Call asynchronous method in constructor

Question

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 lt List lt Writing gt  gt  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 lt Writing gt  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  lt  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

User · Answer

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;

User · Answer

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 lt Task gt  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   gt  await func     null              public static void RunAsync lt T gt  Func lt T  Task gt  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   gt  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

User · Answer

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

User · Answer

A quick way to execute some time-consuming operation in any constructor is by creating an action and run them asynchronously  new Action  async     gt  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

User · Answer

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

User · Answer

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 lt Exception gt  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  quot Thrown in task quot               private void Completed                   some thing             private void HandleError Exception ex                  handle error

User · Answer

You could try AsyncMVVM   Page2 xaml    lt PhoneApplicationPage x Class  Page2                        xmlns  http   schemas microsoft com winfx 2006 xaml presentation  gt       lt ListView ItemsSource   Binding Writings     gt   lt  PhoneApplicationPage gt    Page2 xaml cs   public partial class Page2       InitializeComponent        DataContext   new ViewModel2        ViewModel2 cs   public class ViewModel2  AsyncBindableBase       public IEnumerable lt Writing gt  Writings               get   return Property Get GetWritingsAsync                private async Task lt IEnumerable lt Writing gt  gt  GetWritingsAsync                 string jsonData   await JsonDataManager GetJsonAsync  1            JObject obj   JObject Parse jsonData           JArray array    JArray obj  posts             for  int i   0  i  lt  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

User · Answer

You can also do just like this   Task Run      gt  this FunctionAsync    Wait      Note  Be careful about thread blocking

User · Answer

My preferred approach      caution  fire and forget Task Run async      gt  await someAsyncFunc

User · Answer

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 lt string gt  GetMessageAsync           return await Service GetMessageFromAPI         private async Task GetResultAsync Task lt string gt  resultTask        if  resultTask IsFaulted          Log resultTask Exception      eles      do what ever you need from the result         https   docs microsoft com en-us dotnet standard asynchronous-programming-patterns consuming-the-task-based-asynchronous-pattern

User · Answer

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  Example below  public class MyClass       public static async Task lt MyClass gt  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           Basicly 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 noone could  accidently  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 lt Page2 gt  Create                 var page   new Page2            await page getWritings            return page             List lt Writing gt  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  lt  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            And instead of doing  var page   new Page2      You would be doing  var page   await Page2 Create

User · Answer

Try to replace this   myLongList ItemsSource   writings    with this  Dispatcher BeginInvoke      gt  myLongList ItemsSource   writings

User · Answer

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

[c#] Call asynchronous method in constructor?

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