I'm working on a UWP project and there's something funky going on with how errors are being presented to me. I don't know if it's VS2017 or how UWP is set up.
I have a piece of code that goes online and retrieves json content, sometimes the code works and sometimes it doesn't. It works when I use Expander control from UWP Community toolkit, and fails when I want to switch to GridView. When it doesn't work, it fails on GetStringAsync method of HttpClient. The strange behavior is that the exception isn't thrown in the method where the problem occurs, the code actually redirects me back without giving an error and as soon as it gets to the property that's supposed to have a value that isn't null, I get a null exception.
This is where the problem happens:
string httpContent = "";
using (HttpClient httpClient = new HttpClient())
{
try
{
httpContent = await httpClient.GetStringAsync(uri);
}
catch (Exception e)
{
// TODO: handle errors
var x = "";
}
}
This piece of code is called from within the view model. It starts with a constructor and RefreshServerKanesWrathDataAsync is the method where json is parsed.
public CncOnlinePageViewModel()
{
cnconline = new CncOnline();
cnconline.RefreshServerKanesWrathDataAsync();
}
The second I get to GetStringAsync, the code just goes back to the constructor like nothing happened, however the method never completes, it just exits back to the constructor, and therefore fails to update observable collections with data. I then get a null exception.
I wanted to test this with VS2015, but I updated some controls that are apparently only supported withing VS2017, so I can't run the code in other versions.
I also ran into an issue with the code prior to this problem, where I tried to access files in a directory without using a token. The behavior was exactly the same, the code wasn't telling me that I didn't have access to the directory I wanted to read, it was just throwing me out of the method back into the location that made the call to read the directory. Just like with the current problem, I would then run into a null exception, which wasn't where the main problem was.
I added Template10 and UWP community toolkit to the project, if that matters.
You shouldn't call an async method from a constructor unless you're willing to provide a callback.
public CncOnlinePageViewModel()
{
cnconline = new CncOnline();
var t = cnconline.RefreshServerKanesWrathDataAsync(); // assuming returns Task<string>
t.ContinueWith(OnCompleted);
}
private void OnCompleted(Task<string> task)
{
if (task.IsFaulted)
{
// Check error
var exception = task.Exception;
}
else if (task.IsCanceled)
{
// User hit cancel?
}
else
{
// All good!
var result = task.Result;
}
}
Here's a sample where RefreshServerKanesWrathDataAsync() returns just Task (not Task<result>)
public CncOnlinePageViewModel()
{
cnconline = new CncOnline();
var t = cnconline.RefreshServerKanesWrathDataAsync(); // assuming returns Task
t.ContinueWith(OnCompleted);
}
private void OnCompleted(Task task)
{
if (task.IsFaulted)
{
// Check error
var exception = task.Exception;
}
else if (task.IsCanceled)
{
// User hit cancel?
}
else
{
// All good!
}
}
On a side note, you may also need to have Visual Studio 2017 break when any exception is thrown. In VS2017, go to Debug->Windows->Exception Settings and make sure Common Language Runtime Exceptions has a check. If it has a filled box, click the box until it turns into a checkmark.
Also..., you can tap into an event raised when any task has an unobserved exception. You can do so in the constructor of App.xaml.cs
public App()
{
TaskScheduler.UnobservedTaskException += OnUnobservedException;
}
private static void OnUnobservedException(object sender, UnobservedTaskExceptionEventArgs e)
{
// Put break point here.
var ex = e.Exception;
// This will keep your app alive, but only do it if it's safe to continue.
e.SetObserved();
}
Related
Before I use Nito.MVVM, I used plain async/await and it was throwing me an aggregate exception and I could read into it and know what I have. But since Nito, my exceptions are ignored and the program jumps from async code block and continue executes. I know that it catch exceptions because when I put a breakpoint on catch(Exception ex) line it breaks here but with ex = null. I know that NotifyTask has properties to check if an exception was thrown but where I put it, it checks when Task is uncompleted, not when I need it.
View model:
public FileExplorerPageViewModel(INavigationService navigationService)
{
_navigationService = navigationService;
_manager = new FileExplorerManager();
Files = NotifyTask.Create(GetFilesAsync("UniorDev", "GitRemote/GitRemote"));
}
Private method:
private async Task<ObservableCollection<FileExplorerModel>> GetFilesAsync(string login, string reposName)
{
return new ObservableCollection<FileExplorerModel>(await _manager.GetFilesAsync(login, reposName));
}
Manager method(where exception throws):
public async Task<List<FileExplorerModel>> GetFilesAsync(string login, string reposName)
{
//try
//{
var gitHubFiles = await GetGitHubFilesAsync(login, reposName);
var gitRemoteFiles = new List<FileExplorerModel>();
foreach ( var file in gitHubFiles )
{
if ( file.Type == ContentType.Symlink || file.Type == ContentType.Submodule ) continue;
var model = new FileExplorerModel
{
Name = file.Name,
FileType = file.Type.ToString()
};
if ( model.IsFolder )
{
var nextFiles = await GetGitHubFilesAsync(login, reposName);
var count = nextFiles.Count;
}
model.FileSize = file.Size.ToString();
gitRemoteFiles.Add(model);
}
return gitRemoteFiles;
//}
//catch ( WebException ex )
//{
// throw new Exception("Something wrong with internet connection, try to On Internet " + ex.Message);
//}
//catch ( Exception ex )
//{
// throw new Exception("Getting ExplorerFiles from github failed! " + ex.Message);
//}
}
With try/catch or without it has the same effect. This behavior is anywhere where I have NotifyTask.
Update
There is no event, that fires when exception occurred, but there is Property Changed event, so I used it and added this code:
private void FilesOnPropertyChanged(object sender, PropertyChangedEventArgs propertyChangedEventArgs)
{
throw new Exception("EXCEPTION");
bool failed;
if ( Files.IsFaulted )
failed = true;
}
And exception not fires.
I added throw exception in App class (main class) and it fired. And when I have exceptions that come from XAML, it also fires. So maybe it not fires when it comes from a view model, or something else. I have no idea. Will be very happy for some help with it.
Update
We deal with exception = null, but the question is still alive. What I wanna add, that I rarely this issue, when the app is starting to launch on the physic device. I read some info about it, and it doesn't seem to be related, but maybe it is:
I'm not entirely sure what your desired behavior is, but here's some information I hope you find useful.
NotifyTask is a data-bindable wrapper around Task. That's really all it does. So, if its Task faults with an exception, then it will update its own data-bindable properties regarding that exception. NotifyTask is intended for use when you want the UI to respond to a task completing, e.g., show a spinner while the task is in progress, an error message if the task faults, and data if the task completes successfully.
If you want your application to respond to the task faulting (with code, not just a UI update), then you should use try/catch like you have commented out in GetFilesAsync. NotifyTask doesn't change how those exceptions work; they should work just fine.
I know that it catch exceptions because when I put a breakpoint on catch(Exception ex) line it breaks here but with ex = null.
That's not possible. I suggest you try it again.
I know that NotifyTask has properties to check if an exception was thrown but where I put it, it checks when Task is uncompleted, not when I need it.
If you really want to (asynchronously) wait for the task to complete and then check for exceptions, then you can do so like this:
await Files.TaskCompleted;
var ex = Files.InnerException;
Or, if you just want to re-raise the exception:
await Files.Task;
Though I must say this usage is extremely unusual. The much more proper thing to do is to have a try/catch within your GetFilesAsync.
Specifically I am doing C# development using Xamarin.Forms, however working on the native Android side writing a GPS wrapper class which will be usable in the Xamarin.Forms side via dependency injection. The calls should be the same between C# and Java in regards to Android for the most part.
Essentially, I have this method in my Geolocator object (which implements ILocationListener) on the Android side:
public async Task<Tuple<bool, string, GPSData>> GetGPSData() {
gpsData = null;
var success = false;
var error = string.Empty;
if (!manager.IsProviderEnabled(LocationManager.GpsProvider)) {
//request permission or location services enabling
//set error
} else {
manager.RequestSingleUpdate(LocationManager.GpsProvider, this, null);
success = true;
}
return new Tuple<bool, string, GPSData>(success, error, gpsData);
}
and
public void OnLocationChanged(Location location) {
gpsData = new GPSData(location.Latitude, location.Longitude);
}
I want to be able to call GetGPSData and have it return the tuple, which for now the only important thing about the Tuple is gpsData is filled in. I know it can take seconds to find a fix, so I want this method to be async and be awaitable in the Xamarin.Forms side once I actually need the value.
My issue is I can't figure out a way to have manager.RequestSingleUpdate work synchronously, or any work around. You call that method, and then eventually OnLocationChanged fires off. I tried throwing in a disgusting, barbaric
while (gpsData == null);
after the call to force it not to continue until OnLocationChanged was fired, however when I put that line in, OnLocationChanged never gets called. I'm assuming that is because OnLocationChanged is invoked on the same thread instead of being a background thread.
Is there any way for me to take this scenario and have GetGPSData not return until OnLocationChanged has fired off?
Thanks
EDIT: To add, this method will not be regularly called. It's spontaneous and rare, so I don't want to use RequestLocationUpdates, get regular updates and return the most recent one because that would require always having GPS on, while would rain batteries unnecessarily.
You can do what you want with a TaskCompletionSource. I had the same issue and this is how I solved it:
TaskCompletionSource<Tuple<bool, string, GPSData> tcs;
// No need for the method to be async, as nothing is await-ed inside it.
public Task<Tuple<bool, string, GPSData>> GetGPSData() {
tcs = new TaskCompletionSource<Tuple<bool, string, GPSData>>();
gpsData = null;
var success = false;
var error = string.Empty;
if (!manager.IsProviderEnabled(LocationManager.GpsProvider)) {
//request permission or location services enabling
//set error
tcs.TrySetException(new Exception("some error")); // This will throw on the await-ing caller of this method.
} else {
manager.RequestSingleUpdate(LocationManager.GpsProvider, this, null);
success = true;
}
//return new Tuple<bool, string, GPSData>(success, error, gpsData); <-- change this to:
return this.tcs.Task;
}
And:
public void OnLocationChanged(Location location) {
gpsData = new GPSData(location.Latitude, location.Longitude);
// Here you set the result of TaskCompletionSource. Your other method completes the task and returns the result to its caller.
tcs.TrySetResult(new Tuple<bool, string, GPSData>(false, "someString", gpsData));
}
I'm trying to debug an asynchronous call from a test script inside my .NET webservice, but the breakpoints inside my async call are never getting hit. I even tried putting a Debugger.Break() inside of it. Below is the calling code...
HttpClient client = new HttpClient();
client.BaseAddress = new Uri(ConfigurationManager.AppSettings["BaseAddress"]);
string uri = "/api/Rd_Regions";
// Below is the line of code I want to step into, but it won't step into the 'client.GetAsync(uri)'...
HttpResponseMessage response = await client.GetAsync(uri);
if (response.IsSuccessStatusCode)
{
// Convert the result into business object
// Do stuff...
}
else do other stuff...
and the part of the webservice that should be getting called where the breakpoints are is here, the first is the context of the web api, followed by the method being called. I'd be happy if it stopped in either...
public partial class PIMSContext : DbContext
{
public PIMSContext()
: base(new OracleConnection(Security.ConfigurationReader.GetAppSetting("PIMS")), true)
//: base(new OracleConnection(ConfigurationManager.ConnectionStrings["PIMS"].ConnectionString), true)
etc....
And here is the method that is ultimately called:
// GET: api/RD_REGIONS
public IQueryable<RD_REGIONS> GetRD_REGIONS()
{
// I want the debugger to stop here!
Debugger.Break();
return db.RD_REGIONS;
}
Am I missing something? Is it not possible to step into this asynchronous call?
Any insight is appreciated.
Forgot to update with the answer earlier - it turns out I was accidentally debugging in Release mode (VS2015). Switching to Debug mode fixed it - all breakpoints started behaving as expected.
If i have understood you correctly - all breakpoints listed above are not getting hit? Not in await client.GetAsync(uri); neither in web-service GetRD_REGIONS() ? Is the code following after client.GetAsync not reached?
If it's true - maybe this method is never completed?
await client.GetAsync(uri);
Than it's possible that you are getting an exception in this place. Try to surround your GetAsync method with try/catch and place the breakpoint inside the catch block. Something like this:
...
HttpResponseMessage response = null;
try {
response = await client.GetAsync(uri);
}
catch (Exception e) {
throw e; //breakpoint goes here
}
...
Sometimes, because of your method is asynchronous, unhandled exceptions can't be registered globally by debugger or event-log. You can get this exception only with try/catch.
I'm currently seeing a problem whereby my await method is just hanging, and causing the response to just hang, not doing anything until I kill the request. This is evident in both Chrome debug tools and Fiddler.
I have the following API action defined:
[Route("state/{stateCode}")]
[LogApiCallFilter]
public async Task<IList<MapPlaceDTO>> GetWithinState(string stateCode)
{
//
// Additional code truncated for SO
// Via debugging I know that the 'state' variable below is correct
//
IList<Place> places = await _placeManager.GetPlacesInState(state);
// Instantiate the list of places.
IList<MapPlaceDTO> mapPlaces = new List<MapPlaceDTO>();
// Iterate through the places and add to the map place list
foreach (Place place in places)
{
mapPlaces.Add(MapPlaceDTO.FromPlace(place));
}
return mapPlaces;
}
When I step through that code in debug mode for a unit test for the GetWithinState action, the IList<Place> places = await _placeManager.GetPlacesInState(state); method runs without exception, however I am not able to hover over the places variable to inspect it, nothing happens. Nor can I add it to the watch list, I get the following message:
error CS0103: The name 'places' does not exist in the current context
Interestingly however, if I run the exact same code within a "PlaceManager" unit test, outside of the Web API project, the test runs fine, and I can inspect the places variable.
[Fact(DisplayName = "Can_Get_All_Places_Within_State")]
[Trait("Category", "Place Manager")]
public async Task Can_Get_All_Places_Within_State()
{
State state = new State()
{
ShortName = "VIC",
Name = "Victora",
CountryCode = "AU"
};
IList<Place> places = await _placeManager.GetPlacesInState(state);
Assert.NotNull(places);
Assert.True(places.Count > 0);
}
This is the code that runs within the PlaceManager.GetPlacesInState method:
public async Task<IList<Place>> GetPlacesInState(State state)
{
if (state == null)
{
throw new ArgumentNullException("state", "The 'state' parameter cannot be null.");
}
// Build the cache key
string cacheKey = String.Format("places_state_{0}", state.Id);
// Get the places from the cache (if they exist)
IList<Place> places = CacheManager.GetItem<IList<Place>>(cacheKey);
// Get the places from the database.
if (places == null)
{
// Get the places from the database
places = await _repository.Find(i => i.State.ToLower() == state.ShortName.ToLower() && i.Country.ToLower() == state.CountryCode.ToLower());
// If there are places, then add to the cache for next time
if (places != null && places.Count > 0)
{
CacheManager.AddItem(cacheKey, places);
}
}
// return the places
return (places != null ? places : new List<Place>());
}
Does anyone have any idea why this may be occurring within the API method, but is working fine in unit tests?
As mentioned by Alexei Levenkov above, I was causing a deadlock using the getDataAsync().Result code.
While mapping my Place object to a MapPlaceDTO object, the Place object has a get property to load the place type, which would call an async function in the following way:
public PlaceType PlaceType
{
get
{
return getPlaceType().Result;
}
}
Once I removed that property and just called the GetPlaceType method directly using the await keyword, everything started working correctly. Thanks Alexei!
I got a REST service running in AWS which returns some information. When I access the service from within my app, all works well. When I use the exact same code in a scheduledtask, it doesn't.
The weird thing is, there is no error, no exception, no hint as to what is going on other than that when stepping through the code in Visual Studio, when hitting F10 on the line where JsonConvert.DeserializeObject is called it doesn't step over the line executing it, but it is the same as hitting F5.
My code is:
private void infoRetrieval_DownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
{
if (e.Error == null)
{
string response = e.Result;
if (response != null)
{
try
{
CDateTime info = null;
CJsonDateTime r = JsonConvert.DeserializeObject<CJsonDateTime>(response);
info = new CDateTime(r);
if (info != null)
{
// Indicate that there is a new info
}
}
catch (Exception exc)
{
// The exception can't be handled in a meaningful way, so it's ignored.
}
}
}
}
The code that does the actual call to the webservice looks like this, and it is also the exact same code I use both in my App and in my agent as part of the OnInvoke method:
infoRetrievalClient = new WebClient();
infoRetrievalClient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(infoRetrieval_DownloadCompleted);
infoRetrievalClient.DownloadStringAsync(new Uri(<the URI to the webservice>));
Note that I am using the exact same code in my App as well as my ScheduledTaskAgent. Furthermore, I checked and the string retrieved in both cases is exact the same. Problem here is just that I can't parse the JSON and use the data.
The call to NotifyComplete is in the OnInvoke.
Any help is highly appreciated as I'm currently stuck.
Iwan
I fixed it, it turned out that Igor's question put me on the right track. I moved the NotifyComplete to the callback of the web-request and now it is all processing nicely.
Thanks Igor.
Iwan