I'm fairly new to async/await programming and sometimes I feel that I understand it, and then all of a sudden something happens and throws me for a loop.
I'm trying this out in a test winforms app and here is one version of a snippet that I have. Doing it this way will block the UI
private async void button1_Click(object sender, EventArgs e)
{
int d = await DoStuffAsync(c);
Console.WriteLine(d);
}
private async Task<int> DoStuffAsync(CancellationTokenSource c)
{
int ret = 0;
// I wanted to simulator a long running process this way
// instead of doing Task.Delay
for (int i = 0; i < 500000000; i++)
{
ret += i;
if (i % 100000 == 0)
Console.WriteLine(i);
if (c.IsCancellationRequested)
{
return ret;
}
}
return ret;
}
Now, when I make a slight change by wrapping the body of "DoStuffAsync()" in a Task.Run it works perfectly fine
private async Task<int> DoStuffAsync(CancellationTokenSource c)
{
var t = await Task.Run<int>(() =>
{
int ret = 0;
for (int i = 0; i < 500000000; i++)
{
ret += i;
if (i % 100000 == 0)
Console.WriteLine(i);
if (c.IsCancellationRequested)
{
return ret;
}
}
return ret;
});
return t;
}
With all that said, what is the proper way to handle this scenario?
When you write such code:
private async Task<int> DoStuffAsync()
{
return 0;
}
This way you are doing things synchronously, because you are not using await expression.
Pay attention to the warning:
This async method lacks 'await' operators and will run synchronously.
Consider using the 'await' operator to await non-blocking API calls,
or 'await Task.Run(...)' to do CPU-bound work on a background thread.
Based on the warning suggestion you can correct it this way:
private async Task<int> DoStuffAsync()
{
return await Task.Run<int>(() =>
{
return 0;
});
}
To learn more about async/await you can take a look at:
Async and Await by Stephen Cleary
Asynchronous Programming with Async and Await from msdn
You just need to change the DoStuffAsync task little bit, as below.
private async Task<int> DoStuffAsync(CancellationTokenSource c)
{
return Task<int>.Run(()=> {
int ret = 0;
// I wanted to simulator a long running process this way
// instead of doing Task.Delay
for (int i = 0; i < 500000000; i++)
{
ret += i;
if (i % 100000 == 0)
Console.WriteLine(i);
if (c.IsCancellationRequested)
{
return ret;
}
}
return ret;
});
}
Related
C# 8 adds support for asynchronuous iterator blocks, so you can await things and return an IAsyncEnumarator instead of an IEnumerable:
public async IAsyncEnumerable<int> EnumerateAsync() {
for (int i = 0; i < 10; i++) {
yield return i;
await Task.Delay(1000);
}
}
With a non-blocking consuming code that looks like this:
await foreach (var item in EnumerateAsync()) {
Console.WriteLine(item);
}
This will result in my code running for about 10 seconds. However, sometimes I want to break out of the await foreach before all elements are consumed. With an breakhowever, we would need to wait until the current awaited Task.Delay has finished. How can we break immediately out of that loop without waiting for any dangling async tasks?
The use of a CancellationToken is the solution since that is the only thing that can cancel the Task.Delay in your code. The way we get it inside your IAsyncEnumerable is to pass it as a parameter when creating it, so let's do that:
public async IAsyncEnumerable<int> EnumerateAsync(CancellationToken cancellationToken = default) {
for (int i = 0; i < 10; i++) {
yield return i;
await Task.Delay(1000, cancellationToken);
}
}
With the consuming side of:
// In this example the cancellation token will be caneled after 2.5 seconds
var cts = new CancellationTokenSource(TimeSpan.FromSeconds(2.5));
await foreach (var item in EnumerateAsync(cts.Token)) {
Console.WriteLine(item);
}
Sure, this will cancel the enumeration after 3 elements were returned, but will end in an TaskCanceledException thrown out of Task.Delay. To gracefully exit the await foreach we have to catch it and break on the producing side:
public async IAsyncEnumerable<int> EnumerateAsync(CancellationToken cancellationToken = default) {
for (int i = 0; i < 10; i++) {
yield return i;
try {
await Task.Delay(1000, cancellationToken);
} catch (TaskCanceledException) {
yield break;
}
}
}
Note
As of now this is still in preview and is subject to possible change. If you are interested in this topic you can watch a discussion of the C# language team about CancellationToken in an IAsyncEnumeration.
I have a Parallel.For loop which I use to peform a lot of HTTP request at a certain point when a scheduled task occurs like this:
Parallel.For(0, doc.GetElementsByTagName("ItemID").Count, i => {
var xmlResponse = PerformHttpRequestMethod();
});
Is there any way for me to set the loop to pause after the counter value hits 2,4,6,8,10 and so on...
so every 2 method calls it performs, sleep for 2 minutes lets say..
Is there any way I could achieve this ?
I recommend you to use Task.Delay.
Now your method is asyncronous using async/await.
public async Task DoSomething()
{
int i = 0;
while (i < doc.GetElementsByTagName("ItemID").Count)
{
Task.Run(() => PerformHttpRequestMethod());
if(i%2==0){
await Task.Delay(TimeSpan.FromMinutes(2));
//or simply:
await Task.Delay(120000);//120000 represents 2 minutes.
}
i++;
}
}
OR simply if you want to use for loop.
public async Task DoSomething()
{
for (int i = 0; i < doc.GetElementsByTagName("ItemID").Count; i++)
{
Task.Run(() => PerformHttpRequestMethod());
if(i%2==0){
await Task.Delay(TimeSpan.FromMinutes(2));
}
}
}
How would this 2nd example look if I'd want to do iterations from 0 to 4 then sleep 5 to 9 and so on... ?
public async Task DoSomething()
{
for (int i = 0; i < doc.GetElementsByTagName("ItemID").Count; i=i+5)
{
if( i%10 == 0 ){
for( int j=i;j<=i+4;j++){
Task.Run(() => PerformHttpRequestMethod());
}
}
else{
for(int j=i;j<=i+4;j++){
await Task.Delay(TimeSpan.FromMinutes(2));
}
}
}
}
Let's test the correctitude of algorithm.
i=0 -> (0%10==0 ->true) ,then will execute Task.Run(() => PerformHttpRequestMethod()) for i=(0,4)
i=5 -> (5%10==0 ->false), then will execute await Task.Delay(TimeSpan.FromMinutes(2)); for i=(5,9).
And so on...
I don't really see the point of using a Parallel.For if you want to sleep for x number of minutes or seconds every other iteration...how about using a plain old for loop?:
for(int i = 0; i < doc.GetElementsByTagName("ItemID").Count; ++i)
{
var xmlResponse = PerformHttpRequestMethod();
if (i % 2 == 0)
{
System.Threading.Thread.Sleep(TimeSpan.FromMinutes(2));
}
}
Or maybe you want to keep track of the how many iterations that are currently in flight?:
int inFlight = 0;
Parallel.For(0, doc.GetElementsByTagName("ItemID").Count, i => {
System.Threading.Interlocked.Increment(ref inFlight);
if (inFlight % 2 == 0)
System.Threading.Thread.Sleep(TimeSpan.FromMinutes(2));
var xmlResponse = PerformHttpRequestMethod();
System.Threading.Interlocked.Decrement(ref inFlight);
});
You can do that by combining a Parallel.For and a normal for loop:
for(var i = 0;i<doc.GetElementsByTagName("ItemID").Count;i = i+2)
{
Parallel.For(0, 2, i => {
var xmlResponse = PerformHttpRequestMethod();
});
Thread.Sleep(2000);
}
I would like to write a method which accept several parameters, including an action and a retry amount and invoke it.
So I have this code:
public static IEnumerable<Task> RunWithRetries<T>(List<T> source, int threads, Func<T, Task<bool>> action, int retries, string method)
{
object lockObj = new object();
int index = 0;
return new Action(async () =>
{
while (true)
{
T item;
lock (lockObj)
{
if (index < source.Count)
{
item = source[index];
index++;
}
else
break;
}
int retry = retries;
while (retry > 0)
{
try
{
bool res = await action(item);
if (res)
retry = -1;
else
//sleep if not success..
Thread.Sleep(200);
}
catch (Exception e)
{
LoggerAgent.LogException(e, method);
}
finally
{
retry--;
}
}
}
}).RunParallel(threads);
}
RunParallel is an extention method for Action, its look like this:
public static IEnumerable<Task> RunParallel(this Action action, int amount)
{
List<Task> tasks = new List<Task>();
for (int i = 0; i < amount; i++)
{
Task task = Task.Factory.StartNew(action);
tasks.Add(task);
}
return tasks;
}
Now, the issue: The thread is just disappearing or collapsing without waiting for the action to finish.
I wrote this example code:
private static async Task ex()
{
List<int> ints = new List<int>();
for (int i = 0; i < 1000; i++)
{
ints.Add(i);
}
var tasks = RetryComponent.RunWithRetries(ints, 100, async (num) =>
{
try
{
List<string> test = await fetchSmthFromDb();
Console.WriteLine("#" + num + " " + test[0]);
return test[0] == "test";
}
catch (Exception e)
{
Console.WriteLine(e.StackTrace);
return false;
}
}, 5, "test");
await Task.WhenAll(tasks);
}
The fetchSmthFromDb is a simple Task> which fetches something from the db and works perfectly fine when invoked outside of this example.
Whenever the List<string> test = await fetchSmthFromDb(); row is invoked, the thread seems to be closing and the Console.WriteLine("#" + num + " " + test[0]); not even being triggered, also when debugging the breakpoint never hit.
The Final Working Code
private static async Task DoWithRetries(Func<Task> action, int retryCount, string method)
{
while (true)
{
try
{
await action();
break;
}
catch (Exception e)
{
LoggerAgent.LogException(e, method);
}
if (retryCount <= 0)
break;
retryCount--;
await Task.Delay(200);
};
}
public static async Task RunWithRetries<T>(List<T> source, int threads, Func<T, Task<bool>> action, int retries, string method)
{
Func<T, Task> newAction = async (item) =>
{
await DoWithRetries(async ()=>
{
await action(item);
}, retries, method);
};
await source.ParallelForEachAsync(newAction, threads);
}
The problem is in this line:
return new Action(async () => ...
You start an async operation with the async lambda, but don't return a task to await on. I.e. it runs on worker threads, but you'll never find out when it's done. And your program terminates before the async operation is complete -that's why you don't see any output.
It needs to be:
return new Func<Task>(async () => ...
UPDATE
First, you need to split responsibilities of methods, so you don't mix retry policy (which should not be hardcoded to a check of a boolean result) with running tasks in parallel.
Then, as previously mentioned, you run your while (true) loop 100 times instead of doing things in parallel.
As #MachineLearning pointed out, use Task.Delay instead of Thread.Sleep.
Overall, your solution looks like this:
using System.Collections.Async;
static async Task DoWithRetries(Func<Task> action, int retryCount, string method)
{
while (true)
{
try
{
await action();
break;
}
catch (Exception e)
{
LoggerAgent.LogException(e, method);
}
if (retryCount <= 0)
break;
retryCount--;
await Task.Delay(millisecondsDelay: 200);
};
}
static async Task Example()
{
List<int> ints = new List<int>();
for (int i = 0; i < 1000; i++)
ints.Add(i);
Func<int, Task> actionOnItem =
async item =>
{
await DoWithRetries(async () =>
{
List<string> test = await fetchSmthFromDb();
Console.WriteLine("#" + item + " " + test[0]);
if (test[0] != "test")
throw new InvalidOperationException("unexpected result"); // will be re-tried
},
retryCount: 5,
method: "test");
};
await ints.ParallelForEachAsync(actionOnItem, maxDegreeOfParalellism: 100);
}
You need to use the AsyncEnumerator NuGet Package in order to use the ParallelForEachAsync extension method from the System.Collections.Async namespace.
Besides the final complete reengineering, I think it's very important to underline what was really wrong with the original code.
0) First of all, as #Serge Semenov immediately pointed out, Action has to be replaced with
Func<Task>
But there are still other two essential changes.
1) With an async delegate as argument it is necessary to use the more recent Task.Run instead of the older pattern new TaskFactory.StartNew (or otherwise you have to add Unwrap() explicitly)
2) Moreover the ex() method can't be async since Task.WhenAll must be waited with Wait() and without await.
At that point, even though there are logical errors that need reengineering, from a pure technical standpoint it does work and the output is produced.
A test is available online: http://rextester.com/HMMI93124
I want to perform multiple loops in the same time using async Task (I don't want use Parallel.ForEach)
I do :
static async void Run()
{
await MultiTasks();
}
static async Task MultiTasks()
{
var Loop1 = Loop1Async();
var Loop2 = Loop2Async();
await Loop1;
await Loop2;
}
static async Task Loop1Async()
{
for (int i = 0; i < 500; i++)
{
Console.WriteLine("Loop 1 : " + i);
}
}
static async Task Loop2Async()
{
for (int i = 0; i < 500; i++)
{
Console.WriteLine("Loop 2 : " + i);
}
}
Run() is called in my Main method.
But the two loop is not executed in the same time. Once the first lap is completed, the second begins
Why and how to do this ?
You have the fundamental misunderstanding common to beginner users of await.
Let me be very clear about this. Await does not make something asynchronous. It asynchronously waits for an asynchronous operation to complete.
You are awaiting a synchronous operation. There is absolutely nothing asynchronous about the methods you are calling; await does not make them asynchronous. Rather, if they were asynchronous, then the await would return control to the caller immediately, so that the caller could continue to do work. When the asynchronous work finishes, then at some point in the future the remainder of the method is executed, with the awaited result.
Loop1Async and Loop2Async in fact are synchronous. Consider using WriteLineAsync method.
You can also use Task.WhenAll in MultiTasks.
Try .WhenAll(...)
static async Task MultiTasks()
{
var Loop1 = Loop1Async();
var Loop2 = Loop2Async();
await Task.WhenAll(Loop1, Loop2);
}
As others have noted your async methods do not currently yield execution.
You need something that will allow threads to yield back to the system so they can actually run in parallel. Something like this...
static async Task Loop1Async()
{
for (int i = 0; i < 500; i++)
{
Console.WriteLine("Loop 1 : " + i);
await Task.Yield();
}
}
static async Task Loop2Async()
{
for (int i = 0; i < 500; i++)
{
Console.WriteLine("Loop 2 : " + i);
await Task.Yield();
}
}
Your Loop1Async and Loop2Async methods does not have an await inside. For a method to be async it needs to have 1 or more await
I have this "simple" test code... (Don't bother the strange use of the Class methods...)
I am trying to grasp the Task<> intricacies... I think I have a little understanding of Task<>.Start()/Task<>.Result pattern (maybe as it resembles more the 'old' Thread.Start()?) but as soon as it seems to me to grasp something (and so I throw in the await keyword)... then all entangles again :-(
Why my code returns immediately after the first task completes? Why it doesn't wait on the Task.WhenAll()?
static BigInteger Factorial(BigInteger factor)
{
BigInteger factorial = 1;
for (BigInteger i = 1; i <= factor; i++)
{
factorial *= i;
}
return factorial;
}
private class ChancesToWin
{
private int _n, _r;
public ChancesToWin(int n, int r)
{
_n = n;
_r = r;
}
private Task<BigInteger> CalculateFactAsync(int value)
{
return Task.Factory.StartNew<BigInteger>(() => Factorial(value));
}
public async Task<BigInteger> getFactN()
{
BigInteger result = await CalculateFactAsync(_n);
return result;
}
public async Task<BigInteger> getFactN_R()
{
BigInteger result = await CalculateFactAsync(_n - _r);
return result;
}
public async Task<BigInteger> getFactR()
{
BigInteger result = await CalculateFactAsync(_r);
return result;
}
}
private async static void TaskBasedChancesToWin_UseClass()
{
int n = 69000;
int r = 600;
List<Task<BigInteger>> tasks = new List<Task<BigInteger>>();
ChancesToWin ctw = new ChancesToWin(n, r);
tasks.Add(ctw.getFactN());
tasks.Add(ctw.getFactN_R());
tasks.Add(ctw.getFactR());
// The getFactR() returns first of the other two tasks... and the code exit!
BigInteger[] results = await Task.WhenAll(tasks);
// I don't get here !!!!
BigInteger chances = results[0] / results[1] * results[2];
//Debug.WriteLine(chances);
}
static void Main(string[] args)
{
TaskBasedChancesToWin_UseClass();
}
Async methods run synchronously until the first await when they return control to the calling method, usually returning a task representing the rest of the asynchronous operation. TaskBasedChancesToWin_UseClass doesn't return a task so the caller can't wait for it to complete. That's why you shouldn't use async void outside of event handlers.
Since Main doesn't wait for the operation your application ends before the operation had a chance to complete.
You would usually wait with await but since you Main can't be an async method you can block synchronously with Wait on the task returned from TaskBasedChancesToWin_UseClass:
async static Task TaskBasedChancesToWin_UseClass()
{
// ...
}
static void Main()
{
TaskBasedChancesToWin_UseClass().Wait();
}