I have specific problem with WebClient in my Windows Phone app (using MVVM)
private string _lastCurrencyRatesJson;
private bool _lastCurrencyRatesJsonLoaded = false;
private void GetLastCoursesFromApiAsync()
{
var uri = new Uri(string.Format(OperationGetLastCourses, AppSettings.ApiEndpoint, AppSettings.ApiKey));
var client = new WebClient { Encoding = Encoding.UTF8 };
client.DownloadStringCompleted += client_DownloadStringCompleted;
client.DownloadStringAsync(uri);
}
void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
_lastCurrencyRatesJson = e.Result;
_lastCurrencyRatesJsonLoaded = true;
}
public List<CurrencyRate> GetLastCourses()
{
var worker = new Thread(GetLastCoursesFromApiAsync);
worker.Start();
while (!_lastCurrencyRatesJsonLoaded)
{
}
.....
The problem is that client_DownloadStringCompleted is never fired BUT when I change GetLastCourses this way:
public List<CurrencyRate> GetLastCourses()
{
var worker = new Thread(GetLastCoursesFromApiAsync);
worker.Start();
// whetever here, but any while...
client_DownloadStringCompleted is fired and data are obtained. It means, connectivity is ok.
I had very similar problems with DownloadStringTaskAsyn. Example:
private async Task<string> GetCoursesForDayFromApiAsJson(DateTime date)
{
var uri = new Uri(string.Format(OperationGetCoursesForDay, AppSettings.ApiEndpoint, AppSettings.ApiKey, date.ToString(DateFormat)));
var client = new WebClient { Encoding = Encoding.UTF8 };
return await client.DownloadStringTaskAsync(uri);
}
Again, at the line with await is application waiting for the data but the DownloadStringTaskAsync is never finished and my UI is still loading.
Any ideas what could be wrong?
SITUATION ONE DAY AGO
So, it looks that WP application is working just with one thread. It means, current thread have to be "finished" and then is DownloadStringTaskAsync finished and the code under the await executed. When I want to work with Task.Result I can not. Never.
When I create another Thread and I am trying to wait for thread completetion (using Join()), created Thread is never finsihed and the code after Join() is never executed.
There is any example on the Internet and I absolutely don't know, why exists some DownloadStringTaskAsync when it is not applicable.
You're blocking the UI thread by your while loop and at the same time, the DownloadStringCompleted event wants to execute on the UI loop. This causes a deadlock, so nothing happens. What you need to do is to let GetLastCourses() return (and whatever method calls that), so that the event handler can execute. This means that the code that handles the results should be in that event handler (not in GetLastCourses()).
With async-await, you didn't provide all of your code, but it's likely that you're encountering pretty much the same issue by calling Wait() or Result on the returned Task. If replace that with await, you code will work. Though that requires you to make all your code from GetCoursesForDayFromApiAsJson() up async.
I'd recommend to use the HttpClient class from Microsoft NuGet package and use the async/await downloading pattern rather than using event-based WebClient class:
Uri uri = new Uri(string.Format(OperationGetLastCourses, AppSettings.ApiEndpoint, AppSettings.ApiKey));
using (HttpClient client = new HttpClient())
{
string result = await client.GetStringAsync(uri);
}
Related
The program asks a person if he wants to check the last closing price of Bitcoin and whenever he pushes the button it should first say "Loading..." and wait until the price is received and assign it to the .Text property of the label. When I run the code in a console app the application closes before actually receiving the needed information (writes only "Loading..." ) but if I add ReadKey() the price information shows up. I guess something similar happens to the windows forms app which tries to assign a missing value to the text property of value, hence the program crashes after displaying "Loading...".
public static async Task<string> ApiCall(string apikey)
{
RestClient client = new RestClient("https://api.polygon.io/v2/aggs/ticker/X:BTCUSD/prev?adjusted=true&apiKey=");//write your api key
RestRequest request = new RestRequest($"?api-key={apikey}", Method.GET);
IRestResponse response = await client.ExecuteAsync(request);
return response.Content;
}
public static async Task<string> apiReceiver(string last_closed)
{
Task<string> apiCallTask = getAPI.ApiCall("[apikey]");
string result = apiCallTask.Result;
dynamic array = JsonConvert.DeserializeObject(result);
last_closed = array.results[0].c;
return last_closed;
}
public static async Task dataWait(Label lab, string last_closed)
{
lab.Text = "Loading info ...";
lab.Text = await apiReceiver(last_closed);
}
private async void button1_Click(object sender, EventArgs e)
{
string last_closed = "";
await getAPI.dataWait(label1, last_closed);
}
Why aren't you awaiting getAPI.ApiCall("[apikey]");? Using .Result before the task is completed, will result in a deadlock. The winforms has a SynchronizationContext set on that thread. Meaning that after the await, you're back on the UI thread and allowed and therefor able to modify UI-controls.
When you use .Result on a task, if it's not finished, it will wait there (block the thread). The problem is that when the task is ready, it will be posted on the UI thread, but never be executed, because the thread still blocked.
The difference between winforms and console. The console hasn't got a SynchronizationContext set, so the rest of the method (after the await) is posted on the threadpool. You're allowed to call Console.Writeline on anythread.
So use await here, so the thread isn't blocked.
public static async Task<string> apiReceiver(string last_closed)
{
string result = await getAPI.ApiCall("[apikey]");
dynamic array = JsonConvert.DeserializeObject(result);
last_closed = array.results[0].c;
return last_closed;
}
Here's some information:
source
However, if that code is run in a UI Application, for example when a button is clicked like in the following example:
Then the application will freeze and stop working, we have a deadlock. Of course, users of our library will complain because it makes the application unresponsive.
More about SynchronizationContext read codeproject.com Understanding-the-SynchronizationContext
I have been making a client that installs a program I am also making. The problem is when I go to download the files. Sometimes, it gets stuck. I get thrown a error stating
System.Runtime.InteropServices.ExternalException: 'A generic error occurred in GDI+.'
...and my UI freaks out (Buttons go blank, labels disapper, images go missing, ect). I have researched this error and it seems it happens with images, but I am downloading zip files that contain no images. I have found that it might have something to do with 5 files I am extracting from the zip files but thats not entirely accurate because it doesnt always happen and I have no real way to determine EXACTLY whats causing it. I suspect its because I cannot download so much in a short period of time but I do not know if this is why exactly either.
Also to add to this, the files still complete downloading when in debug mode, they ever continue through the async process, awaiting properly and everything.
I have tried narrowing down what file is causing it but I dont have any evidence to support it is a specific file. I have also tried spliting up the zip files to see if its the size of how much Im downloading at once, still no luck.
These are the download functions.
The RunWorkerTaskAsync() is a custom reference I created to allow a worker to be "awaited". I privide the code below.(I take no credit as its pieces of code I have pulled from others)
private async Task DownloadLibs()
{
Response.Text = "Updating Libraries...";
this.Update();
string url = #"http://akumamc.com/AkumaMC/Libraries.zip";
if (!string.IsNullOrEmpty(url))
{
Uri uri = new Uri(url);
string fileName = System.IO.Path.GetFileName(uri.AbsolutePath);
await DLclient.DownloadFileTaskAsync(uri, #"C:\temp\" + fileName);
DLclient.Dispose();
}
FileZipName = #"C:\temp\Libraries.zip";
FileZipPath = #"C:\temp\.minecraft";
Response.Text = "Extracting Libraries...";
this.Update();
await extractFile.RunWorkerTaskAsync();
}
private async Task DownloadMods()
{
Response.Text = "Updating Mods (1/2)...";
this.Update();
string url = #"http://akumamc.com/AkumaMC/Mods.zip";
if (!string.IsNullOrEmpty(url))
{
Uri uri = new Uri(url);
string fileName = System.IO.Path.GetFileName(uri.AbsolutePath);
await DLclient.DownloadFileTaskAsync(uri, #"C:\temp\" + fileName);
DLclient.Dispose();
}
FileZipName = #"C:\temp\Mods.zip";
FileZipPath = #"C:\temp\.minecraft";
Response.Text = "Extracting Mods (1/2)...";
this.Update();
await extractFile.RunWorkerTaskAsync();
}
private async Task DownloadExtras()
{
Response.Text = "Updating Mods (2/2)...";
this.Update();
string url = #"http://akumamc.com/AkumaMC/Mods2.zip";
if (!string.IsNullOrEmpty(url))
{
Uri uri = new Uri(url);
string fileName = System.IO.Path.GetFileName(uri.AbsolutePath);
await DLclient.DownloadFileTaskAsync(uri, #"C:\temp\" + fileName);
DLclient.Dispose();
}
FileZipName = #"C:\temp\Mods2.zip";
FileZipPath = #"C:\temp\.minecraft";
Response.Text = "Extracting Mods (2/2)...";
this.Update();
await extractFile.RunWorkerTaskAsync();
}
RunWorkerTaskAsync:
public static Task<object> RunWorkerTaskAsync(this BackgroundWorker backgroundWorker)
{
var tcs = new TaskCompletionSource<object>();
RunWorkerCompletedEventHandler handler = null;
handler = (sender, args) =>
{
if (args.Cancelled)
tcs.TrySetCanceled();
else if (args.Error != null)
tcs.TrySetException(args.Error);
else
tcs.TrySetResult(args.Result);
};
backgroundWorker.RunWorkerCompleted += handler;
try
{
backgroundWorker.RunWorkerAsync();
}
catch
{
backgroundWorker.RunWorkerCompleted -= handler;
throw;
}
return tcs.Task;
}
I expect the files to download without the form causing UI glitches and crashing.
EDIT: Link to author's client code (taken from comment below)
This is a summary of my comments beneath the OP's question
System.Runtime.InteropServices.ExternalException: 'A generic error occurred in GDI+.'
So the "interop" error implies some form of component object model (COM) problem and the things that stick out are:
the use of some 3rd party library that may be using COM
your RunWorkerTaskAsync extension method seems to be making multiple calls to BackgroundWorker.RunWorkerAsync without first checking that the worker is busy.
BackgroundWorker.RunWorkerAsync() is a void, it does not return a Task and so can't be used in async/await. Therefore your extension method is essentially kicking off a background worker without waiting for it to complete. Your extension method RunWorkerTaskAsync() (which isn't entirely async) returns immediately to those that called it.
You need to for the worker to complete before calling RunWorkerAsync again.
A possible fix:
Inside your extension method, check BackgroundWorker.IsBusy before telling it to run. (A better way is to wait for RunWorkerCompleted and kicking off a new one there)
Call RunWorkerAsync
Because you want to wait for this "task" to complete before returning control to say DownloadMods(), your extension method will need to monitor RunWorkerCompleted. This is kinda ugly as it goes against the original best practices of BackgroundWorker where everything is event-driven.
Alternatives
Considering you are using async/await anyway, why use BackgroundWorker at all? Consider wrapping up the essence of your extension method into a new method and call it via Task.Run().
You can still have a async Task ExtractFilesAsync method that runs in a child task (because we used Task.Run() it will also be a child thread) can report progress.
Something like (pseudo code):
await Task.Run ( async () => await
UnzipFilesAsync ( p =>
{
myProgressBar.BeginInvoke (new Action( () =>
myprogressBar.Progress = p; ));
});
.
.
.
UnzipFilesAsync (Action<int> progressCallback)
{
.
.
.
int percent = ...;
progressCallback (percent);
}
Tell me more about async progress bar updates
By the way, you shouldn't call MessageBox.Show or update the UI directly in a child thread, even if the call is a dialog with its own message pump.
The Windows Forms GUI thread keeps stalling for a second now and then during async HttpClient SendAsync. Except for that, everything works fine (I get data). I want to do frequent and hopefully parallel requests to different servers to update the screen frequently. I am trying to launch multiple requests that post the responses for processing by the GUI thread later. The code below is a simplification of my code.
I've checked the time before and after SendAsync and see it is sometimes up to 2 seconds while the GUI window is frozen (can't be moved, scrolled, etc) and the polling timer is inactive (counter never incremented).
Using async Task DoWork did not help.
class Worker
{
HttpClient client = new HttpClient();
bool busy = false;
string data = "";
//public async Task DoWork()
public async void DoWork()
{
if ( busy ) return;
busy = true;
HttpRequestMessage request = new HttpRequestMessage(method, requestUrl);
HttpResponseMessage response = await client.SendAsync( request );
data = ... from response ...
busy = false;
}
}
int counter;
private void Update(object sender, EventArgs e)
{
++counter;
foreach ( Worker worker in workers )
worker.DoWork();
}
...
List<Worker> workers = ...
var poll = new System.Windows.Forms.Timer();
poll.Tick += Update;
poll.Interval = 250;
poll.Enabled = true;
...
If you are using Windows.Forms you may consider BackgroundWorker
SendAsync is sufficiently asynchronous (even if it uses threads internally).
Updating UI itself is really slow and the cause of UI freezing.
Maybe I did not search correctly here in the forum because I did not find a similar problem.
Well, my problem is when I try to execute an async method inside a thread.
When I run the method (Register) without the thread it works perfectly!
Below is an example of the scenario.
private SyncProcess _sync = new SyncProcess();
private static HttpClient _httpClient = new HttpClient();
private Thread _thread;
public class SyncProcess : ISyncProcess
{
public event CompleteHandler OnComplete = delegate { };
// another properties ...
public void Run()
{
// import rules
// ...
OnComplete();
}
}
public void TestImport()
{
Register(idsync, "start"); // here register works fine
_sync.OnComplete += ImportComplete;
_thread = new Thread(() =>
{
try
{
_sync.Run();
}
catch (Exception ex)
{
// not fall here
}
});
//
_thread.Start();
}
private void ImportComplete()
{
// other end-of-import rules
// ...
Register(idsync, "complete"); // here register not works
}
public async Task<string> Register(int idsync, string type)
{
string url = "myurl";
var stringContent = new FormUrlEncodedContent(new[] { new KeyValuePair<string, string>("json", "myjson") });
var response = await _httpClient.PostAsync(url + type, stringContent);
if (response.IsSuccessStatusCode)
{
// do something
}
return "";
}
The problem occurs when I call the method (Register) inside the thread, another thing is that is that it does not generate error does not fall into the try, the debugging simply terminates. I've tried adding try code everywhere but never crashed in catch.
Debug always aborts on the following line:
var response = await _httpClient.PostAsync(url + type, stringContent);
What should I do in this case?
Updated the code returning string in the Register method, but the same error remains.
Thanks any suggestions!
Your problem is due to the use of async void, which should be avoided. One of its problems is that you can't catch exceptions using try/catch.
Event handlers in C# are a "fire-and-forget" kind of language feature. In particular, asynchronous event handlers must use async void, and this means the event-publishing code cannot see those exceptions. If you want to allow async event handlers and handle exceptions (or other results) from them, you can use a "deferral" solution or make your event handler delegate return Task instead.
Async void will not allow you to catch any exceptions and will terminate your application when one is thrown. Exceptions are only observed and handled as normal exceptions when using task instead of void.
You can read all about it here link
I can not answer your first question on why it works without the thread without more information. I can guarantee you thought that it has nothing to do with multi threading as far as I know since the main thread is also just a thread like any other.
I have a web request (HttpRequest) which triggers a third library scanning method on my server that has an event handler attached to it:
scanner.OnScanComplete += scanner_OnScanComplete;
The web request will invoke scanner.Scan(files) but how can I force (or hook) the request to wait and get the results from scanner_OnScanComplete when the scan process is complete so it can return data to clients without having to send another web request to get this data?
void DoWork(HttpRequst request, var files)
{
var scanner = new Scanner()
scanner.OnScanComplete += scanner_OnScanComplete;
scan(files)
}
void scanner_OnScanComplete(object sender, EventArgs e)
{
var scanCompleted = true;
//Return scanCompleted somehow to the DoWork thread above
}
Do you have to use a HttpHandler or can you use other api's?
If you can use MVC4 or later then you can use an asynchronous Action Method to do this easily. Look here for an example of how to use them.
In addition to using an async Action Method you may need a way to await the event from the scanner. Using a Task Completion source as in this answer may be a good way to do that.
One way to do what you want is to store the completion of the task in a boolean member.
The boolean shall be marked volatile to avoid threading issues.
The risk of the approach is to lead to timeouts on client side if the scan processing is too long.
private volatile bool _finished;
void DoWork(HttpRequst request, var files)
{
var scanner = new Scanner();
scanner.OnScanComplete += scanner_OnScanComplete;
_finished= false;
scan(files)
while (!_finished) // wait for the scan completion
System.Threading.Thread.Sleep(1000); // avoid consuming 100% cpu
var scanData = Dothescanwork();
//Return scanData somehow to the DoWork thread above
}
void scanner_OnScanComplete(object sender, EventArgs e)
{
_finished= true;
}