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I'm following up on my previous async question.
There are a TON of examples out there about how to consume an Async method, yet none of then actually show you what an async method signature looks like. For example, you see plenty of Entity Framework calls like
public async Task<List<Contact>> GetContactsAsync()
{
return await this.dbContext.Contacts.ToListAsync();
}
but how is ToListAsync actually defined? I'm assuming it returns a Task<list<t>>.
I came across this article, where it appears that all I need to do is return Task.FromResult<t>. Based on that, is this correct?
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Calling Test()");
Test();
Console.WriteLine("After Test");
Console.ReadLine();
}
private async static void Test()
{
Console.WriteLine("Calling GetSomeData()");
var results = await GetSomeData();
Console.WriteLine($"Results: {results}");
}
private static Task GetSomeData()
{
int x = 0;
for (; x (x);
}
Thread.Sleep(5000);
return Task.FromResult(x);
}
When I run this, it does not appear to run async. The console output is:
Calling Test()
Calling GetSomeData()
Results: 900000000
After Test
with a long pause at the call to GetSomeData(). I expected the Test method to return right away, and then, when GetSomeData completes, I would see the results.
Thanks
If it's not broken, fix it until it is.
Everything makes sense in someone's mind.
Ya can't fix stupid.
modified 7-Oct-21 17:25pm.
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The article you linked to is a poor example - it shows how to wrap a synchronous result in a Task<T>, not how to implement an asynchronous method. That's usually not a sensible thing to do:
Should I expose asynchronous wrappers for synchronous methods? - .NET Parallel Programming[^]
Your code has been mangled, and also has several severe problems. For a start: Avoid async void methods[^]
Try:
static async Task Main()
{
Console.WriteLine("Calling Test()");
Task t = Test();
Console.WriteLine("After Test");
await t;
}
private static async Task Test()
{
Console.WriteLine("Calling GetSomeData()");
var results = await GetSomeData();
Console.WriteLine($"Results: {results}");
}
private static async Task<int> GetSomeData()
{
int x = 0;
for (; x < 9000; x++)
{
await Task.Delay(5);
}
return x;
} Output:
Calling Test()
Calling GetSomeData()
After Test
{delay}
Results: 9000
NB: If you're waiting for five seconds on every iteration of 900,000,000 loops, you'll have to wait roughly 142 years for your code to finish.
Waiting five seconds for every iteration of 9,000 loops would take 12½ hours to finish.
Reducing it to five milliseconds for 9,000 loops should technically let the code complete in 45 seconds. However, due to the Windows timer resolution it will almost certainly take longer.
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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OK, so Making Test return Task was the issue?
You're right, my posting was messed up. The GetSomeData method really looked like this:
private static Task<int> GetSomeData()
{
int x = 0;
for (; x < 900000000; x++)
{
}
Thread.Sleep(5000);
return Task.FromResult<int>(x);
}
If it's not broken, fix it until it is.
Everything makes sense in someone's mind.
Ya can't fix stupid.
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That method isn't async; it blocks the caller, and then returns a result wrapped in a Task<T> instance.
If you make it async, you can use await Task.Delay(5000); instead of Thread.Sleep(5000);, and the caller will be able to continue after the loop has finished.
To let the caller continue before the loop has finished, you'd need to yield control before starting the loop.
private static async Task<int> GetSomeData()
{
await Task.Yield();
int x = 0;
for (; x < 900_000_000; x++)
{
}
await Task.Delay(5000);
return x;
}
A Tour of Task, Part 10: Promise Tasks[^]
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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GOt it. Thanks!
If it's not broken, fix it until it is.
Everything makes sense in someone's mind.
Ya can't fix stupid.
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Richard Deeming wrote: o let the caller continue before the loop has finished, you'd need to yield control before starting the loop.
Wouldn't you want to do this all the time?
If it's not broken, fix it until it is.
Everything makes sense in someone's mind.
Ya can't fix stupid.
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No, why would you? Just makes the processor waste time switching between things slowing down everything. It will not even start a new thread, just make the called code wait, instead of the caller.
An important thing to know is: Async/await is NOT concurrent programming. I will not start any new threads - it will not cause anything to be processed at the same time... only the method you call with an "await" can start work in parallel (by scheduling IO to complete on the IO completion threads, thread pool, it's own thread, or responding to events from other parts of the code potentially running on a different thread).
Sure, it can be used along with concurrency - and the way it is implemented can result in you running code concurrently that you did not expect - but that is a side effect and one of the things that makes async programming so hard.
Async on the server is not "invented to make the programmers life much easier". It is invented to get extra performance out of servers as they will waste less time context switching. It has been available for many many years (decades), but was horrible to program. Now we have async/await it is no longer horrible... just difficult. This is something you will notice as soon as you leave the safe path of demo applications.
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Thanks for all the info. I appreciate it.
The reason I'm exploring this is because I'm working on a WPF app that calls SQL on my client's in-house server. Until now it's been a typcial synchronius, client-server model.
But now I'm going to be adding in a Xamarin Forms or .Net Maui app that allows my client to do work in the field. This means creating a web api and making calls async.
So, I'm trying to understand how to write both the back end and front end methods so that I don't hang the UI.
If it's not broken, fix it until it is.
Everything makes sense in someone's mind.
Ya can't fix stupid.
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As mentioned by others - do not worry about the server side being async. The web host will simply spin up more threads to handle the calls concurrently - and the client software will not be able to detect if you use async or not on the server.
Sure there is a limit to how many threads you can have running - but most likely the SQL server will give up before your web server. If not, then it is some serious heavy processing you are running - and either you should let someone more experienced deal with it or just run more servers. Typically you would run two servers anyway if high availability is required.
Just to be clear: You do not have to start threads yourself for the web server to run requests concurrently. The web server will do that for you.
If you do not need to get every bit of performance out of the server, the main advantage in using async on the server side is that all modern libraries expect it - and some might no longer have sync method calls available. So if you write your code async, you will never have sync code calling async methods. Sure it can be done if needed, but you need to know what you are doing to avoid deadlocks. Async code calling sync methods is never an issue.
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SEE AND CONTINUE DISCUSSION on the WPF Forum
I have a UI with a scrollviewer like this:
I want to scroll the viewer to the top when the ItemSource changes.
<ScrollViewer x:Name="Scroller" Grid.Row="2" Margin="0,0">
<StackPanel>
<ItemsControl x:Name="containers"
ItemsSource="{Binding Whatever, NotifyOnTargetUpdated=True}"
TargetUpdated="Containers_OnTargetUpdated">
</ItemsControl>
<StackPanel>
<ScrollViewer>
public partial class MyPage: UserControl
{
public MyPage()
{
InitializeComponent();
}
private void Containers_OnTargetUpdated(object? Sender, DataTransferEventArgs E)
{
var MyScroller = (ScrollViewer) this.FindName("Scroller");
MyScroller.ScrollToHome();
}
}
This works but it's probably not the best way to do it (ie. finding a control by name).
Can I pass the ScrollViewer "object" to the TargetUpdated callback/event ? so that I don't have to referencing it by name?
It seems clunky.
I don't know exactly what magic incantations I need to google for.
CI/CD = Continuous Impediment/Continuous Despair
modified 8-Oct-21 9:03am.
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You could try the WPF forum.
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(obviously, I should have seen that forum)
thanks.
CI/CD = Continuous Impediment/Continuous Despair
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The sender parameter should be the ItemsControl, in which case you just need to walk up the visual tree to find the ScrollViewer.
private void Containers_OnTargetUpdated(object? sender, DataTransferEventArgs e)
{
var current = sender as DependencyObject;
var scroller = current as ScrollViewer;
while (scroller == null && current != null)
{
current = VisualTreeHelper.GetParent(current);
scroller = current as ScrollViewer;
}
if (scroller != null)
{
scroller.ScrollToHome();
}
}
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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Thanks, it works.
(sorry if I did not answer before).
![Rose | [Rose]](https://codeproject.freetls.fastly.net/script/Forums/Images/rose.gif)
CI/CD = Continuous Impediment/Continuous Despair
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Hello fellow members. Im trying to understand why my code is not catching NullReferenceException and i cant figure it out.
I'm using this code to run async function
private async Task SearchMostLessViewsAsync(DateTime dateFrom, DateTime dateTo, int amountOfNumbersSelected, int frequencyOption, int fromDrawNumber, int toDrawNumber)
{
Microsoft.AppCenter.Analytics.Analytics.TrackEvent($"{typeof(FragmentDrawsNumbersFrequency).Name}.{nameof(SearchMostLessViewsAsync)}",
new Dictionary<string, string>()
{
{nameof(dateFrom), dateFrom.ToString()},
{nameof(dateTo), dateTo.ToString()},
{nameof(amountOfNumbersSelected), amountOfNumbersSelected.ToString()},
{nameof(frequencyOption), frequencyOption.ToString()},
{nameof(fromDrawNumber), fromDrawNumber.ToString()},
{nameof(toDrawNumber), toDrawNumber.ToString()},
}
);
((MainActivity)Activity).DisplayLoadingMessage(true, GetString(Resource.String.Common_SearchTitle),
GetString(Resource.String.Common_SearchMessage));
var task = Task.Run(async () =>
{
var textResult = await SearchLeast(dateFrom, dateTo, amountOfNumbersSelected, frequencyOption, fromDrawNumber, toDrawNumber).ConfigureAwait(false);
if (!string.IsNullOrEmpty(textResult))
{
UpdateHistoryList(textResult);
DatabaseFunctions.SaveHistoryList(HistoryList, Settings.DrawsNumbersFrequencyHistoryListViewKey);
((MainActivity)Activity).ShowSearchResults(textResult);
}
}, ApplicationState.GetCancellationToken());
try
{
await task.ConfigureAwait(false);
}
catch (TaskCanceledException tce)
{
Console.WriteLine($"{nameof(TaskCanceledException)} thrown with message: {tce.Message}");
}
catch (System.ObjectDisposedException ode)
{
Console.WriteLine($"{nameof(System.ObjectDisposedException)} thrown with message: {ode.Message}");
}
catch (System.OperationCanceledException e)
{
Console.WriteLine($"{nameof(System.OperationCanceledException)} thrown with message: {e.Message}");
}
catch (NullReferenceException nre)
{
Console.WriteLine($"{nameof(NullReferenceException)} thrown with message: {nre.Message}");
}
finally
{
ApplicationState.DisposeCancellationTokenSource();
((MainActivity)Activity).DisplayLoadingMessage(false);
}
((MainActivity)Activity).DisplayLoadingMessage(false);
}
But sometimes i get this exception
FragmentDrawsNumbersFrequency.SearchMostLessViewsAsync (System.DateTime dateFrom, System.DateTime dateTo, System.Int32 amountOfNumbersSelected, System.Int32 frequencyOption, System.Int32 fromDrawNumber, System.Int32 toDrawNumber)
FragmentDrawsNumbersFrequency.ShowMostLessViews ()
FragmentDrawsNumbersFrequency.<OnCreateView>b__50_8 (System.Object sender, System.EventArgs e)
AsyncMethodBuilderCore+<>c.<ThrowAsync>b__7_0 (System.Object state)
SyncContext+<>c__DisplayClass2_0.<Post>b__0 ()
Thread+RunnableImplementor.Run ()
IRunnableInvoker.n_Run (System.IntPtr jnienv, System.IntPtr native__this)
(wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.61(intptr,intptr)
I'm not sure why. Shouldn't the try catch wrap catch nullreference exceptions? Something tells me i miss something else here
modified 2-Oct-21 8:56am.
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The exception is referring to this method: SearchMostLessViewsAsync. That's not anywhere in the code you're showing here.
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As Pete has said, the error isn't in the code you show, nor is the method that throws the error directly called in any of that code.
Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.
But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
modified 2-Oct-21 8:14am.
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Richard? Are you saying I'm a complete and utter Richard? 
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No I'm saying I'm a complete and utter pillock ... 
Sorry ... fixed. 
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
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what means to be a Richard?  i didnt get the joke
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Griff called me Richard. He's edited the post to use my name now.
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I made a mistake, and referred to Pete as Richard, is all. No joke, just a mistake on my part.
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
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oh i though it had a deeper meaning i didn't get
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oh yes you are right, sorry i have used the wrong part of the code. i edit the question again.
I cant reproduce the bug in my device, it is happening in around 2% of the devices that is being used, this code is from an app that is available from google play already
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You're making the assumption that the method that is throwing the null reference is inside your task. What's interesting is that you have failed to apply any forms of input guards to SearchMostLessViewsAsync and you're assuming you actually have values coming in. It wouldn't surprise me if the null reference you are seeing occurred in this call:
Microsoft.AppCenter.Analytics.Analytics.TrackEvent($"{typeof(FragmentDrawsNumbersFrequency).Name}.{nameof(SearchMostLessViewsAsync)}",
new Dictionary<string, string>()
{
{nameof(dateFrom), dateFrom.ToString()},
{nameof(dateTo), dateTo.ToString()},
{nameof(amountOfNumbersSelected), amountOfNumbersSelected.ToString()},
{nameof(frequencyOption), frequencyOption.ToString()},
{nameof(fromDrawNumber), fromDrawNumber.ToString()},
{nameof(toDrawNumber), toDrawNumber.ToString()},
}
);
If any of those is null, you'll get a null reference because you're calling ToString() on it. The bottom line is that you shouldn't make assumptions about where exceptions are occurring; look at the whole part of the code.
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