retVal = Task.Run(async () => await MyAsyncMethod(param1)).Result;

DoSomethingAsync()

await

21 Answers
21

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

async

await

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:

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

Task<int> longRunningTask = LongRunningOperationAsync();

LongRunningOperation

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

await longRunningTask

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).

longRunningTask

MyMethodAsync()

longRunningTask

MyMethodAsync()

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.

longRunningTask

await longRunningTask

Task.Delay(1000)

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";



Task.Delay

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 uderstand why this method must be marked as "async'.

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

async

await

await

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.

async

//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.

async

await

async

async

Sleep()

DoSomethingAsync()

button1_Click()

DoSomethingAsync()

Thread.Sleep()

Task.Delay() does not.

unusually

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.

Task.Delay(1000)

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.

Task.Delay

await a

b

c

d

e

a

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.

await

await

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)


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.

ConfigureAwait(false)

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.

await

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;

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.

await

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,

Thus, not thread is blocked.

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

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

async

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 ...

System.Threading.Thread.Sleep

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

async

await

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);





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

By utilizing async/await in MVC controller, it is possible to increase thread pool utilization and achieve a 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 sees a large number of concurrent requests at
start-up or has 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.

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 numbewr 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;



task.Wait();

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.

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:

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

At #6 step: AccessTheWebAsync() has run out of work that it can do without a result from getStringTask. Therefore, AccessTheWebAsync uses an await operator to suspend its progress and give control back(yield) to the caller. AccessTheWebAsync returns a Task(of string return value) to the caller. The task represents a promise to produce a string result. But when will it return the call? a 2nd call again?

The caller of AccessTheWebAsync() did nothing but wait(it could have done some internal tasks and then waited if needed). So caller waiting for AccessTheWebAsync, and AccessTheWebAsync is waiting for GetStringAsync at the moment.

Remember, the method was already returned, it cannot return again(no second time). So how will the caller know? It is all about Tasks! Task was returned. Task was waited for (not method, not value). Value will be set in Task. Task status will be set to complete. Caller just monitors Task. 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?

async

await

Tasks

Because learning Task automatically covers the other 2. For the sake of learning at least. Of course, this is an answer to your question about async and await.

Task

async

await

Quickly get through syntax sugar: 5 mins

Before conversion(original method)

internal static int Method(int arg0, int arg1)

int result = arg0 + arg1;
IO(); // Do some long running IO.
return result;


internal static int Method(int arg0, int arg1)

int result = arg0 + arg1;
IO(); // Do some long running IO.
return result;


Another 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;


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.. an integer.

Calling a Task is slightly tricky. Let us call MethodTask()

internal static async Task<int> MethodAsync(int arg0, int arg1)

int result = await HelperMethods.MethodTask(arg0, arg1);
return result;


internal static async Task<int> MethodAsync(int arg0, int arg1)

int result = await HelperMethods.MethodTask(arg0, arg1);
return result;


We are 'awaiting' task to be finished. Hence the await. Since we use await, we must use async(mandatory) and MethodAsync with 'Async' as the prefix (coding standard). Further reads for later here

await

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. 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.

Task

MethodAsync()

Task

MethodTask()

Tasks

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)

MethodAsync

ToListAsync

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!

code got changed like this

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


Method signature changed from

Contract GetContract(int contractnumber)

Contract GetContract(int contractnumber)

to

async Task<Contract> GetContractAsync(int contractnumber)

async Task<Contract> GetContractAsync(int contractnumber)

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

GetContractAsync(123456);

GetContractAsync(123456).Result;

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. Which you know how. Still easy!

Where to Next?
There is a wonderful quick video we could watch about Converting Synchronous Calls to Asynchronous in ASP.Net Core, because that is likely the direction one would go after reading this.

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;




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

Task

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

Async

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

Task

async

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

await

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.

async

Task

Task

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.

In the following code, the HttpClient method GetByteArrayAsync returns a Task, getContentsTask. The task is a promise to produce the actual byte array when the task is complete. The await operator is applied to getContentsTask to suspend execution in SumPageSizesAsync until getContentsTask is complete. In the meantime, control is returned to the caller of SumPageSizesAsync. When getContentsTask is finished, the await expression evaluates to a byte array.

private async Task SumPageSizesAsync()

// To use the HttpClient type in desktop apps, you must include a using directive and add a
// reference for the System.Net.Http namespace.
HttpClient client = new HttpClient();
// . . .
Task<byte> getContentsTask = client.GetByteArrayAsync(url);
byte urlContents = await getContentsTask;

// Equivalently, now that you see how it works, you can write the same thing in a single line.
//byte urlContents = await client.GetByteArrayAsync(url);
// . . .



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();






Actually Async / Await are a pair of keywords which 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]
);



This method has several disadvantages. Errors are not passed on and it's extremely 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:

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]
);
Thread.Sleep(1000);
// 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 are 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. If it's about asp.net or Windows applications. Blocking your main thread due a network call is a bad thing. If you do this, your app will get unresponsive or might crash.

Check out ms docs to get some examples.

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.

async / await

Task.Run

The term "await" is literal, so whatever thread you call it on will wait for the result of the method before continuing.

await

async/await

await

await

await

await

async/await

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()
All files (*.*)

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





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 every 5 seconds
async void 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();