When should an action be marked as async?

When should an action be marked as async? - c#

I have a controller action in mvc 4 app:
public ActionResult Index()
{
GetStartedModel gsModel = new GetStartedModel();
return View(gsModel);
}
and ViewModel:
public class GetStartedModel
{
public IEnumerable<SelectListItem> listA { get; set; }
public IEnumerable<SelectListItem> listB { get; set; }
public GetStartedModel()
{
TestDataWebServiceHelper service = new TestDataWebServiceHelper();
this.GetData(service);
}
private async void SetData(TestDataWebServiceHelper service)
{
listA = await this.SetListA(service);
listB = await this.SetListB(service);
}
private async Task<IEnumerable<SelectListItem>> SetListA(TestDataWebServiceHelper service)
{
List<String> rawList = new List<String>();
rawList = await service.GetValuesAsync("json");
return rawList.Select(x => new SelectListItem { Text = x, Value = x });
}
private async Task<IEnumerable<SelectListItem>> SetListB(TestDataWebServiceHelper service)
{
List<String> rawList = new List<String>();
rawList = await service.GetValuesAsync("json");
return rawList.Select(x => new SelectListItem { Text = x, Value = x });
}
}
When I call this controller action I receive following error:
An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, ensure that the Page is marked <%# Page Async="true" %>.
So, questions:
Should I somehow mark controller or action or page itself as asynchronous to allow this model initialization?
Is it possible to encapsulate all initialization logic to viewmodel and not to pop it to the controller?
What is the reason of that error? Seem like it related to WebForms, but I use Razor engine.

There are two problems in your code:
You shouldn't start async void operations from constructor like this. In fact, you usually shouldn't start any async operations from constructor and you also shouldn't use async void methods at all (except for event handlers).
I think that in your case, async factory method instead of an constructor makes the most sense:
private GetStartedModel()
{}
public static async Task<GetStartedModel> Create()
{
var service = new TestDataWebServiceHelper();
var result = new GetStartedModel();
listA = await result.SetListA(service);
listB = await result.SetListB(service);
return result;
}
For more details, see Stephen Cleary's post on async constructors.
You need to make your controller action async too:
public async Task<ActionResult> Index()
{
var gsModel = await GetStartedModel.Create()
return View(gsModel);
}

There a few things to take care about "async" are as follows :
The method should only be "async" when it needs to wait for the action to be performed by the code written in that.
if there is nothing to be "await" in the method, than that method should not declared as "async".
if there is any event (or else event handler) to be marked as async then return type should be "void" or any specific return type
but if any other (general) method should marked as "async" then the return type should be Task or Task< return-type > .
Now coming to your questions,
definitely you can mark "Controller", "Event" & "Page" as async (I'm
not damn sure about marking the page as async coz i never used it)
and by that those method will take rest until the action will be
performed completely written in that method.
And that's the actual system to encapsulate whole the initialization
logic in a viewModel.
for that make a Floder, named it as "ViewModels" and put all of the viewModel Codes in that folder and whenever needed, use that.
You should put your this code in a ViewModel:
private async void SetData(TestDataWebServiceHelper service)
{
listA = await this.SetListA(service);
listB = await this.SetListB(service);
}
private async Task<IEnumerable<SelectListItem>> SetListA(TestDataWebServiceHelper service)
{
List<String> rawList = new List<String>();
rawList = await service.GetValuesAsync("json");
return rawList.Select(x => new SelectListItem { Text = x, Value = x });
}
private async Task<IEnumerable<SelectListItem>> SetListB(TestDataWebServiceHelper service)
{
List<String> rawList = new List<String>();
rawList = await service.GetValuesAsync("json");
return rawList.Select(x => new SelectListItem { Text = x, Value = x });
}
and then you should call it whenever needed.
(Hope this helps...)

Related

C# ASP.NET Is there an alternative to duplicating error checking on every controller action

If I have a controller with quite a few actions, doing similar things e.g. Gets data from an api, does something with it, returns view with updated model. Is there a better way to handle errors. Currently I do this on a number of action methods, obviously this duplication doesn't feel right but I can't think of an alternative. Thanks
public async Task<IActionResult> method(string id)
{
var result = await _flightRepository.GetLightById(id);
if (!result.Valid)
{
return View("ErrorPage", result.Error.Message);
}
var viewModel = new FlightViewModel
{
Flight = result.Result
};
return View(viewModel);
}
Basically I want to somehow encapsulate the error handling logic to return the error view if valid is false, otherwise populate viewmodel and return view. The valid property returns true if there is an error with the request (this is done in api layer)
Thanks for any help

You should use Filters to reuse code among your actions.
For example in your case your filter could be:
public class FlightValidator: ActionFilterAttribute
{
private readonly string _flightIdRouteKey; // e.g 23
private readonly string _errorViewName; // e.g "ErrorPage"
private readonly IFlightRepository _flightRepo;
public FlightValidator(string flightIdRouteKey, string errorViewName, IFlightRepository flightRepository)
{
_flightIdRouteKey = flightIdRouteKey;
_errorViewName = errorViewName;
_flightRepo = flightRepository;
}
public override async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
{
int flightId = (int)context.RouteData.Values[_flightIdRouteKey];
var result = await _flightRepo.GetFlightById(flightId);
if (!result.Valid)
{
context.Result = new ViewResult
{
ViewName = _errorViewName,
ViewData = new ViewDataDictionary(result.ViewData)
{
Model = model
}
};
return;
}
await next();
}
}
Now you can use the filter like this:
[TypeFilter(typeof(FlightValidator), Arguments = new object[] { "id", "ErrorPage"})]
public async Task<IActionResult> method(string id)
{
var viewModel = new FlightViewModel
{
Flight = result.Result
};
return View(viewModel);
}
I suggest you to refer Filters docs for more information about filters.

If you use a static method like this:
static async Task<IActionResult> ProcessAsync<T>(
Func<Task<RepositoryResult<T>>> process,
Func<T, IActionResult> ifValid
)
{
var result = await process();
if (!result.Valid)
{
return View("ErrorPage", result.Error.Message);
}
return ifValid(result.Result);
}
Then you can use it like this:
public Task<IActionResult> method(string id)
{
return ProcessAsync(
() => _flightRepository.GetLightById(id),
flight =>
{
var viewModel = new FlightViewModel
{
Flight = flight
};
return View(viewModel);
}
);
}
The static method can be defined anywhere you wish - in a base Controller class, or in a different namespace entirely (in which case you would import it with using static Some.Namespace.With.Class;.
Additionally, I suggest you use the convention of an Async suffix for asynchronous methods - so because GetLightById returns Task<T> I suggest renaming it GetLightByIdAsync.

Can we call an async method from a constructor? [duplicate]

This question already has answers here:
Can constructors be async?
(15 answers)
Closed 3 years ago.
I'm needing to call a third party async method from a mvc app. The name of this async method is ForceClient.QueryAsync. It is from an open source project: https://github.com/developerforce/Force.com-Toolkit-for-NET/.
Below works fine, the model.Opportunity contains expected info when the process is in the View stage of the mvc:
public async Task<ActionResult> MyController(string Id) {
. . . .
MyModel model = new MyModel();
var client = new ForceClient(instanceUrl, accessToken, apiVersion);
var qry = await client.QueryAsync<MyModel.SFOpportunity>(
"SELECT Name, StageName FROM Opportunity where Id='" + Id + "'");
model.Opportunity = qry.Records.FirstOrDefault();
. . . .
return View(viewName, myModel);
}
But below does not work. The model.Opportunity is null when the process is in the View stage. I did some debugging and see that the flow goes like this:
1) Step1
2) Step2
3) In the View stage. At this point the model.Opportunity is null, which I need it to be populated.
4) Step3.
public async Task<ActionResult> MyController(string Id) {
. . . .
MyModel myModel = await Task.Run(() =>
{
var result = new MyModel(Id);
return result;
}); // =====> Step 1
. . . .
return View(viewName, myInfoView);
}
public class MyModel
{
public SFOpportunity Opportunity { get; set; }
public MyModel(string id)
{
setOpportunityAsync(id);
}
private async void setOpportunityAsync(string id)
{
. . .
var client = new ForceClient(instanceUrl, accessToken, apiVersion);
var qry = await client.QueryAsync<MyModel.SFOpportunity>(
"SELECT Name, StageName FROM Opportunity where Id='" + id + "'"); // ======> Step2
Opportunity = qry.Records.FirstOrDefault(); // =====> step3
}
So, my question is what do I need to do to get it to execute the steps in the following sequence:
1) Step1
2) Step2
3) Step3
4) In the View stage. At this point the model.Opportunity is should be populated.

You cannot have async constructors.
One alternative is to have async factory methods:
public class MyModel
{
public SFOpportunity Opportunity { get; set; }
private MyModel() { }
public static async Task<MyModel> CreateAsync(string id)
{
var result = new MyModel();
await result.setOpportunityAsync(id);
return result;
}
private async Task setOpportunityAsync(string id)
{
...
}
}

The constructor for MyModel does not (and can not) await setOpportunityAsync because the constructor itself isn't (and can't be) asynchronous. Otherwise you would be able to await the call to the constructor itself, but you can't. So the async method likely won't be finished executing right after the constructor is called. It will be finished... whenever it's finished.
Here's a smaller test class to illustrate the behavior:
public class HasConstructorWithAsyncCall
{
public HasConstructorWithAsyncCall()
{
MarkConstructorFinishedAsync();
}
public bool ConstructorHasFinished { get; private set; }
async void MarkConstructorFinishedAsync()
{
await Task.Delay(500);
ConstructorHasFinished = true;
}
}
What is the value of ConstructorHasFinished immediately after an instance is constructed? Here's a unit test:
[TestMethod]
public void TestWhenConstructorFinishes()
{
var subject = new HasConstructorWithAsyncCall();
Assert.IsFalse(subject.ConstructorHasFinished);
Thread.Sleep(600);
Assert.IsTrue(subject.ConstructorHasFinished);
}
The test passes. The constructor returns while MarkConstructorFinishedAsync hasn't completed, so ConstructorHasFinished is false. Half a second later it finishes, and the value is true.
You can't mark a constructor async so you can't await anything in the constructor.
In general we wouldn't put anything long-running like data retrieval in a constructor, including anything we would call asynchronously. If we do then we must either call it synchronously or know that the completion of the constructor doesn't mean that it's completely "constructed."

Make WebMethod Async Webform

I have tons of [WebMethod] in whole application and I want all my [WebMethod] to be async. I decorated all my[WebMethod] signatures with async and await keywords and then I run my app. It didn't work out as expected. Let say I have following code
Code Behind
[WebMethod]
public static async Task<List<XXX>> GetXXX()
{
return await new BusinessLogic().GetXXX().ConfigureAwait(false);
}
Business Layer
internal async Task<List<XXX>> GetXXX()
{
var obj = await objDAL.GetXXX().ConfigureAwait(false);
List<XXX> listDO = new List<XXX>();
foreach (var item in obj)
{
listDO.Add(new Countries
{
XXXName = item.name,
XXXID = item.id
});
}
return listDO;
objDAL.GetXXX
internal async Task<List<tblXXX>> GetXXX()
{
using (ABCEntities ctx = new ABCEntities())
{
return await ctx.tblXXX.ToListAsync().ConfigureAwait(false);
}
}
I have put debuggers in the whole path the code above come to return await ctx.tblXXX.ToListAsync(); and then the debugger loses control and the system became non-responsive.
I have found old solutions that doesn't use the power of async
and await.
Question is how I can make my WebMethod Function asyc.
Thanks

Timing issue between async initialisation and results loading in application startup

Seeking some input on a behaviour I'm noticing in my code below. This is my first attempt at async/await using Xamarin Forms and I have perused hundreds of posts, blogs and articles on the subject including the writings from Stephen Cleary on async from constructors and best practices to avoid locking. Although I am using a MVVM framework I assume my issue is more generic than that so I'll ignore it for the moment here.
If I am still missing something or there are ways to improve what I'm trying to do ... happy to listen and learn.
At a high level the logic is as follows:
Application starts and initialises
During initialisation verify database exist and if not - create the SQLite DB. Currently I force this every time to simulate a new application and pre-populate it with some sample data for development purposes
After initialisation completed load results set and display
This works most of the time but I have noticed 2 infrequent occurrences due to the async handling of the database initialisation and pre-populating:
Occasionally not all sample records created are displayed once the app started up - I assume this is because the pre-population phase has not completed when the results are loaded
Occasionally I get an error that one of the tables have not been created - I assume this is because the database initialisation has not completed when the results are loaded
The code - simplified to show the flow during initialisation and startup:
----------- VIEW / PAGE MODEL ----------------
public class MyListItemsPageModel
{
private ObservableRangeCollection<MyListItem> _myListItems;
private Command loadItemsCommand;
public MyListItemsPageModel()
{
_myListItems = new ObservableRangeCollection<MyListItem>();
}
public override void Init(object initData)
{
if (LoadItemsCommand.CanExecute(null))
LoadItemsCommand.Execute(null);
}
public Command LoadItemsCommand
{
get
{
return loadItemsCommand ?? (loadItemsCommand = new Command(async () => await ExecuteLoadItemsAsyncCommand(), () => { return !IsBusy; }));
}
}
public ObservableRangeCollection<MyListItem> MyListItems {
get { return _myListItems ?? (_myListItems = new ObservableRangeCollection<MyListItem>()); }
private set {
_myListItems = value;
}
}
private async Task ExecuteLoadItemsAsyncCommand() {
if (IsBusy)
return;
IsBusy = true;
loadItemsCommand.ChangeCanExecute();
var _results = await MySpecificDBServiceClass.LoadAllItemsAsync;
MyListItems = new ObservableRangeCollection<MyListItem>(_results.OrderBy(x => x.ItemName).ToList());
IsBusy = false;
loadItemsCommand.ChangeCanExecute();
}
}
----------- DB Service Class ----------------
// THERE IS A SPECIFIC SERVICE LAYER BETWEEN THIS CLASS AND THE PAGE VIEW MODEL HANDLING THE CASTING OF TO THE SPECIFIC DATA TYPE
// public class MySpecificDBServiceClass : MyGenericDBServiceClass
public class MyGenericDBServiceClass<T>: IDataAccessService<T> where T : class, IDataModel, new()
{
public SQLiteAsyncConnection _connection = FreshIOC.Container.Resolve<ISQLiteFactory>().CreateConnection();
internal static readonly AsyncLock Mutex = new AsyncLock();
public DataServiceBase()
{
// removed this from the constructor
//if (_connection != null)
//{
// IsInitialized = DatabaseManager.CreateTableAsync(_connection);
//}
}
public Task<bool> IsInitialized { get; private set; }
public virtual async Task<List<T>> LoadAllItemsAsync()
{
// Temporary async/await initialisation code. This will be moved to the start up as per Stephen's suggestion
await DBInitialiser();
var itemList = new List<T>();
using (await Mutex.LockAsync().ConfigureAwait(false))
{
itemList = await _connection.Table<T>().ToListAsync().ConfigureAwait(false);
}
return itemList;
}
}
----------- DB Manager Class ----------------
public class DatabaseManager
{
static double CURRENT_DATABASE_VERSION = 0.0;
static readonly AsyncLock Mutex = new AsyncLock();
private static bool IsDBInitialised = false;
private DatabaseManager() { }
public static async Task<bool> CreateTableAsync(SQLiteAsyncConnection CurrentConnection)
{
if (CurrentConnection == null || IsDBInitialised)
return IsDBInitialised;
await ProcessDBScripts(CurrentConnection);
return IsDBInitialised;
}
private static async Task ProcessDBScripts(SQLiteAsyncConnection CurrentConnection)
{
using (await Mutex.LockAsync().ConfigureAwait(false))
{
var _tasks = new List<Task>();
if (CURRENT_DATABASE_VERSION <= 0.1) // Dev DB - recreate everytime
{
_tasks.Add(CurrentConnection.DropTableAsync<Table1>());
_tasks.Add(CurrentConnection.DropTableAsync<Table2>());
await Task.WhenAll(_tasks).ConfigureAwait(false);
}
_tasks.Clear();
_tasks.Add(CurrentConnection.CreateTableAsync<Table1>());
_tasks.Add(CurrentConnection.CreateTableAsync<Table2>());
await Task.WhenAll(_tasks).ConfigureAwait(false);
_tasks.Clear();
_tasks.Add(UpgradeDBIfRequired(CurrentConnection));
await Task.WhenAll(_tasks).ConfigureAwait(false);
}
IsDBInitialised = true;
}
private static async Task UpgradeDBIfRequired(SQLiteAsyncConnection _connection)
{
await CreateSampleData();
return;
// ... rest of code not relevant at the moment
}
private static async Task CreateSampleData()
{
IDataAccessService<MyListItem> _dataService = FreshIOC.Container.Resolve<IDataAccessService<MyListItem>>();
ObservableRangeCollection<MyListItem> _items = new ObservableRangeCollection<MyListItem>(); ;
_items.Add(new MyListItem() { ItemName = "Test 1", ItemCount = 14 });
_items.Add(new MyListItem() { ItemName = "Test 2", ItemCount = 9 });
_items.Add(new MyListItem() { ItemName = "Test 3", ItemCount = 5 });
await _dataService.SaveAllItemsAsync(_items).ConfigureAwait(false);
_items = null;
_dataService = null;
IDataAccessService<Sample> _dataService2 = FreshIOC.Container.Resolve<IDataAccessService<AnotherSampleTable>>();
ObservableRangeCollection<Sample> _sampleList = new ObservableRangeCollection<Sample>(); ;
_sampleList.Add(new GuestGroup() { SampleName = "ABC" });
_sampleList.Add(new GuestGroup() { SampleName = "DEF" });
await _dataService2.SaveAllItemsAsync(_sampleList).ConfigureAwait(false);
_sampleList = null;
_dataService2 = null;
}
}

In your DataServiceBase constructor, you're calling DatabaseManager.CreateTableAsync() but not awaiting it, so by the time your constructor exits, that method has not yet completed running, and given that it does very little before awaiting, it's probably barely started at that point. As you can't effectively use await in a constructor, you need to remodel things so you do that initialisation at some other point; e.g. perhaps lazily when needed.
Then you also want to not use .Result/.Wait() whenever possible, especially as you're in an async method anyway (e.g. ProcessDBScripts()), so instead of doing
var _test = CurrentConnection.DropTableAsync<MyListItem>().Result;
rather do
var _test = await CurrentConnection.DropTableAsync<MyListItem>();
You also don't need to use Task.Run() for methods that return Task types anyway. So instead of
_tasks.Add(Task.Run(() => CurrentConnection.CreateTableAsync<MyListItem>().ConfigureAwait(false)));
_tasks.Add(Task.Run(() => CurrentConnection.CreateTableAsync<AnotherSampleTable>().ConfigureAwait(false)));
just do
_tasks.Add(CurrentConnection.CreateTableAsync<MyListItem>()));
_tasks.Add(CurrentConnection.CreateTableAsync<AnotherSampleTable>()));

sellotape has correctly diagnosed the code problem: the constructor is starting an asynchronous method but nothing is (a)waiting for it to complete. A simple fix would be to add await IsInitialized; to the beginning of LoadAllItemsAsync.
However, there's also a design problem:
After initialisation completed load results set and display
That's not possible on Xamarin, or any other modern UI platform. You must load your UI immediately and synchronously. What you should do is immediately display a splash/loading page and start the asynchronous initialization work. Then, when the async init is completed, update your VM/UI with your "real" page. If you just have LoadAllItemsAsync await IsInitialized, then your app will sit there for some time showing the user zero data before it "fills in".
You may find my NotifyTask<T> type (available on NuGet) useful here if you want to show a splash/spinner instead of zero data.

Call asynchronous method in constructor?

Summary: I would like to call an asynchronous method in a constructor. Is this possible?
Details: I have a method called getwritings() that parses JSON data. Everything works fine if I just call getwritings() in an async method and put await to left of it. However , when I create a LongListView in my page and try to populate it I'm finding that getWritings() is surprisingly returning null and the LongListView is empty.
To address this problem, I tried changing the return type of getWritings() to Task<List<Writing>> and then retrieving the result in the constructor via getWritings().Result. However, doing that ends up blocking the UI thread.
public partial class Page2 : PhoneApplicationPage
{
List<Writing> writings;
public Page2()
{
InitializeComponent();
getWritings();
}
private async void getWritings()
{
string jsonData = await JsonDataManager.GetJsonAsync("1");
JObject obj = JObject.Parse(jsonData);
JArray array = (JArray)obj["posts"];
for (int i = 0; i < array.Count; i++)
{
Writing writing = new Writing();
writing.content = JsonDataManager.JsonParse(array, i, "content");
writing.date = JsonDataManager.JsonParse(array, i, "date");
writing.image = JsonDataManager.JsonParse(array, i, "url");
writing.summary = JsonDataManager.JsonParse(array, i, "excerpt");
writing.title = JsonDataManager.JsonParse(array, i, "title");
writings.Add(writing);
}
myLongList.ItemsSource = writings;
}
}

The best solution is to acknowledge the asynchronous nature of the download and design for it.
In other words, decide what your application should look like while the data is downloading. Have the page constructor set up that view, and start the download. When the download completes update the page to display the data.
I have a blog post on asynchronous constructors that you may find useful. Also, some MSDN articles; one on asynchronous data-binding (if you're using MVVM) and another on asynchronous best practices (i.e., you should avoid async void).

You can also do just like this:
Task.Run(() => this.FunctionAsync()).Wait();
Note: Be careful about thread blocking!

I'd like to share a pattern that I've been using to solve these kinds of problems. It works rather well I think. Of course, it only works if you have control over what calls the constructor.
public class MyClass
{
public static async Task<MyClass> Create()
{
var myClass = new MyClass();
await myClass.Initialize();
return myClass;
}
private MyClass()
{
}
private async Task Initialize()
{
await Task.Delay(1000); // Do whatever asynchronous work you need to do
}
}
Basically what we do is we make the constructor private and make our own public static async method that is responsible for creating an instance of MyClass. By making the constructor private and keeping the static method within the same class we have made sure that no one could "accidentally" create an instance of this class without calling the proper initialization methods.
All the logic around the creation of the object is still contained within the class (just within a static method).
var myClass1 = new MyClass() // Cannot be done, the constructor is private
var myClass2 = MyClass.Create() // Returns a Task that promises an instance of MyClass once it's finished
var myClass3 = await MyClass.Create() // asynchronously creates and initializes an instance of MyClass
Implemented on the current scenario it would look something like:
public partial class Page2 : PhoneApplicationPage
{
public static async Task<Page2> Create()
{
var page = new Page2();
await page.getWritings();
return page;
}
List<Writing> writings;
private Page2()
{
InitializeComponent();
}
private async Task getWritings()
{
string jsonData = await JsonDataManager.GetJsonAsync("1");
JObject obj = JObject.Parse(jsonData);
JArray array = (JArray)obj["posts"];
for (int i = 0; i < array.Count; i++)
{
Writing writing = new Writing();
writing.content = JsonDataManager.JsonParse(array, i, "content");
writing.date = JsonDataManager.JsonParse(array, i, "date");
writing.image = JsonDataManager.JsonParse(array, i, "url");
writing.summary = JsonDataManager.JsonParse(array, i, "excerpt");
writing.title = JsonDataManager.JsonParse(array, i, "title");
writings.Add(writing);
}
myLongList.ItemsSource = writings;
}
}
Instead of doing
var page = new Page2();
you would be using:
var page = await Page2.Create();

A quick way to execute some time-consuming operation in any constructor is by creating an action and run them asynchronously.
new Action( async() => await InitializeThingsAsync())();
Running this piece of code will neither block your UI nor leave you with any loose threads. And if you need to update any UI (considering you are not using MVVM approach), you can use the Dispatcher to do so as many have suggested.
A Note: This option only provides you a way to start an execution of a method from the constructor if you don't have any init or onload or navigated overrides. Most likely this will keep on running even after the construction has been completed. Hence the result of this method call may NOT be available in the constructor itself.

My preferred approach:
// caution: fire and forget
Task.Run(async () => await someAsyncFunc());

Try to replace this:
myLongList.ItemsSource = writings;
with this
Dispatcher.BeginInvoke(() => myLongList.ItemsSource = writings);

To put it simply, referring to Stephen Cleary https://stackoverflow.com/a/23051370/267000
your page on creation should create tasks in constructor and you should declare those tasks as class members or put it in your task pool.
Your data are fetched during these tasks, but these tasks should awaited in the code i.e. on some UI manipulations, i.e. Ok Click etc.
I developped such apps in WP, we had a whole bunch of tasks created on start.

You could try AsyncMVVM.
Page2.xaml:
<PhoneApplicationPage x:Class="Page2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
<ListView ItemsSource="{Binding Writings}" />
</PhoneApplicationPage>
Page2.xaml.cs:
public partial class Page2
{
InitializeComponent();
DataContext = new ViewModel2();
}
ViewModel2.cs:
public class ViewModel2: AsyncBindableBase
{
public IEnumerable<Writing> Writings
{
get { return Property.Get(GetWritingsAsync); }
}
private async Task<IEnumerable<Writing>> GetWritingsAsync()
{
string jsonData = await JsonDataManager.GetJsonAsync("1");
JObject obj = JObject.Parse(jsonData);
JArray array = (JArray)obj["posts"];
for (int i = 0; i < array.Count; i++)
{
Writing writing = new Writing();
writing.content = JsonDataManager.JsonParse(array, i, "content");
writing.date = JsonDataManager.JsonParse(array, i, "date");
writing.image = JsonDataManager.JsonParse(array, i, "url");
writing.summary = JsonDataManager.JsonParse(array, i, "excerpt");
writing.title = JsonDataManager.JsonParse(array, i, "title");
yield return writing;
}
}
}

Don't ever call .Wait() or .Result as this is going to lock your app.
Don't spin up a new Task either, just call the ContinueWith
public class myClass
{
public myClass
{
GetMessageAsync.ContinueWith(GetResultAsync);
}
async Task<string> GetMessageAsync()
{
return await Service.GetMessageFromAPI();
}
private async Task GetResultAsync(Task<string> resultTask)
{
if (resultTask.IsFaulted)
{
Log(resultTask.Exception);
}
eles
{
//do what ever you need from the result
}
}
}
https://learn.microsoft.com/en-us/dotnet/standard/asynchronous-programming-patterns/consuming-the-task-based-asynchronous-pattern

A little late to the party, but I think many are struggling with this...
I've been searching for this as well. And to get your method/action running async without waiting or blocking the thread, you'll need to queue it via the SynchronizationContext, so I came up with this solution:
I've made a helper-class for it.
public static class ASyncHelper
{
public static void RunAsync(Func<Task> func)
{
var context = SynchronizationContext.Current;
// you don't want to run it on a threadpool. So if it is null,
// you're not on a UI thread.
if (context == null)
throw new NotSupportedException(
"The current thread doesn't have a SynchronizationContext");
// post an Action as async and await the function in it.
context.Post(new SendOrPostCallback(async state => await func()), null);
}
public static void RunAsync<T>(Func<T, Task> func, T argument)
{
var context = SynchronizationContext.Current;
// you don't want to run it on a threadpool. So if it is null,
// you're not on a UI thread.
if (context == null)
throw new NotSupportedException(
"The current thread doesn't have a SynchronizationContext");
// post an Action as async and await the function in it.
context.Post(new SendOrPostCallback(async state => await func((T)state)), argument);
}
}
Usage/Example:
public partial class Form1 : Form
{
private async Task Initialize()
{
// replace code here...
await Task.Delay(1000);
}
private async Task Run(string myString)
{
// replace code here...
await Task.Delay(1000);
}
public Form1()
{
InitializeComponent();
// you don't have to await nothing.. (the thread must be running)
ASyncHelper.RunAsync(Initialize);
ASyncHelper.RunAsync(Run, "test");
// In your case
ASyncHelper.RunAsync(getWritings);
}
}
This works for Windows.Forms and WPF

In order to use async within the constructor and ensure the data is available when you instantiate the class, you can use this simple pattern:
class FooClass : IFooAsync
{
FooClass
{
this.FooAsync = InitFooTask();
}
public Task FooAsync { get; }
private async Task InitFooTask()
{
await Task.Delay(5000);
}
}
The interface:
public interface IFooAsync
{
Task FooAsync { get; }
}
The usage:
FooClass foo = new FooClass();
if (foo is IFooAsync)
await foo.FooAsync;

Brian Lagunas has shown a solution that I really like. More info his youtube video
Solution:
Add a TaskExtensions method
public static class TaskExtensions
{
public static async void Await(this Task task, Action completedCallback = null ,Action<Exception> errorCallBack = null )
{
try
{
await task;
completedCallback?.Invoke();
}
catch (Exception e)
{
errorCallBack?.Invoke(e);
}
}
}
Usage:
public class MyClass
{
public MyClass()
{
DoSomething().Await();
// DoSomething().Await(Completed, HandleError);
}
async Task DoSomething()
{
await Task.Delay(3000);
//Some works here
//throw new Exception("Thrown in task");
}
private void Completed()
{
//some thing;
}
private void HandleError(Exception ex)
{
//handle error
}
}

The answer is simple, If you are developing an UWP app, then add the async function to the Page_Loaded method of the page.

if you want it to wait task to be done you can improve madlars codes like below. (I tried on .net core 3.1 it worked )
var taskVar = Task.Run(async () => await someAsyncFunc());
taskVar.Wait();

You could put the async calls in a separate method and call that method in the constructor.
Although, this may lead to a situation where some variable values not being available at the time you expect them.
public NewTravelPageVM(){
GetVenues();
}
async void GetVenues(){
var locator = CrossGeolocator.Current;
var position = await locator.GetPositionAsync();
Venues = await Venue.GetVenues(position.Latitude, position.Longitude);
}

We Keep Coding

C# (C-Sharp) is a programming language developed by Microsoft that runs on the .NET Framework.

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